Inside the atom

An atom has a dense central nucleus of protons (charge +1, mass ≈ 1 u) and neutrons (charge 0, mass ≈ 1 u), surrounded by electrons (charge −1, mass ≈ 1/1836 u) occupying quantised energy levels.

• Atomic number Z = number of protons; it defines the element.
• Mass number A = protons + neutrons.
• Isotopes — atoms of one element (same Z) with different numbers of neutrons (different A), e.g. ¹²C and ¹⁴C.
• Relative atomic mass Aᵣ — the weighted average of isotope masses by natural abundance.

Electrons do not orbit like planets. The quantum-mechanical model describes them by orbitals — regions of space (s, p, d, f) with characteristic shapes where an electron is likely to be found. Orbitals fill in order of increasing energy (Aufbau principle), each holds at most two electrons of opposite spin (Pauli exclusion), and degenerate orbitals fill singly first (Hund's rule).

From plum pudding to probability clouds

Rutherford's 1911 gold-foil experiment fired alpha particles at thin foil. Most passed straight through, but a few bounced sharply back — impossible if positive charge were spread out (Thomson's "plum pudding"). The conclusion: the atom is mostly empty space with a tiny, dense, positive nucleus. Later, Bohr explained why atoms emit light only at specific wavelengths: electrons occupy fixed energy levels, and light is emitted when an electron drops from a higher to a lower level, carrying away exactly the energy difference as a photon.

The modern picture replaces Bohr's fixed orbits with orbitals — you cannot know both an electron's exact position and momentum (Heisenberg uncertainty), so we speak of probability. An orbital is where the electron is 90% likely to be. This is why chemistry is quantised: energy comes in discrete packets, and that discreteness ultimately governs bonding and colour.

Worked example — relative atomic mass of chlorine

Chlorine exists as two isotopes: ³⁵Cl (75.77% abundant, mass 34.969 u) and ³⁷Cl (24.23%, mass 36.966 u). Calculate Aᵣ.

1. Write the weighted-average formula: Aᵣ = Σ (abundance × isotope mass).
2. Convert percentages to fractions: 0.7577 and 0.2423.
3. Multiply each: 0.7577 × 34.969 = 26.497; 0.2423 × 36.966 = 8.957.
4. Add the contributions: 26.497 + 8.957 = 35.45 u.
5. Interpret: chlorine's Aᵣ ≈ 35.45, closer to 35 because the lighter isotope dominates — consistent with the value on the periodic table.

Also write the ground-state configuration of chlorine (Z = 17): 1s² 2s² 2p⁶ 3s² 3p⁵ — seven electrons in the outer shell, one short of a full octet, which is why Cl is so reactive.