{"id":10558,"date":"2026-09-26T11:34:30","date_gmt":"2026-09-26T10:34:30","guid":{"rendered":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/oxidation-numbers\/"},"modified":"2026-09-26T12:43:28","modified_gmt":"2026-09-26T11:43:28","slug":"oxidation-numbers","status":"publish","type":"page","link":"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/oxidation-numbers\/","title":{"rendered":"Oxidation Numbers"},"content":{"rendered":"\n<section class=\"ols-revision-page ols-nature-of-covalent-bonding-9ch0-page\">\n  <style>\n    .ols-revision-page {\n      --navy: #1C244B;\n      --blue: #2563eb;\n      --soft-blue: #eef4ff;\n      --soft-red: #fff7f7;\n      --soft-purple: #f7f0ff;\n      --soft-green: #f0f7f1;\n      --soft-orange: #fff7ed;\n      --grey-text: #667085;\n      --body-text: #1f2937;\n      --border: rgba(28, 36, 75, 0.14);\n      --shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      --inner-shadow: 0 10px 26px rgba(28, 36, 75, 0.08);\n      font-family: Poppins, Arial, sans-serif;\n      color: var(--navy);\n      background: #ffffff;\n    }\n\n    .ols-revision-page * { box-sizing: border-box; }\n    .ols-main { min-width: 0; max-width: 970px; }\n    .ols-breadcrumbs { display: flex; flex-wrap: wrap; gap: 8px; margin: 10px 0 22px; font-size: 15px; color: var(--grey-text); }\n    .ols-breadcrumbs a, .ols-breadcrumbs span { color: var(--grey-text); text-decoration: none; font-weight: 600; }\n    .ols-breadcrumbs a:hover { color: var(--blue); text-decoration: underline; text-underline-offset: 3px; }\n    .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card { background: #ffffff; border: 1px solid var(--border); border-radius: 28px; box-shadow: var(--shadow); padding: 34px; margin-bottom: 24px; overflow: hidden; }\n    .ols-title-card { background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); }\n    .ols-title-card h1 { margin: 0 0 18px; font-size: clamp(36px, 5vw, 58px); line-height: 1.08; font-weight: 800; letter-spacing: -0.04em; color: #111827; }\n    .ols-page-intro { font-size: 19px; line-height: 1.7; font-weight: 300; color: var(--body-text); margin: 0 0 22px; }\n    .ols-badges { display: flex; flex-direction: column; align-items: flex-start; gap: 12px; margin-bottom: 22px; }\n    .ols-badge { display: inline-flex; align-items: center; padding: 9px 14px; border-radius: 999px; background: #ffffff; border: 1px solid var(--border); font-size: 15px; font-weight: 600; color: var(--navy); }\n    .ols-author { display: flex; align-items: center; gap: 20px; padding: 28px; border-radius: 28px; background: linear-gradient(135deg, #ffffff 0%, #f8fbff 100%); border: 1px solid rgba(28,36,75,0.12); box-shadow: 0 18px 45px rgba(28,36,75,0.10); margin-top: 22px; }\n    .ols-author-avatar-img { width: 92px; height: 92px; border-radius: 50%; object-fit: cover; object-position: center; flex-shrink: 0; border: 4px solid #ffffff; box-shadow: 0 14px 32px rgba(28,36,75,0.18), 0 0 0 1px rgba(28,36,75,0.10); transition: transform 0.25s ease, box-shadow 0.25s ease; }\n    .ols-author:hover .ols-author-avatar-img { transform: scale(1.04); box-shadow: 0 20px 42px rgba(28,36,75,0.22), 0 0 0 1px rgba(28,36,75,0.10); }\n    .ols-author-content { min-width: 0; }\n    .ols-author-title { margin: 0 0 4px; font-size: clamp(24px,3vw,34px); line-height: 1.15; font-weight: 700; color: var(--navy); letter-spacing: -0.03em; }\n    .ols-author-description { margin: 0; font-size: 17px; line-height: 1.7; font-weight: 300; color: var(--grey-text); }\n    .ols-linkedin-pill { display: inline-flex; align-items: center; justify-content: center; gap: 10px; margin-top: 16px; padding: 11px 18px; border-radius: 999px; background: linear-gradient(135deg,#0A66C2,#004182); color: #ffffff; text-decoration: none; font-size: 14px; line-height: 1; font-weight: 700; letter-spacing: 0.01em; box-shadow: 0 10px 22px rgba(10,102,194,0.24); position: relative; overflow: hidden; transition: transform 0.22s ease, box-shadow 0.22s ease; }\n    .ols-linkedin-pill::before { content: \"\"; position: absolute; top: 0; left: -120%; width: 100%; height: 100%; background: linear-gradient(120deg, rgba(255,255,255,0) 0%, rgba(255,255,255,0.28) 50%, rgba(255,255,255,0) 100%); transition: left 0.7s ease; }\n    .ols-linkedin-pill:hover { transform: translateY(-3px) scale(1.03); box-shadow: 0 16px 34px rgba(10,102,194,0.34), 0 0 0 6px rgba(10,102,194,0.10); color: #ffffff; }\n    .ols-linkedin-pill:hover::before { left: 120%; }\n    .ols-linkedin-icon { width: 18px; height: 18px; display: block; flex-shrink: 0; }\n\n    .ols-note-card.soft { background: linear-gradient(135deg, #ffffff 0%, var(--soft-red) 100%); }\n    .ols-note-card.purple { background: linear-gradient(135deg, #ffffff 0%, var(--soft-purple) 100%); }\n    .ols-note-card.orange { background: linear-gradient(135deg, #ffffff 0%, var(--soft-orange) 100%); }\n    .ols-note-title { display: flex; align-items: center; gap: 14px; margin-bottom: 20px; }\n    .ols-note-icon { width: 52px; height: 52px; border-radius: 16px; background: var(--navy); color: #ffffff; display: grid; place-items: center; font-size: 24px; font-weight: 800; flex-shrink: 0; }\n    .ols-note-title h2, .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin: 0; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; color: #111827; letter-spacing: -0.03em; }\n    .ols-h5p-card h2, .ols-related-card h2, .ols-faq-card h2 { margin-bottom: 14px; }\n    .ols-note-card p, .ols-note-card li { font-size: clamp(15px, 1.25vw, 17px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-h5p-card p, .ols-related-card p, .ols-faq-card p { font-size: clamp(15px, 1.3vw, 18px); line-height: 1.65; font-weight: 300; color: var(--body-text); }\n    .ols-note-card strong, .ols-h5p-card strong, .ols-related-card strong, .ols-faq-card strong { font-weight: 700; color: var(--navy); }\n    .ols-note-card ul, .ols-note-card ol { margin: 0; padding-left: 22px; }\n\n    .ols-key-box { margin-top: 20px; background: var(--soft-green); border: 1px solid rgba(63, 143, 70, 0.25); border-radius: 20px; padding: 18px 20px; }\n    .ols-key-box p { margin: 0; font-weight: 400; color: var(--navy); }\n    .ols-definition-box { background: linear-gradient(135deg, #ffffff, var(--soft-purple)); border: 2px dashed rgba(122, 62, 157, 0.35); border-radius: 24px; padding: 22px 24px; margin-top: 22px; }\n    .ols-definition-box p { margin: 0; color: var(--navy); font-weight: 400; }\n\n    .ols-rule-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-rule-item { background: #ffffff; border: 1px solid var(--border); border-left: 5px solid var(--blue); border-radius: 18px; padding: 18px; box-shadow: var(--inner-shadow); }\n    .ols-rule-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.25; color: var(--navy); }\n    .ols-rule-item p { margin: 0; font-size: 15px; line-height: 1.6; }\n\n    .ols-figure-card { margin: 26px auto 4px; border: 1px solid var(--border); border-radius: 26px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); max-width: 100%; }\n    .ols-figure-card.medium { max-width: 820px; }\n    .ols-figure-card.compact { max-width: 700px; }\n    .ols-figure-image { width: 100%; min-height: 260px; display: flex; align-items: center; justify-content: center; background: #ffffff; padding: 20px; }\n    .ols-figure-image img { max-width: 100%; height: auto; display: block; }\n    .ols-figure-caption { padding: 18px 24px 22px; background: linear-gradient(135deg, #ffffff, #f8fbff); border-top: 1px solid var(--border); }\n    .ols-figure-caption p { margin: 0; color: #5f6b85; font-size: clamp(15px, 1.3vw, 17px); line-height: 1.6; font-weight: 300; font-style: italic; }\n    .ols-zoom-card, .ols-zoom-card * { box-sizing: border-box; }\n    .ols-zoom-card { display: block !important; margin: 26px auto 34px !important; border: 1px solid rgba(28, 36, 75, 0.14) !important; border-radius: 26px !important; background: #ffffff !important; box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10) !important; overflow: visible !important; position: relative !important; z-index: 1 !important; isolation: isolate !important; }\n    .ols-zoom-card.medium { max-width: 820px; }\n    .ols-zoom-card.compact { max-width: 700px; }\n    .ols-zoom-card.wide { max-width: 940px; }\n    .ols-zoom-card.slim { max-width: 600px; }\n    .ols-zoom-card:hover { z-index: 50 !important; }\n    .ols-zoom-card-image { display: flex !important; align-items: center !important; justify-content: center !important; width: 100% !important; min-height: 240px !important; padding: 20px !important; background: #ffffff !important; overflow: visible !important; border-top-left-radius: 26px !important; border-top-right-radius: 26px !important; position: relative !important; z-index: 2 !important; }\n    .ols-zoom-card img.ols-zoomable-img { display: block !important; max-width: 100% !important; height: auto !important; border-radius: 18px !important; cursor: zoom-in !important; pointer-events: auto !important; user-select: none !important; -webkit-user-drag: none !important; transform: translateZ(0) scale(1) !important; transform-origin: center center !important; transition: transform 0.32s ease, box-shadow 0.32s ease, filter 0.32s ease !important; position: relative !important; z-index: 2 !important; }\n    @media (hover: hover) and (pointer: fine) { .ols-zoom-card img.ols-zoomable-img:hover { transform: translateZ(0) scale(1.35) !important; box-shadow: 0 28px 70px rgba(28, 36, 75, 0.34) !important; filter: saturate(1.02) contrast(1.01) !important; z-index: 100 !important; } }\n    .ols-zoom-card-caption { padding: 18px 22px 20px !important; background: linear-gradient(135deg, #ffffff, #f8fbff) !important; border-bottom-left-radius: 26px !important; border-bottom-right-radius: 26px !important; position: relative !important; z-index: 1 !important; }\n    .ols-zoom-card-caption p { margin: 0 !important; color: #5f6b85 !important; font-size: 15px !important; line-height: 1.65 !important; font-weight: 300 !important; font-style: italic !important; font-family: Poppins, Arial, sans-serif !important; }\n    .ols-image-lightbox { position: fixed; inset: 0; z-index: 999999; display: none; align-items: center; justify-content: center; padding: 34px; background: rgba(10, 15, 35, 0.86); backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px); }\n    .ols-image-lightbox.is-open { display: flex; }\n    .ols-image-lightbox-inner { position: relative; width: min(96vw, 1500px); max-height: 92vh; display: flex; align-items: center; justify-content: center; }\n    .ols-image-lightbox-img { display: block; max-width: 100%; max-height: 92vh; height: auto; width: auto; border-radius: 22px; background: #ffffff; box-shadow: 0 32px 90px rgba(0, 0, 0, 0.45); object-fit: contain; }\n    .ols-image-lightbox-close { position: absolute; top: -18px; right: -18px; width: 46px; height: 46px; border: 0; border-radius: 50%; background: #ffffff; color: var(--navy); font-family: Poppins, Arial, sans-serif; font-size: 28px; line-height: 1; font-weight: 700; cursor: pointer; box-shadow: 0 16px 34px rgba(0, 0, 0, 0.28); display: flex; align-items: center; justify-content: center; transition: transform 0.2s ease, background 0.2s ease, color 0.2s ease; }\n    .ols-image-lightbox-close:hover { transform: scale(1.08); background: var(--blue); color: #ffffff; }\n    @media (max-width: 760px) { .ols-zoom-card { border-radius: 22px !important; overflow: hidden !important; } .ols-zoom-card-image { min-height: auto !important; padding: 12px !important; overflow: hidden !important; border-top-left-radius: 22px !important; border-top-right-radius: 22px !important; } .ols-zoom-card img.ols-zoomable-img, .ols-zoom-card img.ols-zoomable-img:hover { transform: none !important; box-shadow: none !important; } .ols-zoom-card-caption { border-bottom-left-radius: 22px !important; border-bottom-right-radius: 22px !important; } .ols-image-lightbox { padding: 16px; } .ols-image-lightbox-inner { width: 100%; max-height: 88vh; } .ols-image-lightbox-img { max-height: 88vh; border-radius: 16px; } .ols-image-lightbox-close { top: 10px; right: 10px; width: 42px; height: 42px; font-size: 26px; } }\n\n\n    .ols-table-wrap { overflow: hidden; border-radius: 22px; border: 1px solid var(--border); background: #ffffff; margin-top: 18px; }\n    .ols-table { width: 100%; border-collapse: collapse; table-layout: fixed; }\n    .ols-table th { background: var(--navy); color: #ffffff; padding: 16px; text-align: left; font-size: clamp(14px, 1.2vw, 17px); font-weight: 600; overflow-wrap: anywhere; }\n    .ols-table td { padding: 16px; border-bottom: 1px solid var(--border); font-size: clamp(14px, 1.15vw, 16px); line-height: 1.45; color: var(--body-text); vertical-align: top; overflow-wrap: anywhere; }\n    .ols-table tr:last-child td { border-bottom: none; }\n\n    .ols-h5p-card { background: linear-gradient(135deg, #ffffff 0%, #f7f0ff 100%); }\n    .ols-h5p-frame { margin-top: 22px; padding: 18px; border-radius: 24px; background: #ffffff; border: 1px solid var(--border); box-shadow: inset 0 0 0 1px rgba(28, 36, 75, 0.03); }\n\n    .ols-faq-list { display: grid; gap: 14px; margin-top: 18px; }\n    .ols-faq-item { background: #ffffff; border: 1px solid var(--border); border-radius: 18px; padding: 18px 20px; box-shadow: var(--inner-shadow); }\n    .ols-faq-item h3 { margin: 0 0 8px; font-size: 20px; line-height: 1.3; color: var(--navy); }\n    .ols-faq-item p { margin: 0; font-size: 16px; line-height: 1.65; color: var(--body-text); }\n\n    .ols-related-grid { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 16px; margin-top: 18px; }\n    .ols-related-item { border: 1px solid var(--border); border-radius: 18px; padding: 18px; background: #ffffff; color: var(--navy); text-decoration: none; font-weight: 600; line-height: 1.5; transition: transform 0.2s ease, box-shadow 0.2s ease; }\n    .ols-related-item:hover { transform: translateY(-2px); box-shadow: 0 10px 22px rgba(28, 36, 75, 0.08); }\n\n    .ols-course-cta-covalent {\n      width: 100%;\n      margin: 30px 0 24px;\n      font-family: Poppins, Arial, sans-serif;\n    }\n    .ols-course-cta-covalent, .ols-course-cta-covalent * { box-sizing: border-box; }\n    .ols-course-cta-card {\n      overflow: hidden;\n      border-radius: 30px;\n      border: 1px solid var(--border);\n      background: radial-gradient(circle at top left, rgba(37, 99, 235, 0.16), transparent 34%), linear-gradient(135deg, #ffffff 0%, #f8fbff 100%);\n      box-shadow: var(--shadow);\n      padding: 30px;\n    }\n    .ols-course-cta-top {\n      display: grid;\n      grid-template-columns: minmax(260px, 0.9fr) minmax(0, 1.1fr);\n      gap: 28px;\n      align-items: center;\n      margin-bottom: 24px;\n    }\n    .ols-course-cta-image-link { display: block; text-decoration: none; border-radius: 24px; }\n    .ols-course-cta-image {\n      width: 100%;\n      border-radius: 24px;\n      overflow: hidden;\n      border: 1px solid var(--border);\n      background: #ffffff;\n      box-shadow: var(--inner-shadow);\n      transition: transform 0.35s ease, box-shadow 0.35s ease;\n    }\n    .ols-course-cta-image img { width: 100%; height: auto; display: block; transition: transform 0.45s ease; }\n    .ols-course-cta-image-link:hover .ols-course-cta-image { transform: translateY(-8px) scale(1.015); box-shadow: 0 26px 60px rgba(28, 36, 75, 0.18), 0 0 0 1px rgba(37, 99, 235, 0.12); }\n    .ols-course-cta-image-link:hover .ols-course-cta-image img { transform: scale(1.03); }\n    .ols-course-cta-header-row { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 14px; margin-bottom: 18px; }\n    .ols-course-cta-kicker { display: inline-flex; align-items: center; padding: 8px 14px; border-radius: 999px; background: #ffffff; border: 1px solid rgba(37, 99, 235, 0.18); color: var(--blue); font-size: 14px; line-height: 1.2; font-weight: 700; box-shadow: var(--inner-shadow); }\n    .ols-course-cta-covalent h2 { margin: 0 0 14px; font-size: clamp(28px, 3.5vw, 42px); line-height: 1.15; font-weight: 800; letter-spacing: -0.03em; color: #111827; }\n    .ols-course-cta-intro { margin: 0 0 24px; color: var(--body-text); font-size: clamp(16px, 1.4vw, 18px); line-height: 1.7; font-weight: 300; }\n    .ols-course-cta-features { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 14px; margin: 0 0 30px; }\n    .ols-course-feature { background: rgba(255, 255, 255, 0.78); border: 1px solid var(--border); border-radius: 18px; padding: 16px 18px; }\n    .ols-course-feature h3 { margin: 0 0 6px; color: var(--navy); font-size: 18px; line-height: 1.3; font-weight: 800; }\n    .ols-course-feature p { margin: 0; color: var(--body-text); font-size: 15px; line-height: 1.6; font-weight: 300; }\n    .ols-course-cta-bottom { display: flex; justify-content: center; padding-top: 22px; border-top: 1px solid var(--border); }\n    .ols-course-button { display: inline-flex; align-items: center; justify-content: center; padding: 15px 28px; border-radius: 999px; background: var(--navy); color: #ffffff; text-decoration: none; font-size: 16px; line-height: 1.2; font-weight: 800; box-shadow: 0 12px 26px rgba(28, 36, 75, 0.18); transition: transform 0.2s ease, background 0.2s ease, box-shadow 0.2s ease; }\n    .ols-course-button:hover { transform: translateY(-2px); background: var(--blue); color: #ffffff; box-shadow: 0 16px 32px rgba(37, 99, 235, 0.24); }\n    .ols-course-button-top { flex-shrink: 0; padding: 12px 22px; font-size: 15px; }\n\n    .ols-attribution-card {\n      background: linear-gradient(135deg, #ffffff, #f8fbff);\n      border: 1px solid rgba(28, 36, 75, 0.14);\n      border-radius: 28px;\n      box-shadow: 0 18px 45px rgba(28, 36, 75, 0.10);\n      padding: 34px;\n      margin-bottom: 24px;\n      overflow: hidden;\n      font-family: Poppins, Arial, sans-serif;\n      box-sizing: border-box;\n    }\n    .ols-attribution-card, .ols-attribution-card * { box-sizing: border-box; }\n    .ols-attribution-card p { margin: 0; font-size: 14px; line-height: 1.65; font-weight: 300; color: #667085; }\n    .ols-attribution-card strong { font-weight: 700; color: #1C244B; }\n\n    @media (max-width: 1050px) {\n      .ols-main { max-width: none; }\n      .ols-related-grid { grid-template-columns: repeat(2, minmax(0, 1fr)); }\n      .ols-course-cta-top { grid-template-columns: 1fr; }\n      .ols-course-cta-image-link { max-width: 520px; margin: 0 auto; }\n    }\n\n    @media (max-width: 760px) {\n      .ols-title-card, .ols-note-card, .ols-h5p-card, .ols-related-card, .ols-faq-card, .ols-attribution-card { padding: 24px 18px; border-radius: 22px; }\n      .ols-note-title { align-items: flex-start; }\n      .ols-related-grid, .ols-course-cta-features { grid-template-columns: 1fr; }\n      .ols-figure-card { border-radius: 22px; }\n      .ols-figure-image { min-height: 210px; padding: 14px; }\n      .ols-figure-caption { padding: 16px; }\n      .ols-table-wrap { border: none; background: transparent; }\n      .ols-table, .ols-table thead, .ols-table tbody, .ols-table th, .ols-table td, .ols-table tr { display: block; width: 100%; }\n      .ols-table { border-collapse: separate; border-spacing: 0; }\n      .ols-table thead { display: none; }\n      .ols-table tr { margin-bottom: 14px; border: 1px solid var(--border); border-radius: 18px; overflow: hidden; background: #ffffff; box-shadow: var(--inner-shadow); }\n      .ols-table td { border-bottom: 1px solid var(--border); padding: 14px 16px; overflow-wrap: anywhere; }\n      .ols-table td:last-child { border-bottom: none; }\n      .ols-table td::before { content: attr(data-label); display: block; margin-bottom: 6px; font-size: 13px; 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}\n<\/style>\n\n  <aside class=\"ols-sidebar\">\n  <div class=\"ols-sidebar-header\">\n    <h3>Revision Notes<\/h3>\n    <p>A Level Chemistry<\/p>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>2.1.5 Redox<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/\">2.1.5 Overview<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/oxidation-numbers\/\">Oxidation Numbers<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/disproportionation\/\">Disproportionation<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/redox-classification\/\">Redox Classification<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n\n  <div class=\"ols-topic-group\">\n    <h4>Other Sections<\/h4>\n\n    <ul class=\"ols-topic-list\">\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-2-group-2\/\">3.1.2 Group 2<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-3-the-halogens\/\">3.1.3 The Halogens<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-4-qualitative-analysis\/\">3.1.4 Qualitative Analysis<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-4-acids\/\">2.1.4 Acids<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-1-1-periodicity\/\">3.1.1 Periodicity<\/a>\n      <\/li>\n      <li>\n        <a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/3-2-1-enthalpy-changes\/\">3.2.1 Enthalpy Changes<\/a>\n      <\/li>\n    <\/ul>\n  <\/div>\n<\/aside>\n\n<script>\r\n(function() {\r\n  function normalisePath(path) {\r\n    return String(path || '')\r\n      .split('?')[0]\r\n      .split('#')[0]\r\n      .replace(\/\\\/+$\/, '')\r\n      .toLowerCase();\r\n  }\r\n\r\n  function highlightActive() {\r\n    var sidebar = document.querySelector('.ols-sidebar');\r\n    if (!sidebar) return false;\r\n\r\n    var currentPath = 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href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/\">OCR<\/a> \/\n<a href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/\">2.1.5 Redox<\/a> \/\n<span>Oxidation Numbers<\/span>\n<\/nav>\n\n      <header class=\"ols-title-card\">\n        <h1>Oxidation Numbers<\/h1>\n        <p class=\"ols-page-intro\">A concise revision guide to oxidation numbers: what the number means, the rules for assigning it, working it out in compounds and ions (including peroxides and metal hydrides), Roman numerals in names, and writing formulae from oxidation numbers.<\/p>\n\n        <div class=\"ols-badges\">\n<div class=\"ols-badge\">Paper 1 and 3<\/div>\n<div class=\"ols-badge\">2.1.5: Redox<\/div>\n<div class=\"ols-badge\">H432\/01 and H432\/03<\/div>\n<\/div>\n\n        <div class=\"ols-author\">\n\n    <img decoding=\"async\"\n      class=\"ols-author-avatar-img\"\n      src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/05\/Author-Profile.jpeg\"\n      alt=\"Dr. Mohammed Al-Fatah\"\n    >\n\n    <div class=\"ols-author-content\">\n\n      <h2 class=\"ols-author-title\">\n        Written by:<br><span>Dr. Mohammed Al-Fatah<\/span>\n      <\/h2>\n\n      <p class=\"ols-author-description\">\n        Chemistry specialist revision notes for A Level Chemistry.\n      <\/p>\n\n      <a class=\"ols-linkedin-pill\" href=\"https:\/\/www.linkedin.com\/in\/doctormohammedfatah\/\" target=\"_blank\" rel=\"noopener noreferrer\">\n        <svg class=\"ols-linkedin-icon\" viewBox=\"0 0 24 24\" fill=\"currentColor\" aria-hidden=\"true\">\n          <path d=\"M4.98 3.5C4.98 4.88 3.86 6 2.48 6S0 4.88 0 3.5 1.12 1 2.48 1s2.5 1.12 2.5 2.5zM.5 8h4V24h-4V8zm7 0h3.8v2.2h.1c.5-.9 1.8-2.2 3.9-2.2 4.2 0 5 2.8 5 6.4V24h-4v-7.6c0-1.8 0-4.2-2.6-4.2s-3 2-3 4v7.8h-4V8z\"\/>\n        <\/svg>\n        View LinkedIn Profile\n      <\/a>\n\n    <\/div>\n\n  <\/div>\n      <\/header>\n\n      <section class=\"ols-h5p-card ols-h5p-inline ols-h5p-recap\">\n<span class=\"ols-h5p-kicker\">Before you start<\/span>\n<h2>GCSE Recap: Losing and Gaining Electrons<\/h2>\n<p>Three quick questions on the GCSE ideas this page builds on: ions, charges and the reactivity series.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"850\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">1<\/div>\n<h2>What an Oxidation number Is<\/h2>\n<\/div>\n<p>An <strong>oxidation number<\/strong> is a number given to each atom in a substance that shows how many electrons it has gained, lost or shared compared with the free element. It is a bookkeeping tool: it treats every bond as if it were fully ionic, so the more electronegative atom is given all the shared electrons. Chemists use oxidation numbers to follow electrons through a reaction without drawing every dot and cross.<\/p>\n<p>The sign matters. A positive oxidation number means the atom has lost control of electrons; a negative oxidation number means it has gained control. In sodium chloride the sodium ion is +1 and the chloride ion is \u22121, and those are also the real charges. In hydrogen chloride the bond is covalent, but chlorine is more electronegative, so hydrogen is counted as +1 and chlorine as \u22121 even though neither is a full ion.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Definition:<\/strong> The oxidation number of an atom is the charge it would have if every bond in the substance were ionic, with the shared electrons given to the more electronegative atom.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">2<\/div>\n<h2>The Rules for Assigning Them<\/h2>\n<\/div>\n<p>The rules are applied in order. When two rules seem to clash, the one higher in the list wins, which is why the exceptions for hydrogen and oxygen come last.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Rule<\/th><th>What it says<\/th><th>Example<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>1<\/strong><\/td><td>An atom in an uncombined element has an oxidation number of 0<\/td><td>Na, O\u2082, S\u2088, Cl\u2082 are all 0<\/td><\/tr>\n<tr><td><strong>2<\/strong><\/td><td>The oxidation numbers in a neutral compound add up to 0; in an ion they add up to the charge on the ion<\/td><td>MgCl\u2082: +2 + 2(\u22121) = 0; SO\u2084\u00b2\u207b: +6 + 4(\u22122) = \u22122<\/td><\/tr>\n<tr><td><strong>3<\/strong><\/td><td>Fluorine is always \u22121<\/td><td>F in OF\u2082 is \u22121, so O is +2<\/td><\/tr>\n<tr><td><strong>4<\/strong><\/td><td>Group 1 metals are always +1 and Group 2 metals always +2<\/td><td>K in KMnO\u2084 is +1; Ca in CaH\u2082 is +2<\/td><\/tr>\n<tr><td><strong>5<\/strong><\/td><td>Hydrogen is +1, except in metal hydrides where it is \u22121<\/td><td>H in H\u2082O is +1; H in NaH is \u22121<\/td><\/tr>\n<tr><td><strong>6<\/strong><\/td><td>Oxygen is \u22122, except in peroxides where it is \u22121 and in OF\u2082 where it is +2<\/td><td>O in H\u2082O\u2082 is \u22121<\/td><\/tr>\n<tr><td><strong>7<\/strong><\/td><td>Chlorine is \u22121 unless it is bonded to oxygen or fluorine<\/td><td>Cl in NaCl is \u22121; Cl in ClO\u207b is +1<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The peroxide and hydride exceptions are there because the usual value would break rule 2. In hydrogen peroxide, H\u2082O\u2082, two hydrogens at +1 leave \u22122 to be shared by two oxygens, so each is \u22121. In sodium hydride, NaH, sodium must be +1, so hydrogen has to be \u22121.<\/p>\n<div class=\"ols-zoom-card\">\n<div class=\"ols-zoom-card-image\">\n<a class=\"ols-lightbox-link\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-oxidation-numbers-rules.png\" aria-label=\"Open image full screen\">\n<img decoding=\"async\" class=\"ols-zoomable-img ols-lightbox-target\" src=\"https:\/\/www.onlinelearningsystem.net\/xyz\/wp-content\/uploads\/2026\/09\/redox-groups-oxidation-numbers-rules.png\" alt=\"Rules for assigning oxidation numbers with worked examples for H2O2, NaH, Cr2O7 2- and MnO4 -\">\n<\/a>\n<\/div>\n<div class=\"ols-zoom-card-caption\"><p>The rules in order, with the four exceptions that questions test most: peroxides, metal hydrides, dichromate(VI) and manganate(VII).<\/p><\/div>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam focus:<\/strong> Learn the order. Questions are built around the exceptions, and the reason each exception exists is always &#8220;so that the oxidation numbers add up to the overall charge&#8221;.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Assigning the Rules<\/h2>\n<p>Decide which rule fixes each atom in a set of compounds and ions you have not met on this page.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-851\" class=\"h5p-iframe\" data-content-id=\"851\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Summary: Which Rule Fixes It?\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card purple\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">3<\/div>\n<h2>Working Out an Oxidation number<\/h2>\n<\/div>\n<p>To find the oxidation number of one element in a formula, give every other element its usual value and let the unknown make the total come out right.<\/p>\n<ol>\n<li>Write the oxidation number of every element you are sure of (Group 1, Group 2, fluorine, then oxygen and hydrogen).<\/li>\n<li>Multiply each value by the number of atoms of that element in the formula.<\/li>\n<li>Set the total equal to the overall charge (0 for a compound) and solve for the unknown.<\/li>\n<\/ol>\n<p><strong>Worked example 1:<\/strong> sulfur in sulfuric acid, H\u2082SO\u2084. Hydrogen 2 \u00d7 (+1) = +2, oxygen 4 \u00d7 (\u22122) = \u22128, so +2 + S \u2212 8 = 0 and S = <strong>+6<\/strong>.<\/p>\n<p><strong>Worked example 2:<\/strong> chromium in the dichromate(VI) ion, Cr\u2082O\u2087\u00b2\u207b. Oxygen 7 \u00d7 (\u22122) = \u221214, so 2Cr \u2212 14 = \u22122 and 2Cr = +12, giving Cr = <strong>+6<\/strong>. Notice that the answer is the value per chromium atom, not the total.<\/p>\n<p><strong>Worked example 3:<\/strong> nitrogen in the ammonium ion, NH\u2084\u207a. Hydrogen 4 \u00d7 (+1) = +4, so N + 4 = +1 and N = <strong>\u22123<\/strong>.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Common mistake:<\/strong> Dividing at the wrong point. In Cr\u2082O\u2087\u00b2\u207b the seven oxygens contribute \u221214 in total, and the +12 that remains is shared between two chromium atoms. Always quote the oxidation number per atom.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Calculating Oxidation numbers<\/h2>\n<p>Type the oxidation number of the named element in each species. None of them appears in the worked examples above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-852\" class=\"h5p-iframe\" data-content-id=\"852\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Redox and Groups Quick Answers: Calculating Oxidation Numbers\"><\/iframe><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">4<\/div>\n<h2>Peroxides and Metal Hydrides<\/h2>\n<\/div>\n<p>These two families are the standard exam traps, so they deserve their own section. A <strong>peroxide<\/strong> contains the O\u2082\u00b2\u207b ion or an O\u2013O single bond, and each oxygen is \u22121. Hydrogen peroxide, H\u2082O\u2082, sodium peroxide, Na\u2082O\u2082, and barium peroxide, BaO\u2082, all contain oxygen at \u22121. Compare barium peroxide with barium oxide, BaO, where oxygen is the usual \u22122.<\/p>\n<p>A <strong>metal hydride<\/strong> is a compound of hydrogen with a reactive metal, such as sodium hydride, NaH, calcium hydride, CaH\u2082, or lithium aluminium hydride, LiAlH\u2084. The metal is more electropositive than hydrogen, so hydrogen takes the electron and is \u22121. These compounds react with water to release hydrogen gas, because the H\u207b ion is a powerful reducing agent.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Compound<\/th><th>Element<\/th><th>oxidation number<\/th><th>Reason<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>H\u2082O\u2082<\/strong><\/td><td>O<\/td><td>\u22121<\/td><td>peroxide: two H at +1 leave \u22122 for two O<\/td><\/tr>\n<tr><td><strong>Na\u2082O\u2082<\/strong><\/td><td>O<\/td><td>\u22121<\/td><td>two Na at +1 leave \u22122 for two O<\/td><\/tr>\n<tr><td><strong>NaH<\/strong><\/td><td>H<\/td><td>\u22121<\/td><td>Na must be +1<\/td><\/tr>\n<tr><td><strong>CaH\u2082<\/strong><\/td><td>H<\/td><td>\u22121<\/td><td>Ca must be +2, so 2H = \u22122<\/td><\/tr>\n<tr><td><strong>OF\u2082<\/strong><\/td><td>O<\/td><td>+2<\/td><td>fluorine is always \u22121<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Remember:<\/strong> A formula with O\u2082 in it is a peroxide only when the metal is Group 1 or 2 and the oxygen count does not fit the usual \u22122. Barium peroxide BaO\u2082 is a peroxide; manganese(IV) oxide MnO\u2082 is not, because Mn can be +4.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card soft\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">5<\/div>\n<h2>Roman Numerals in Names<\/h2>\n<\/div>\n<p>Many elements form compounds in more than one oxidation number, so the name shows the value as a <strong>Roman numeral<\/strong> in brackets immediately after the element it refers to. Iron(II) chloride is FeCl\u2082 and iron(III) chloride is FeCl\u2083. The numeral is written without a sign because it is always positive.<\/p>\n<p>The same convention names ions that contain oxygen. Manganate(VII) is MnO\u2084\u207b, with manganese at +7; chlorate(I) is ClO\u207b, with chlorine at +1; chlorate(V) is ClO\u2083\u207b, with chlorine at +5. Sulfate(VI) and sulfate(IV) distinguish SO\u2084\u00b2\u207b from SO\u2083\u00b2\u207b, although the older names sulfate and sulfite are still widely used.<\/p>\n<div class=\"ols-table-wrap\">\n<table class=\"ols-table\">\n<thead>\n<tr><th>Name<\/th><th>Formula<\/th><th>oxidation number shown<\/th><\/tr>\n<\/thead>\n<tbody>\n<tr><td><strong>copper(I) oxide<\/strong><\/td><td>Cu\u2082O<\/td><td>Cu +1<\/td><\/tr>\n<tr><td><strong>copper(II) oxide<\/strong><\/td><td>CuO<\/td><td>Cu +2<\/td><\/tr>\n<tr><td><strong>potassium manganate(VII)<\/strong><\/td><td>KMnO\u2084<\/td><td>Mn +7<\/td><\/tr>\n<tr><td><strong>sodium chlorate(I)<\/strong><\/td><td>NaClO<\/td><td>Cl +1<\/td><\/tr>\n<tr><td><strong>potassium dichromate(VI)<\/strong><\/td><td>K\u2082Cr\u2082O\u2087<\/td><td>Cr +6<\/td><\/tr>\n<tr><td><strong>lead(IV) oxide<\/strong><\/td><td>PbO\u2082<\/td><td>Pb +4<\/td><\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<div class=\"ols-key-box\">\n<p><strong>Exam wording:<\/strong> The numeral refers to the element immediately before the bracket. In potassium manganate(VII) it is manganese that is +7, not potassium.<\/p>\n<\/div>\n<\/article>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">6<\/div>\n<h2>Writing Formulae from Oxidation numbers<\/h2>\n<\/div>\n<p>Given the oxidation numbers, a formula is written so that the positive and negative values cancel. Find the lowest whole-number ratio of atoms that makes the total zero.<\/p>\n<p><strong>Worked example:<\/strong> the formula of chromium(III) sulfate. Chromium is +3 and the sulfate ion is \u22122. The lowest common multiple of 3 and 2 is 6, so two chromium ions (+6) balance three sulfate ions (\u22126): <strong>Cr\u2082(SO\u2084)\u2083<\/strong>.<\/p>\n<p><strong>Worked example:<\/strong> the formula of vanadium(V) oxide. Vanadium is +5 and oxygen is \u22122. Two vanadium atoms (+10) balance five oxygens (\u221210): <strong>V\u2082O\u2085<\/strong>.<\/p>\n<p>The reverse question is just as common: &#8220;Name the compound PbO\u2082.&#8221; Oxygen is \u22122 and there are two of them, so lead is +4 and the name is lead(IV) oxide.<\/p>\n<div class=\"ols-key-box\">\n<p><strong>Key idea:<\/strong> The ratio in the formula is the ratio that cancels the charges, then simplified. Brackets are needed round a compound ion when there is more than one of it.<\/p>\n<\/div>\n<\/article>\n<section class=\"ols-h5p-card ols-h5p-inline\">\n<span class=\"ols-h5p-kicker\">Check your understanding<\/span>\n<h2>Check: Names and Formulae<\/h2>\n<p>Match Roman-numeral names to formulae and write formulae from oxidation numbers for compounds not shown above.<\/p>\n<div class=\"ols-h5p-frame\"><div class=\"h5p-content\" data-content-id=\"853\"><\/div><\/div>\n<\/section>\n<article class=\"ols-note-card\">\n<div class=\"ols-note-title\">\n<div class=\"ols-note-icon\">7<\/div>\n<h2>Common Exam Points<\/h2>\n<\/div>\n<h3>Deduce the oxidation number of an element in a compound or ion<\/h3><p>Give every other element its usual value, multiply by the number of atoms, and make the total equal the overall charge.<\/p>\n<h3>Explain why oxygen is \u22121 in hydrogen peroxide<\/h3><p>The two hydrogens are +1 each, so the two oxygens must total \u22122 to make the compound neutral, which is \u22121 each.<\/p>\n<h3>Give the oxidation number shown by a Roman numeral<\/h3><p>The numeral is the oxidation number of the element written immediately before the bracket, and it is always positive.<\/p>\n<h3>Do not say<\/h3><p>&#8220;The oxidation number of chromium in Cr\u2082O\u2087\u00b2\u207b is +12&#8221;; &#8220;hydrogen is always +1&#8221;; &#8220;the oxidation number of an element in a molecule such as O\u2082 is \u22122&#8221;.<\/p>\n<\/article>\n<section class=\"ols-faq-card\">\n<h2>FAQs<\/h2>\n<p>Use these quick answers to check the oxidation number rules that come up most often in OCR A questions.<\/p>\n\n<div class=\"ols-faq-list\">\n<div class=\"ols-faq-item\">\n<h3>What is an oxidation number?<\/h3>\n<p>A number that shows how many electrons an atom has gained, lost or shared compared with the free element, counting every bond as if it were ionic. It is a bookkeeping tool, not always a real charge.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Why is hydrogen \u22121 in sodium hydride?<\/h3>\n<p>Sodium must be +1, and the compound is neutral, so hydrogen has to be \u22121. Hydrogen takes the electron because it is more electronegative than the metal.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Is the oxidation number of chromium in Cr\u2082O\u2087\u00b2\u207b +12?<\/h3>\n<p>No. The two chromium atoms total +12, so each chromium is +6. Always quote the value per atom.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>What does the Roman numeral in a name tell me?<\/h3>\n<p>The oxidation number of the element written immediately before the bracket. Iron(III) chloride contains iron at +3 and is FeCl\u2083.<\/p>\n<\/div>\n\n<div class=\"ols-faq-item\">\n<h3>Can an oxidation number be a fraction?<\/h3>\n<p>In a few compounds the average value is a fraction, such as sulfur in S\u2084O\u2086\u00b2\u207b (+2.5), because the atoms are not all in the same environment. Exam questions avoid these unless they say so.<\/p>\n<\/div>\n<\/div>\n<\/section>\n<section class=\"ols-related-card\">\n<h2>Related 2.1.5 Redox Pages<\/h2>\n<p>Use these pages to connect the ideas across 2.1.5 Redox and the rest of the course.<\/p>\n<div class=\"ols-related-grid\">\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/electron-transfer-and-half-equations\/\">Electron Transfer and Half-Equations<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/disproportionation\/\">Disproportionation<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/redox-classification\/\">Redox Classification<\/a>\n<a class=\"ols-related-item\" href=\"https:\/\/www.onlinelearningsystem.net\/xyz\/revision-notes\/a-level-chemistry\/ocr-a\/2-1-5-redox\/\">2.1.5 Redox Overview<\/a>\n<\/div>\n<\/section>\n<section class=\"ols-attribution-card\">\n        <p><strong>Copyright and author footprint:<\/strong> This OLS revision page was written for Online Learning System by <strong>Dr. Mohammed Al-Fatah<\/strong>. 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