<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      background-color: #f8f9fa;
      color: #1e2945;
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      padding: 12px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: center;
      min-height: 100vh;
      user-select: none;
    }

    .container {
      width: 100%;
      max-width: 560px;
      display: flex;
      flex-direction: column;
      gap: 12px;
    }

    .main-grid {
      display: grid;
      grid-template-columns: 1fr 130px;
      gap: 10px;
      width: 100%;
      height: 275px;
    }

    /* Simulation Canvas */
    .stage {
      position: relative;
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      overflow: hidden;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    .stage-header {
      position: absolute;
      top: 8px;
      left: 10px;
      right: 10px;
      display: flex;
      justify-content: space-between;
      align-items: baseline;
      gap: 6px;
      z-index: 10;
      pointer-events: none;
    }

    .stage-label {
      font-size: 13px;
      font-weight: 700;
      color: #1e293b;
      white-space: nowrap;
      transition: color 0.35s ease;
    }

    .sub-label {
      font-size: 11px;
      color: #64748b;
      font-weight: 500;
      white-space: nowrap;
      transition: color 0.35s ease;
    }

    /* Attraction cue arrows */
    .attract-indicator {
      position: absolute;
      top: 34px;
      left: 50%;
      transform: translateX(-50%);
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 4px;
      font-size: 10px;
      font-weight: 700;
      color: #0369a1;
      background: #f0f9ff;
      border: 1px solid #bae6fd;
      padding: 2px 8px;
      border-radius: 12px;
      letter-spacing: 0.2px;
      opacity: 1;
      transition: opacity 0.35s ease, transform 0.35s ease;
      pointer-events: none;
      z-index: 5;
      white-space: nowrap;
    }

    .bonded .attract-indicator {
      opacity: 0;
      transform: translateX(-50%) translateY(-4px);
    }

    /* Atoms & System */
    .atom-system {
      position: relative;
      width: 100%;
      height: 140px;
      display: flex;
      align-items: center;
      justify-content: center;
      margin-top: 10px;
    }

    .atom {
      position: absolute;
      width: 58px;
      height: 58px;
      border-radius: 50%;
      background: radial-gradient(circle at 35% 35%, #e0f2fe 0%, #bae6fd 55%, #38bdf8 100%);
      border: 2px solid #0284c7;
      display: flex;
      align-items: center;
      justify-content: center;
      font-weight: 800;
      font-size: 18px;
      color: #0369a1;
      box-shadow: 0 4px 12px rgba(2, 132, 199, 0.18);
      transition: transform 0.7s cubic-bezier(0.34, 1.4, 0.64, 1);
      z-index: 2;
    }

    .atom::after {
      content: 'e⁻';
      position: absolute;
      font-size: 9px;
      font-weight: 700;
      color: #0284c7;
      background: #ffffff;
      width: 15px;
      height: 15px;
      border-radius: 50%;
      border: 1px solid #38bdf8;
      display: flex;
      align-items: center;
      justify-content: center;
      top: -3px;
      right: 4px;
      box-shadow: 0 1px 3px rgba(0,0,0,0.1);
    }

    .atom-left {
      transform: translateX(-56px);
    }

    .atom-right {
      transform: translateX(56px);
    }

    .bonded .atom-left {
      transform: translateX(-18px);
    }

    .bonded .atom-right {
      transform: translateX(18px);
    }

    /* Shared electron cloud overlap indication */
    .covalent-zone {
      position: absolute;
      width: 44px;
      height: 44px;
      border-radius: 50%;
      background: rgba(2, 132, 199, 0.2);
      opacity: 0;
      transform: scale(0.6);
      transition: opacity 0.5s ease 0.2s, transform 0.5s ease 0.2s;
      z-index: 1;
    }

    .bonded .covalent-zone {
      opacity: 1;
      transform: scale(1);
    }

    /* Energy Waves Burst */
    .burst-container {
      position: absolute;
      width: 100%;
      height: 100%;
      pointer-events: none;
      display: flex;
      align-items: center;
      justify-content: center;
      z-index: 0;
    }

    .wave {
      position: absolute;
      border-radius: 50%;
      border: 2px solid #f59e0b;
      opacity: 0;
      transform: scale(0.3);
    }

    .bonded .wave-1 {
      animation: ripple 1.9s infinite ease-out;
    }

    .bonded .wave-2 {
      animation: ripple 1.9s infinite 0.65s ease-out;
    }

    @keyframes ripple {
      0% {
        width: 40px;
        height: 40px;
        opacity: 0.95;
        transform: scale(0.5);
        border-color: #f59e0b;
      }
      60% {
        opacity: 0.6;
      }
      100% {
        width: 190px;
        height: 190px;
        opacity: 0;
        transform: scale(1.35);
        border-color: #fbbf24;
      }
    }

    .release-tag {
      position: absolute;
      bottom: 10px;
      background: #fef3c7;
      color: #92400e;
      border: 1px solid #fcd34d;
      padding: 4px 10px;
      border-radius: 20px;
      font-size: 11px;
      font-weight: 700;
      opacity: 0;
      transform: translateY(6px);
      transition: opacity 0.4s ease 0.2s, transform 0.4s ease 0.2s;
      display: flex;
      align-items: center;
      gap: 4px;
      z-index: 10;
    }

    .bonded .release-tag {
      opacity: 1;
      transform: translateY(0);
    }

    /* Energy Meter Column */
    .meter-card {
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 8px 6px;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      position: relative;
      min-width: 0;
    }

    .meter-title {
      font-size: 10px;
      font-weight: 700;
      color: #64748b;
      text-transform: uppercase;
      letter-spacing: 0.4px;
      text-align: center;
      line-height: 1.2;
    }

    .meter-body {
      position: relative;
      width: 100%;
      flex: 1;
      display: flex;
      align-items: center;
      justify-content: center;
      margin: 6px 0;
    }

    .meter-track {
      position: relative;
      width: 20px;
      height: 100%;
      max-height: 135px;
      background: #f1f5f9;
      border-radius: 10px;
      border: 1px solid #cbd5e1;
      display: flex;
      flex-direction: column;
      justify-content: flex-end;
      overflow: hidden;
      margin-left: -40px;
    }

    .meter-fill {
      width: 100%;
      height: 82%;
      background: linear-gradient(to top, #10b981 0%, #f59e0b 50%, #ef4444 100%);
      border-radius: 8px;
      transition: height 0.7s cubic-bezier(0.4, 0, 0.2, 1);
    }

    .meter-pointer {
      position: absolute;
      left: calc(50% - 15px);
      bottom: 82%;
      transform: translateY(50%);
      font-size: 11px;
      font-weight: 800;
      color: #ef4444;
      white-space: nowrap;
      transition: bottom 0.7s cubic-bezier(0.4, 0, 0.2, 1), color 0.4s ease;
      display: flex;
      align-items: center;
      gap: 2px;
    }

    .bonded .meter-fill {
      height: 24%;
    }

    .bonded .meter-pointer {
      bottom: 24%;
      color: #059669;
    }

    .delta-badge {
      font-size: 9.5px;
      font-weight: 700;
      color: #047857;
      background: #d1fae5;
      border: 1px solid #a7f3d0;
      padding: 3px 4px;
      border-radius: 6px;
      text-align: center;
      width: 100%;
      line-height: 1.25;
      opacity: 0;
      transform: translateY(2px);
      transition: opacity 0.35s ease 0.2s, transform 0.35s ease 0.2s;
    }

    .bonded .delta-badge {
      opacity: 1;
      transform: translateY(0);
    }

    /* Controls */
    .controls {
      display: flex;
      align-items: center;
      gap: 8px;
      width: 100%;
      justify-content: center;
      flex-wrap: wrap;
    }

    .btn {
      appearance: none;
      border: 1px solid #cbd5e1;
      background: #ffffff;
      color: #334155;
      font-size: 12px;
      font-weight: 600;
      padding: 8px 12px;
      border-radius: 8px;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      gap: 4px;
      transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
      flex: 1;
      min-width: 100px;
    }

    .btn:hover {
      background-color: #f8fafc;
      border-color: #94a3b8;
    }

    .btn:active {
      opacity: 0.85;
    }

    .btn.active {
      background-color: #0284c7;
      color: #ffffff;
      border-color: #0284c7;
    }

    .btn-play {
      background-color: #f1f5f9;
      font-weight: 700;
      flex: 0 0 auto;
      min-width: 76px;
    }

    .btn-play:hover {
      background-color: #e2e8f0;
    }

    /* Responsive scaling for mobile <= 400px */
    @media (max-width: 400px) {
      .main-grid {
        grid-template-columns: 1fr 108px;
        height: 260px;
        gap: 6px;
      }
      .stage-header {
        flex-direction: column;
        align-items: flex-start;
        gap: 1px;
        top: 6px;
        left: 8px;
        right: 8px;
      }
      .stage-label {
        font-size: 11.5px;
      }
      .sub-label {
        font-size: 9.5px;
      }
      .attract-indicator {
        top: 40px;
        font-size: 9px;
        padding: 1px 6px;
      }
      .atom {
        width: 48px;
        height: 48px;
        font-size: 16px;
      }
      .atom-left {
        transform: translateX(-44px);
      }
      .atom-right {
        transform: translateX(44px);
      }
      .bonded .atom-left {
        transform: translateX(-15px);
      }
      .bonded .atom-right {
        transform: translateX(15px);
      }
      .meter-track {
        margin-left: -32px;
        width: 18px;
      }
      .meter-pointer {
        left: calc(50% - 10px);
        font-size: 10px;
      }
      .meter-title {
        font-size: 8.5px;
      }
      .delta-badge {
        font-size: 8.5px;
        padding: 2px;
      }
      .release-tag {
        font-size: 10px;
        padding: 3px 8px;
        bottom: 8px;
      }
      .btn {
        font-size: 11px;
        padding: 7px 8px;
        min-width: 80px;
      }
    }
  </style>
</head>
<body>

  <div class="container">
    <div class="main-grid" id="diagram">
      
      <div class="stage">
        <div class="stage-header">
          <span class="stage-label" id="stateLabel">Isolated H Atoms</span>
          <span class="sub-label" id="stateSub">Unbonded State</span>
        </div>

        <div class="attract-indicator">
          <span>Electrostatic Attraction → ←</span>
        </div>
        
        <div class="atom-system">
          <div class="burst-container">
            <div class="wave wave-1"></div>
            <div class="wave wave-2"></div>
          </div>

          <div class="covalent-zone"></div>
          <div class="atom atom-left">H</div>
          <div class="atom atom-right">H</div>
        </div>

        <div class="release-tag">⚡ Energy Released</div>
      </div>

      
      <div class="meter-card">
        <div class="meter-title">Potential<br>Energy</div>
        
        <div class="meter-body">
          <div class="meter-track">
            <div class="meter-fill"></div>
          </div>
          <div class="meter-pointer" id="pointer">◀ <span id="levelText">High</span></div>
        </div>

        <div class="delta-badge">Exothermic: Released (-ΔH)</div>
      </div>
    </div>

    
    <div class="controls">
      <button class="btn active" id="btnSeparate" onclick="setBonded(false)">1. Separate</button>
      <button class="btn" id="btnBond" onclick="setBonded(true)">2. Form Bond</button>
      <button class="btn btn-play" id="btnAuto" onclick="toggleAuto()">▶ Auto</button>
    </div>
  </div>

  <script>
    const diagram = document.getElementById('diagram');
    const stateLabel = document.getElementById('stateLabel');
    const stateSub = document.getElementById('stateSub');
    const levelText = document.getElementById('levelText');
    const btnSeparate = document.getElementById('btnSeparate');
    const btnBond = document.getElementById('btnBond');
    const btnAuto = document.getElementById('btnAuto');

    let isBonded = false;
    let autoTimer = null;

    function setBonded(bonded) {
      isBonded = bonded;
      if (bonded) {
        diagram.classList.add('bonded');
        stateLabel.textContent = 'H—H Molecule';
        stateSub.textContent = 'Stable Covalent Bond';
        levelText.textContent = 'Low';
        btnBond.classList.add('active');
        btnSeparate.classList.remove('active');
      } else {
        diagram.classList.remove('bonded');
        stateLabel.textContent = 'Isolated H Atoms';
        stateSub.textContent = 'Unbonded State';
        levelText.textContent = 'High';
        btnSeparate.classList.add('active');
        btnBond.classList.remove('active');
      }
    }

    function toggleAuto() {
      if (autoTimer) {
        clearInterval(autoTimer);
        autoTimer = null;
        btnAuto.textContent = '▶ Auto';
      } else {
        btnAuto.textContent = '⏸ Pause';
        setBonded(!isBonded);
        autoTimer = setInterval(() => {
          setBonded(!isBonded);
        }, 2400);
      }
    }
  </script>
</body>
</html>

Breaking Bonds Requires Energy, Making Bonds Releases Energy

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
      background: #ffffff;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      align-items: center;
      justify-content: space-between;
      min-height: 100%;
      padding: 12px;
      user-select: none;
    }

    .controls {
      display: flex;
      gap: 8px;
      margin-bottom: 8px;
      z-index: 10;
    }

    .btn {
      padding: 6px 14px;
      font-size: 14px;
      font-weight: 600;
      border-radius: 20px;
      border: 1.5px solid #d1d5db;
      background: #f8fafc;
      color: #475569;
      cursor: pointer;
      transition: all 0.2s ease;
    }

    .btn.active {
      background: #2563eb;
      color: #ffffff;
      border-color: #2563eb;
      box-shadow: 0 2px 6px rgba(37, 99, 235, 0.25);
    }

    .svg-container {
      width: 100%;
      max-width: 500px;
      aspect-ratio: 500 / 350;
      position: relative;
    }

    svg {
      width: 100%;
      height: 100%;
      overflow: visible;
    }

    .axis { stroke: #94a3b8; stroke-width: 2; stroke-linecap: round; }
    .axis-arrow { fill: #94a3b8; }
    .axis-label { font-size: 12px; fill: #64748b; font-weight: 600; }
    
    .plateau { stroke-width: 3.5; stroke-linecap: round; }
    .plateau-reactants { stroke: #2563eb; }
    .plateau-atoms { stroke: #64748b; stroke-dasharray: 4,4; }
    .plateau-products { stroke: #059669; }

    .energy-arrow { stroke-width: 2.5; stroke-linecap: round; }
    .arrow-break { stroke: #dc2626; fill: #dc2626; }
    .arrow-make { stroke: #16a34a; fill: #16a34a; }
    .arrow-net { stroke: #0284c7; fill: #0284c7; }

    .tag-bg { rx: 5; ry: 5; }
    .label-title { font-size: 12px; font-weight: 700; }
    .label-val { font-size: 11px; font-weight: 600; }

    /* Transitions */
    .animated {
      transition: all 0.4s cubic-bezier(0.4, 0, 0.2, 1);
    }
  </style>
</head>
<body>

  <div class="controls">
    <button id="btnExo" class="btn active" onclick="setProfile('exo')">Exothermic (H₂ + Cl₂)</button>
    <button id="btnEndo" class="btn" onclick="setProfile('endo')">Endothermic</button>
  </div>

  <div class="svg-container">
    <svg viewBox="0 0 500 350">
      <defs>
        
        <marker id="arr-up" markerWidth="6" markerHeight="6" refX="3" refY="3" orient="auto">
          <path d="M0,6 L3,0 L6,6 Z" fill="#dc2626" />
        </marker>
        <marker id="arr-down" markerWidth="6" markerHeight="6" refX="3" refY="3" orient="auto">
          <path d="M0,0 L3,6 L6,0 Z" fill="#16a34a" />
        </marker>
        <marker id="arr-net-down" markerWidth="6" markerHeight="6" refX="3" refY="3" orient="auto">
          <path d="M0,0 L3,6 L6,0 Z" fill="#0284c7" />
        </marker>
        <marker id="arr-net-up" markerWidth="6" markerHeight="6" refX="3" refY="3" orient="auto">
          <path d="M0,6 L3,0 L6,6 Z" fill="#0284c7" />
        </marker>
        <marker id="axis-arr" markerWidth="6" markerHeight="6" refX="3" refY="3" orient="auto">
          <path d="M0,0 L6,3 L0,6 Z" fill="#94a3b8" />
        </marker>
      </defs>

      
      <line x1="50" y1="310" x2="50" y2="25" class="axis" />
      <polygon points="50,18 46,26 54,26" class="axis-arrow" />
      
      <line x1="50" y1="310" x2="480" y2="310" class="axis" />
      <polygon points="487,310 479,306 479,314" class="axis-arrow" />

      <text x="58" y="32" class="axis-label">Potential Energy (kJ/mol)</text>
      <text x="480" y="330" class="axis-label" text-anchor="end">Reaction Progress →</text>

      
      <line id="guideReactant" class="animated" x1="50" y1="210" x2="160" y2="210" stroke="#cbd5e1" stroke-dasharray="3,3" stroke-width="1.5" />
      <line id="guideProduct" class="animated" x1="50" y1="270" x2="440" y2="270" stroke="#cbd5e1" stroke-dasharray="3,3" stroke-width="1.5" />

      
      <line id="platReactants" class="plateau plateau-reactants animated" x1="70" y1="210" x2="160" y2="210" />
      <text id="txtReactants" class="label-title animated" x="115" y="230" text-anchor="middle" fill="#1e40af">H₂ + Cl₂</text>

      
      <line id="platAtoms" class="plateau plateau-atoms animated" x1="200" y1="65" x2="300" y2="65" />
      <text id="txtAtoms" class="label-title animated" x="250" y="52" text-anchor="middle" fill="#475569">2H + 2Cl (Atoms)</text>

      
      <line id="platProducts" class="plateau plateau-products animated" x1="340" y1="270" x2="440" y2="270" />
      <text id="txtProducts" class="label-title animated" x="390" y="290" text-anchor="middle" fill="#065f46">2HCl</text>

      
      
      <line id="arrowBreak" class="energy-arrow arrow-break animated" x1="180" y1="210" x2="180" y2="68" marker-end="url(#arr-up)" />
      
      
      <g id="grpBreak" class="animated" transform="translate(180, 137)">
        <rect x="-70" y="-18" width="65" height="34" rx="4" fill="#fee2e2" />
        <text x="-37" y="-4" class="label-title" fill="#991b1b" text-anchor="middle">Breaking</text>
        <text id="valBreak" x="-37" y="10" class="label-val" fill="#dc2626" text-anchor="middle">+678 kJ</text>
      </g>

      
      <line id="arrowMake" class="energy-arrow arrow-make animated" x1="320" y1="65" x2="320" y2="267" marker-end="url(#arr-down)" />
      
      
      <g id="grpMake" class="animated" transform="translate(320, 167)">
        <rect x="6" y="-18" width="65" height="34" rx="4" fill="#dcfce7" />
        <text x="38" y="-4" class="label-title" fill="#166534" text-anchor="middle">Making</text>
        <text id="valMake" x="38" y="10" class="label-val" fill="#16a34a" text-anchor="middle">-862 kJ</text>
      </g>

      
      <line id="arrowNet" class="energy-arrow arrow-net animated" x1="455" y1="210" x2="455" y2="267" marker-end="url(#arr-net-down)" />
      
      
      <g id="grpNet" class="animated" transform="translate(455, 240)">
        <rect x="-85" y="-16" width="80" height="32" rx="4" fill="#e0f2fe" />
        <text id="valNet" x="-45" y="-3" class="label-title" fill="#0369a1" text-anchor="middle">ΔH = -184</text>
        <text id="tagNet" x="-45" y="10" class="label-val" fill="#0284c7" text-anchor="middle">Exothermic</text>
      </g>
    </svg>
  </div>

  <script>
    function setProfile(type) {
      const btnExo = document.getElementById('btnExo');
      const btnEndo = document.getElementById('btnEndo');
      
      const guideReactant = document.getElementById('guideReactant');
      const guideProduct = document.getElementById('guideProduct');
      
      const platReactants = document.getElementById('platReactants');
      const txtReactants = document.getElementById('txtReactants');
      
      const platAtoms = document.getElementById('platAtoms');
      const txtAtoms = document.getElementById('txtAtoms');
      
      const platProducts = document.getElementById('platProducts');
      const txtProducts = document.getElementById('txtProducts');
      
      const arrowBreak = document.getElementById('arrowBreak');
      const grpBreak = document.getElementById('grpBreak');
      const valBreak = document.getElementById('valBreak');
      
      const arrowMake = document.getElementById('arrowMake');
      const grpMake = document.getElementById('grpMake');
      const valMake = document.getElementById('valMake');
      
      const arrowNet = document.getElementById('arrowNet');
      const grpNet = document.getElementById('grpNet');
      const valNet = document.getElementById('valNet');
      const tagNet = document.getElementById('tagNet');

      if (type === 'exo') {
        btnExo.classList.add('active');
        btnEndo.classList.remove('active');

        // Y Coordinates: Reactants: 210, Atoms: 65, Products: 270
        guideReactant.setAttribute('y1', 210);
        guideReactant.setAttribute('y2', 210);
        guideProduct.setAttribute('y1', 270);
        guideProduct.setAttribute('y2', 270);

        platReactants.setAttribute('y1', 210);
        platReactants.setAttribute('y2', 210);
        txtReactants.setAttribute('y', 230);
        txtReactants.textContent = 'H₂ + Cl₂';

        platAtoms.setAttribute('y1', 65);
        platAtoms.setAttribute('y2', 65);
        txtAtoms.setAttribute('y', 52);
        txtAtoms.textContent = '2H + 2Cl (Atoms)';

        platProducts.setAttribute('y1', 270);
        platProducts.setAttribute('y2', 270);
        txtProducts.setAttribute('y', 290);
        txtProducts.textContent = '2HCl';

        arrowBreak.setAttribute('y1', 210);
        arrowBreak.setAttribute('y2', 68);
        grpBreak.setAttribute('transform', 'translate(180, 137)');
        valBreak.textContent = '+678 kJ';

        arrowMake.setAttribute('y1', 65);
        arrowMake.setAttribute('y2', 267);
        arrowMake.setAttribute('marker-end', 'url(#arr-down)');
        grpMake.setAttribute('transform', 'translate(320, 167)');
        valMake.textContent = '-862 kJ';

        arrowNet.setAttribute('y1', 210);
        arrowNet.setAttribute('y2', 267);
        arrowNet.setAttribute('marker-end', 'url(#arr-net-down)');
        grpNet.setAttribute('transform', 'translate(455, 240)');
        valNet.textContent = 'ΔH = -184';
        tagNet.textContent = 'Exothermic';
      } else {
        btnEndo.classList.add('active');
        btnExo.classList.remove('active');

        // Y Coordinates: Reactants: 270, Atoms: 65, Products: 160
        guideReactant.setAttribute('y1', 270);
        guideReactant.setAttribute('y2', 270);
        guideProduct.setAttribute('y1', 160);
        guideProduct.setAttribute('y2', 160);

        platReactants.setAttribute('y1', 270);
        platReactants.setAttribute('y2', 270);
        txtReactants.setAttribute('y', 290);
        txtReactants.textContent = 'Reactants';

        platAtoms.setAttribute('y1', 65);
        platAtoms.setAttribute('y2', 65);
        txtAtoms.setAttribute('y', 52);
        txtAtoms.textContent = 'Separated Atoms';

        platProducts.setAttribute('y1', 160);
        platProducts.setAttribute('y2', 160);
        txtProducts.setAttribute('y', 145);
        txtProducts.textContent = 'Products';

        arrowBreak.setAttribute('y1', 270);
        arrowBreak.setAttribute('y2', 68);
        grpBreak.setAttribute('transform', 'translate(180, 167)');
        valBreak.textContent = 'Large +ΔH';

        arrowMake.setAttribute('y1', 65);
        arrowMake.setAttribute('y2', 157);
        arrowMake.setAttribute('marker-end', 'url(#arr-down)');
        grpMake.setAttribute('transform', 'translate(320, 112)');
        valMake.textContent = 'Small -ΔH';

        arrowNet.setAttribute('y1', 270);
        arrowNet.setAttribute('y2', 163);
        arrowNet.setAttribute('marker-end', 'url(#arr-net-up)');
        grpNet.setAttribute('transform', 'translate(455, 215)');
        valNet.textContent = 'ΔH > 0';
        tagNet.textContent = 'Endothermic';
      }
    }
  </script>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta name="aspect-ratio" content="4:3">
  <style>
    * { box-sizing: border-box; margin: 0; padding: 0; }
    body {
      font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
      background: #f8f9fa;
      color: #1e2945;
      display: flex;
      flex-direction: column;
      height: 100vh;
      min-height: 380px;
      padding: 16px;
      user-select: none;
    }

    .main-grid {
      display: grid;
      grid-template-columns: 1fr 140px;
      gap: 16px;
      flex: 1;
      min-height: 0;
    }

    @media (max-width: 480px) {
      .main-grid {
        grid-template-columns: 1fr 110px;
        gap: 10px;
      }
    }

    .viewport {
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      position: relative;
      overflow: hidden;
      display: flex;
      flex-direction: column;
    }

    .status-header {
      padding: 12px 16px;
      font-size: 15px;
      font-weight: 700;
      color: #1e2945;
      display: flex;
      justify-content: space-between;
      align-items: center;
      border-bottom: 1px solid #f1f5f9;
    }

    .badge {
      font-size: 12px;
      font-weight: 700;
      padding: 4px 8px;
      border-radius: 6px;
      background: #e2e8f0;
      color: #475569;
      transition: all 0.4s ease;
    }
    .badge.endothermic {
      background: #fee2e2;
      color: #dc2626;
    }

    .stage {
      flex: 1;
      position: relative;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    /* Atoms & Electrons */
    .atoms-container {
      position: relative;
      width: 100%;
      height: 120px;
      display: flex;
      align-items: center;
      justify-content: center;
    }

    .atom {
      width: 76px;
      height: 76px;
      border-radius: 50%;
      background: radial-gradient(circle at 35% 35%, #4ade80, #15803d);
      color: #ffffff;
      display: flex;
      align-items: center;
      justify-content: center;
      font-size: 22px;
      font-weight: 800;
      position: absolute;
      box-shadow: 0 4px 12px rgba(21, 128, 61, 0.25);
      transition: transform 0.6s cubic-bezier(0.34, 1.56, 0.64, 1);
      z-index: 2;
    }

    .atom-left {
      transform: translateX(-42px);
    }
    .atom-right {
      transform: translateX(42px);
    }

    .state-broken .atom-left {
      transform: translateX(-95px);
    }
    .state-broken .atom-right {
      transform: translateX(95px);
    }

    @media (max-width: 480px) {
      .atom { width: 62px; height: 62px; font-size: 18px; }
      .atom-left { transform: translateX(-34px); }
      .atom-right { transform: translateX(34px); }
      .state-broken .atom-left { transform: translateX(-70px); }
      .state-broken .atom-right { transform: translateX(70px); }
    }

    /* Shared Electrons Bond */
    .bond-zone {
      position: absolute;
      width: 44px;
      height: 28px;
      background: rgba(254, 240, 138, 0.85);
      border: 2px dashed #ca8a04;
      border-radius: 16px;
      display: flex;
      align-items: center;
      justify-content: space-around;
      padding: 0 6px;
      z-index: 3;
      transition: opacity 0.4s ease, transform 0.4s ease;
    }

    .electron {
      width: 10px;
      height: 10px;
      background: #eab308;
      border: 1.5px solid #a16207;
      border-radius: 50%;
    }

    .state-broken .bond-zone {
      opacity: 0;
      transform: scale(0.3);
      pointer-events: none;
    }

    /* Energy In Arrow */
    .energy-packet {
      position: absolute;
      top: -40px;
      background: #ef4444;
      color: #ffffff;
      padding: 6px 14px;
      border-radius: 20px;
      font-size: 13px;
      font-weight: 800;
      letter-spacing: 0.5px;
      box-shadow: 0 4px 14px rgba(239, 68, 68, 0.4);
      display: flex;
      align-items: center;
      gap: 6px;
      opacity: 0;
      transform: translateY(-20px) scale(0.9);
      transition: all 0.5s ease;
      z-index: 10;
    }

    .state-absorbing .energy-packet {
      opacity: 1;
      transform: translateY(65px) scale(1);
    }
    .state-broken .energy-packet {
      opacity: 0;
      transform: translateY(110px) scale(0.8);
    }

    /* Energy Meter Column */
    .meter-card {
      background: #ffffff;
      border: 1px solid #e2e8f0;
      border-radius: 12px;
      padding: 12px 10px;
      display: flex;
      flex-direction: column;
      align-items: center;
      position: relative;
    }

    .meter-title {
      font-size: 11px;
      font-weight: 700;
      text-transform: uppercase;
      letter-spacing: 0.5px;
      color: #64748b;
      margin-bottom: 8px;
      text-align: center;
    }

    .meter-track {
      flex: 1;
      width: 24px;
      background: #f1f5f9;
      border-radius: 12px;
      position: relative;
      overflow: hidden;
      display: flex;
      flex-direction: column;
      justify-content: flex-end;
      border: 1px solid #cbd5e1;
    }

    .meter-fill {
      width: 100%;
      height: 25%;
      background: linear-gradient(to top, #3b82f6, #ef4444);
      border-radius: 0 0 10px 10px;
      transition: height 0.6s cubic-bezier(0.4, 0, 0.2, 1);
    }

    .meter-labels {
      position: absolute;
      inset: 8px 36px 8px 8px;
      pointer-events: none;
      display: flex;
      flex-direction: column;
      justify-content: space-between;
      font-size: 10px;
      font-weight: 700;
      color: #94a3b8;
    }

    .meter-delta {
      margin-top: 10px;
      font-size: 11px;
      font-weight: 800;
      color: #dc2626;
      text-align: center;
      line-height: 1.2;
    }

    /* Controls Bar */
    .controls {
      margin-top: 12px;
      display: flex;
      align-items: center;
      justify-content: center;
      gap: 12px;
    }

    .btn {
      background: #ffffff;
      border: 1px solid #cbd5e1;
      padding: 8px 16px;
      border-radius: 8px;
      font-size: 13px;
      font-weight: 600;
      color: #1e2945;
      cursor: pointer;
      display: inline-flex;
      align-items: center;
      gap: 6px;
      transition: background 0.2s, border-color 0.2s;
    }
    .btn:hover {
      background: #f8fafc;
      border-color: #94a3b8;
    }
    .btn:active {
      background: #f1f5f9;
    }

    .stepper {
      display: flex;
      gap: 6px;
    }

    .step-btn {
      padding: 6px 12px;
      font-size: 12px;
      font-weight: 700;
      border-radius: 6px;
      border: 1px solid #e2e8f0;
      background: #ffffff;
      color: #64748b;
      cursor: pointer;
    }
    .step-btn.active {
      background: #2563eb;
      color: #ffffff;
      border-color: #2563eb;
    }
  </style>
</head>
<body>

  <div class="main-grid">
    
    <div class="viewport" id="viewport">
      <div class="status-header">
        <span id="state-text">Stable Cl₂ Molecule</span>
        <span class="badge" id="thermo-badge">Bond Intact</span>
      </div>

      <div class="stage" id="stage">
        
        <div class="energy-packet" id="energy-packet">
          ⚡ Heat Energy In
        </div>

        <div class="atoms-container" id="molecule">
          <div class="atom atom-left">Cl</div>
          <div class="bond-zone" id="bond-zone" title="Shared electron pair">
            <div class="electron"></div>
            <div class="electron"></div>
          </div>
          <div class="atom atom-right">Cl</div>
        </div>
      </div>
    </div>

    
    <div class="meter-card">
      <div class="meter-title">Potential Energy</div>
      <div class="meter-track">
        <div class="meter-fill" id="energy-fill"></div>
      </div>
      <div class="meter-delta" id="delta-text">Low PE<br>(Stable)</div>
    </div>
  </div>

  
  <div class="controls">
    <button class="btn" id="play-btn">⏸ Pause</button>
    <div class="stepper">
      <button class="step-btn active" data-step="0">1. Bonded</button>
      <button class="step-btn" data-step="1">2. Energy In</button>
      <button class="step-btn" data-step="2">3. Broken</button>
    </div>
  </div>

  <script>
    const viewport = document.getElementById('viewport');
    const stateText = document.getElementById('state-text');
    const thermoBadge = document.getElementById('thermo-badge');
    const energyFill = document.getElementById('energy-fill');
    const deltaText = document.getElementById('delta-text');
    const playBtn = document.getElementById('play-btn');
    const stepBtns = document.querySelectorAll('.step-btn');

    let currentStep = 0;
    let isPlaying = true;
    let timer = null;

    const steps = [
      {
        name: "1. Bonded",
        stateClass: "",
        title: "Stable Cl₂ Molecule",
        badge: "Low Energy",
        badgeEndo: false,
        peHeight: "25%",
        peText: "Low PE<br>(Stable)"
      },
      {
        name: "2. Energy In",
        stateClass: "state-absorbing",
        title: "Energy Absorbed (+ΔH)",
        badge: "Endothermic",
        badgeEndo: true,
        peHeight: "60%",
        peText: "Absorbing<br>Energy"
      },
      {
        name: "3. Broken",
        stateClass: "state-broken",
        title: "2 Separate Cl Atoms",
        badge: "+ΔH Stored",
        badgeEndo: true,
        peHeight: "92%",
        peText: "High PE<br>(Free Atoms)"
      }
    ];

    function applyStep(index) {
      currentStep = index;
      const s = steps[currentStep];

      viewport.className = 'viewport ' + s.stateClass;
      stateText.textContent = s.title;
      thermoBadge.textContent = s.badge;
      
      if (s.badgeEndo) {
        thermoBadge.classList.add('endothermic');
      } else {
        thermoBadge.classList.remove('endothermic');
      }

      energyFill.style.height = s.peHeight;
      deltaText.innerHTML = s.peText;

      stepBtns.forEach((btn, idx) => {
        btn.classList.toggle('active', idx === currentStep);
      });
    }

    function nextStep() {
      applyStep((currentStep + 1) % steps.length);
    }

    function startAutoPlay() {
      if (timer) clearInterval(timer);
      timer = setInterval(nextStep, 3200);
      isPlaying = true;
      playBtn.innerHTML = '⏸ Pause';
    }

    function stopAutoPlay() {
      if (timer) clearInterval(timer);
      timer = null;
      isPlaying = false;
      playBtn.innerHTML = '▶ Play';
    }

    playBtn.addEventListener('click', () => {
      if (isPlaying) {
        stopAutoPlay();
      } else {
        nextStep();
        startAutoPlay();
      }
    });

    stepBtns.forEach(btn => {
      btn.addEventListener('click', (e) => {
        stopAutoPlay();
        applyStep(parseInt(e.target.dataset.step, 10));
      });
    });

    // Initialize
    applyStep(0);
    startAutoPlay();
  </script>
</body>
</html>

UK-GCSE-SCIENCE-CHEM-EC

Child standards (3)

Aligned learning resources

Related diagrams