Atomic Structure
& Chemical Bonding
Master atoms, electrons, bonds and nuclear physics through interactive models, animated diagrams, and scored practice questions.
Introduction to Atomic Structure
Every substance is built from atoms — the fundamental building blocks of matter. An atom contains a dense nucleus surrounded by orbiting electrons.
Atomic Number (Z) = Number of protons |
Mass Number (A) = Protons + Neutrons
Neutrons = A − Z |
Neutral atom: Electrons = Protons
Niels Bohr Atomic Model
In 1913, Bohr proposed electrons orbit the nucleus in fixed circular paths called shells or energy levels, each with a maximum electron capacity.
Shell 1 (K) — Maximum 2 electrons
The innermost, lowest-energy shell. Elements: H (1e⁻), He (2e⁻). Full = very stable (Helium).
Shell 2 (L) — Maximum 8 electrons
Second shell. Filled from Li (3) to Ne (10). Formula: 2n² = 2×4 = 8.
Shell 3 (M) — Maximum 8 electrons (for Z≤20)
Can theoretically hold 18, but for elements 1–20 it fills to 8 before the 4th shell starts.
Shell 4 (N) — Begins at Potassium (Z=19)
K = 2,8,8,1 and Ca = 2,8,8,2. The 4th shell opens even though 3rd isn’t completely filled.
Radioactivity & Radiation Types
Radioactivity is the spontaneous emission of particles or electromagnetic energy from an unstable atomic nucleus as it seeks a more stable state.
Comparison: α, β and γ Radiation
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nuclei (⁴He) | Fast electrons (e⁻) | Electromagnetic photons |
| Charge | +2 | −1 | 0 (neutral) |
| Mass (amu) | 4 (heaviest) | ~0.00055 | Massless |
| Speed | ~5–7% of c | Up to 99% of c | Speed of light (c) |
| Penetration | Stopped by paper / skin | Stopped by 5 mm Al | Reduced by thick Pb / concrete |
| Ionisation | Very high (most dangerous internally) | Moderate | Low (most penetrating externally) |
Nuclear Fission vs Fusion
Two fundamentally different nuclear reactions that release extraordinary amounts of energy — far beyond any chemical reaction.
Atomic Energy — Applications
Valence Electrons & Octet Rule
The outermost shell of an atom determines its chemical reactivity. Atoms react to achieve a full outer shell — 8 electrons (octet) or 2 electrons (duplet) for the smallest atoms.
Octet Rule
Atoms tend to gain, lose, or share electrons until their outermost shell contains 8 electrons — like the nearest noble gas. Applies to Period 2 and 3 elements.
Duplet Rule
Hydrogen and Helium are stable with only 2 electrons (full first shell). H forms 1 bond, He has no bonds.
Valence Electrons
Electrons in the outermost shell. Same-group elements have the same valence electron count and similar chemistry. They determine valency, bonding type, and reactivity.
Ions and Ion Formation
When atoms gain or lose electrons to achieve stability, they become electrically charged ions. Cations are positive (lose e⁻); anions are negative (gain e⁻).
💡 Key rule: Metals (left of periodic table) tend to lose electrons → become cations (+).
Non-metals (right of periodic table) tend to gain electrons → become anions (−).
Interactive Periodic Table
Click any element (Z = 1 to 20) to see its full atomic details including electron configuration, valency, and group.
Chemical Bonds & Types
Chemical bonds hold atoms together. Bond type depends on electronegativity difference (ΔEN) between the atoms involved.
Bond Formation Diagrams
Molecular Formulas & Calculators
Learn the crisscross method for writing chemical formulas and calculate exact molecular weights with the interactive tools below.
🔀 Crisscross Method
Select a cation and anion — the tool applies the crisscross method and simplifies the formula automatically.
⚖️ Molecular Weight Calculator
Type a chemical formula using element symbols and numbers. Spaces and brackets not required.
Chemical FormulaReference atomic masses (g/mol)
H=1 · He=4 · Li=7 · Be=9 · B=11 · C=12 · N=14 · O=16 · F=19 · Ne=20
Na=23 · Mg=24 · Al=27 · Si=28 · P=31 · S=32 · Cl=35.5 · Ar=40 · K=39 · Ca=40
Fe=56 · Cu=64 · Zn=65 · Br=80 · Ag=108 · I=127 · Pb=207
Quiz & Practice
Test your knowledge with 10 curriculum-aligned questions. Select the best answer and get instant feedback with explanations.
Frequently Asked Questions
Common questions answered clearly and concisely by our curriculum experts.