*{box-sizing:border-box}
:root{--accent:#4f46e5;--accent2:#3730a3;--bg:#f7f8fc;--card:#fff;--text:#20243a;--muted:#687086;--line:#dde1ed;--soft:#eef0ff;--good:#eaf7ee;--warm:#fff7df}
html{scroll-behavior:smooth}body{margin:0;background:var(--bg);color:var(--text);font:16px/1.65 system-ui,-apple-system,Segoe UI,Roboto,Arial,sans-serif}
button,input,textarea{font:inherit}.topbar{position:sticky;top:0;z-index:20;display:flex;justify-content:space-between;align-items:center;padding:11px 16px;background:rgba(255,255,255,.96);backdrop-filter:blur(9px);border-bottom:1px solid var(--line)}
.brand{display:flex;gap:10px;align-items:center}.logo{width:40px;height:40px;border-radius:12px;background:var(--accent);color:white;display:grid;place-items:center;font-size:22px}.brand small{display:block;color:var(--muted);font-size:.76rem}.menu{border:1px solid var(--line);background:#fff;border-radius:10px;padding:7px 10px}
.nav{display:none;position:sticky;top:63px;z-index:19;background:#fff;border-bottom:1px solid var(--line);padding:8px 14px;gap:5px;overflow:auto}.nav.open{display:flex}.nav a{white-space:nowrap;color:var(--text);text-decoration:none;padding:7px 9px;border-radius:9px}.nav a:hover{background:var(--soft)}
main{max-width:760px;margin:auto}.hero{padding:34px 18px;background:linear-gradient(180deg,#eef0ff,#f7f8fc);border-bottom:1px solid var(--line)}.eyebrow{font-size:.74rem;letter-spacing:.09em;font-weight:800;color:var(--accent)}h1{font-size:clamp(2.1rem,9vw,3.4rem);line-height:1.03;letter-spacing:-.045em;margin:8px 0 14px}.big{font-size:1.12rem;font-weight:650}.section{padding:28px 18px;border-bottom:1px solid var(--line)}.section>h2{font-size:1.5rem;margin:0 0 14px}.btn{display:inline-flex;justify-content:center;align-items:center;border-radius:12px;padding:10px 14px;border:1px solid var(--line);text-decoration:none;font-weight:750;cursor:pointer}.btn.primary{background:var(--accent);border-color:var(--accent);color:#fff}.btn.secondary{background:#fff;color:var(--accent)}
.outcomes li{margin:7px 0}.story,.think,.card{background:var(--card);border:1px solid var(--line);border-radius:16px;padding:17px;margin:11px 0}.think{background:var(--warm)}.prediction{display:block;width:100%;text-align:left;background:#fff;border:1px solid #eadfba;border-radius:10px;padding:10px;margin:7px 0;cursor:pointer}.feedback{margin-top:11px;background:var(--soft);border-radius:10px;padding:10px;min-height:20px}.particle-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:9px}.particle{border:1px solid var(--line);border-radius:12px;padding:12px;text-align:center}.particle b,.particle span,.particle small{display:block}.particle span{font-weight:750;color:var(--accent)}.particle small{color:var(--muted)}.formula{display:flex;gap:8px;flex-wrap:wrap;margin:13px 0}.formula span{background:var(--soft);border:1px solid #dfe2ff;padding:8px 10px;border-radius:10px;font-weight:700}.example{background:#f7f8fc;padding:12px;border-radius:11px}.calculator{display:grid;gap:9px}.calculator label{display:grid;gap:4px}.calculator input,.calculator textarea{border:1px solid var(--line);border-radius:10px;padding:9px;background:#fff;width:100%}
.atom-simple{height:250px;max-width:250px;margin:15px auto;position:relative;display:grid;place-items:center}.orbit{position:absolute;border:2px solid #cbd0e5;border-radius:50%}.o1{width:135px;height:135px}.o2{width:220px;height:220px}.nucleus{width:75px;height:75px;border-radius:50%;background:#ffe7a8;display:grid;place-items:center;text-align:center;font-weight:800;z-index:2}.electron{position:absolute;width:28px;height:28px;border-radius:50%;background:var(--accent);color:#fff;font-style:normal;display:grid;place-items:center;font-size:.75rem;font-weight:800}.e1{top:38px;left:110px}.e2{right:14px;top:111px}.e3{bottom:32px;left:110px}.e4{left:14px;top:111px}.caption{font-size:.82rem;color:var(--muted);text-align:center}
.shell-examples{display:grid;grid-template-columns:repeat(3,1fr);gap:8px}.shell-examples div{border:1px solid var(--line);border-radius:11px;padding:10px;text-align:center}.shell-examples span{display:block;font-size:1.1rem;color:var(--accent);font-weight:800}.ion-flow{display:flex;flex-wrap:wrap;justify-content:center;align-items:center;gap:8px;background:#f7f8fc;padding:10px;border-radius:11px;margin:7px 0}.molecule{display:flex;justify-content:center;align-items:center;gap:6px;margin:15px}.atom{width:48px;height:48px;border-radius:50%;display:grid;place-items:center;font-weight:800}.smallH{background:#dfe8ff}.smallO{background:#ffd9d9}.bond{font-weight:800}
.tinted{background:#f0f2ff}.model-wrap{display:grid;grid-template-columns:1fr 1fr;gap:20px;align-items:center}.atom-interactive{height:270px;position:relative;display:grid;place-items:center}.ring{position:absolute;border:2px solid #c7cbed;border-radius:50%}.r1{width:130px;height:130px}.r2{width:225px;height:225px}.core{width:80px;height:80px;border-radius:50%;background:#ffe7a8;display:grid;grid-template-columns:1fr 1fr;place-items:center;font-weight:800;z-index:2}.orb{position:absolute;width:32px;height:32px;border:0;border-radius:50%;background:var(--accent);color:#fff;font-weight:800;cursor:pointer}.eA{top:28px;left:calc(50% - 16px)}.eB{right:10px;top:calc(50% - 16px)}.eC{bottom:27px;left:calc(50% - 16px)}.legend{display:grid;gap:8px}.legend-btn{background:#fff;border:1px solid var(--line);border-radius:10px;padding:10px;text-align:left;cursor:pointer}
.periodic-mini{display:grid;grid-template-columns:repeat(5,1fr);gap:7px}.periodic-mini button{background:#fff;border:1px solid var(--line);border-radius:9px;padding:9px;cursor:pointer;font-weight:800}.periodic-mini small{color:var(--accent)}
.community-grid,.facts,.words{display:grid;grid-template-columns:repeat(2,1fr);gap:9px}.community-grid>div,.facts>div,.words>div{background:#fff;border:1px solid var(--line);border-radius:11px;padding:11px}.community-grid p{margin:3px 0;color:var(--muted);font-size:.92rem}.facts>div:before{content:"✦ ";color:var(--accent)}.words span{display:block;color:var(--muted);font-size:.9rem}
.quiz{background:#f0f2ff}.quiz-q{background:#fff;border:1px solid var(--line);border-radius:13px;padding:13px;margin:9px 0}.quiz-q p{font-weight:750}.quiz-opt{display:block;border:1px solid var(--line);padding:8px;border-radius:9px;margin:6px 0}.quiz-result{font-weight:800}.checklist{display:grid;gap:8px}.checklist label{background:#f7f8fc;border:1px solid var(--line);padding:9px;border-radius:9px}.callout,.remember{background:var(--soft);border-left:4px solid var(--accent);padding:12px;border-radius:10px}textarea{width:100%;min-height:120px;border:1px solid var(--line);border-radius:11px;padding:10px;resize:vertical}details{background:#fff;border:1px solid var(--line);border-radius:11px;margin:7px 0;padding:11px}summary{cursor:pointer;font-weight:750}.keypoints{display:grid;gap:7px}.remember{margin-top:18px}.remember b{font-size:.75rem;letter-spacing:.08em;color:var(--accent)}.remember p{font-weight:750;margin-bottom:0}footer{text-align:center;color:var(--muted);padding:24px 16px;font-size:.88rem}
@media(max-width:560px){.particle-grid,.model-wrap{grid-template-columns:1fr}.shell-examples,.community-grid,.facts,.words{grid-template-columns:1fr}.periodic-mini{grid-template-columns:repeat(5,1fr)}}
@media(min-width:700px){.menu{display:none}.nav{display:flex;justify-content:center}.nav a{padding:7px 11px}}