body {
    font-family: Arial, sans-serif;
    margin: 0;
    background-color: #f5f7fa;
    color: #1f2937;
}

h1 {
    text-align: center;
    font-size: 48px;
    margin-top: 60px;
}

h2 {
    text-align: center;
}

p {
    max-width: 700px;
    margin: 20px auto;
    line-height: 1.6;
}
header {
    background-color: #1e3a5f;
    padding: 20px 40px;
}

nav {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.logo {
    color: white;
    font-size: 24px;
    font-weight: bold;
    text-decoration: none;
}

.nav-links {
    display: flex;
    gap: 30px;
}

.nav-links a {
    color: white;
    text-decoration: none;
    font-size: 16px;
}

.nav-links a:hover {
    text-decoration: underline;
}
.hero {
    text-align: center;
    padding: 200px 20px;
    background-color: white;
}

.hero h1 {
    font-size: 64px;
    margin: 0 0 20px 0;
}

.hero-subtitle {
    font-size: 20px;
    max-width: 650px;
    margin: 0 auto 35px auto;
}

.hero-button {
    display: inline-block;
    background-color: #1e3a5f;
    color: white;
    padding: 14px 28px;
    border-radius: 6px;
    text-decoration: none;
    font-weight: bold;
}

.hero-button:hover {
    background-color: #162d4a;
}
#units {
    padding: 80px 40px;
}

#units h2 {
    font-size: 36px;
    margin-bottom: 40px;
}

.unit-grid {
    max-width: 1100px;
    margin: 0 auto;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 24px;
}

.unit-card {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    min-height: 180px;
    padding: 28px;
    background-color: white;
    border-radius: 10px;
    text-decoration: none;
    color: #1f2937;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
    transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.unit-number {
    font-size: 14px;
    font-weight: bold;
    color: #1e3a5f;
    letter-spacing: 1px;
}

.unit-card h3 {
    font-size: 20px;
    margin: 15px 0;
}

.card-link {
    font-weight: bold;
    color: #1e3a5f;
}

.unit-card:hover {
    transform: translateY(-5px);
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.12);
}
.unit-page {
    max-width: 900px;
    margin: 0 auto;
    padding: 80px 30px;
}

.unit-page h1 {
    text-align: left;
    font-size: 52px;
    margin: 15px 0 20px 0;
}

.unit-page > p {
    max-width: 750px;
    margin: 0 0 50px 0;
    font-size: 19px;
}

.unit-page h2 {
    text-align: left;
    font-size: 30px;
    margin-top: 50px;
}

.unit-page ul {
    padding-left: 25px;
    line-height: 2;
}

.unit-page li {
    font-size: 18px;
}
.unit-page {
    max-width: 900px;
    margin: 0 auto;
    padding: 80px 30px;
}

.unit-page h1 {
    text-align: left;
    font-size: 52px;
    margin: 15px 0 20px 0;
}

.unit-page > p {
    max-width: 750px;
    margin: 0 0 50px 0;
    font-size: 19px;
}

.unit-page h2 {
    text-align: left;
    font-size: 30px;
    margin-top: 50px;
}

.topic-list {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 20px;
}

.topic-card {
    padding: 25px;
    background-color: white;
    border-radius: 10px;
    text-decoration: none;
    color: #1f2937;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
    transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.topic-card h3 {
    margin-top: 0;
    color: #1e3a5f;
    font-size: 21px;
}

.topic-card p {
    margin: 10px 0 0 0;
    line-height: 1.5;
}

.topic-card:hover {
    transform: translateY(-4px);
    box-shadow: 0 8px 20px rgba(0, 0, 0, 0.12);
}
.lesson {
    max-width: 900px;
    margin: 0 auto 80px auto;
    padding: 45px 30px;
    background-color: white;
    border-radius: 12px;
    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
}

.lesson h2 {
    text-align: left;
    font-size: 34px;
    margin-top: 0;
    color: #1e3a5f;
}

.lesson h3 {
    text-align: left;
    font-size: 24px;
    margin-top: 35px;
}

.lesson p {
    max-width: none;
    margin: 16px 0;
    font-size: 18px;
    line-height: 1.7;
}

.key-concept {
    margin: 25px 0;
    padding: 22px;
    background-color: #eef4fa;
    border-left: 5px solid #1e3a5f;
    border-radius: 6px;
}

.equation {
    margin: 25px 0;
    padding: 18px;
    background-color: #f5f7fa;
    border-radius: 8px;
    text-align: center;
    font-size: 21px;
    font-weight: bold;
}

.example {
    margin-top: 30px;
    padding: 25px;
    background-color: #f8fafc;
    border: 1px solid #dbe3ec;
    border-radius: 8px;
}

.example h4 {
    margin-top: 0;
    color: #1e3a5f;
    font-size: 20px;
}

.fraction {
    display: inline-flex;
    flex-direction: column;
    vertical-align: middle;
    margin: 0 8px;
}

.numerator {
    padding: 0 8px 4px 8px;
    border-bottom: 2px solid #1f2937;
}

.denominator {
    padding-top: 4px;
}

.video-resource {
    display: flex;
    justify-content: space-between;
    align-items: center;
    gap: 30px;
    margin-top: 40px;
    padding: 28px;
    background-color: #eef4fa;
    border: 1px solid #cbd9e8;
    border-radius: 10px;
}

.video-text {
    flex: 1;
}

.video-label {
    color: #1e3a5f;
    font-size: 13px;
    font-weight: bold;
    letter-spacing: 1px;
}

.video-resource h3 {
    margin: 8px 0;
    font-size: 23px;
    color: #1f2937;
}

.video-resource p {
    margin: 0;
    font-size: 17px;
}

.video-button {
    display: inline-block;
    flex-shrink: 0;
    padding: 13px 22px;
    background-color: #1e3a5f;
    color: white;
    text-decoration: none;
    font-weight: bold;
    border-radius: 6px;
}

.video-button:hover {
    background-color: #162d4a;
}

@media (max-width: 750px) {
    .unit-grid,
    .topic-list {
        grid-template-columns: 1fr;
    }

    nav {
        flex-direction: column;
        gap: 18px;
    }

    .nav-links {
        gap: 16px;
        flex-wrap: wrap;
        justify-content: center;
    }

    .hero {
        padding: 120px 20px;
    }

    .hero h1,
    .unit-page h1 {
        font-size: 42px;
    }

    .video-resource {
        flex-direction: column;
        align-items: flex-start;
    }

    .video-button {
        width: 100%;
        box-sizing: border-box;
        text-align: center;
    }
}

.practice-section {
    margin-top: 50px;
    padding-top: 15px;
    border-top: 2px solid #dbe3ec;
}

.practice-question {
    margin-top: 25px;
    padding: 24px;
    background-color: #f8fafc;
    border: 1px solid #dbe3ec;
    border-radius: 8px;
}

.practice-question h4 {
    margin: 0 0 12px 0;
    color: #1e3a5f;
    font-size: 20px;
}

.practice-question details {
    margin-top: 18px;
}

.practice-question summary {
    color: #1e3a5f;
    font-weight: bold;
    cursor: pointer;
}

.practice-question summary:hover {
    text-decoration: underline;
}

.solution {
    margin-top: 20px;
    padding-top: 5px;
}
.spectrum-notes {
    margin: 25px 0;
    padding-left: 28px;
    font-size: 18px;
    line-height: 1.7;
}

.spectrum-notes li {
    margin-bottom: 12px;
}

.mass-spectrum-figure {
    margin: 35px 0;
    padding: 20px;
    background-color: #f8fafc;
    border: 1px solid #dbe3ec;
    border-radius: 10px;
}

.mass-spectrum-figure img {
    display: block;
    width: 100%;
    height: auto;
}

.mass-spectrum-figure figcaption {
    margin-top: 15px;
    color: #4b5563;
    text-align: center;
    font-size: 15px;
    line-height: 1.5;
}
.table-container {
    width: 100%;
    margin: 30px 0;
    overflow-x: auto;
}

.particle-table {
    width: 100%;
    border-collapse: collapse;
    background-color: white;
}

.particle-table th,
.particle-table td {
    padding: 15px;
    border: 1px solid #dbe3ec;
    text-align: left;
}

.particle-table th {
    background-color: #1e3a5f;
    color: white;
}

.particle-table tbody tr:nth-child(even) {
    background-color: #f8fafc;
}
.chemistry-figure {
    margin: 35px 0;
    padding: 20px;
    background-color: #f8fafc;
    border: 1px solid #dbe3ec;
    border-radius: 10px;
}

.chemistry-figure img {
    display: block;
    width: 100%;
    height: auto;
}

.chemistry-figure figcaption {
    margin-top: 15px;
    color: #4b5563;
    text-align: center;
    font-size: 15px;
    line-height: 1.5;
}