Atomic models and atomic number

Thomson's model pictured the atom as a sphere of positive charge with electrons embedded in it, like raisins in a Christmas pudding (the plum pudding model), or seeds in a watermelon.

Rutherford tested this by firing α-particles (helium nuclei) at a very thin gold foil:

• Most passed straight through.
• A few were deflected by small angles.
• Very few (about 1 in 20,000) bounced back.

His conclusions: most of the atom is empty space; the positive charge and nearly all the mass are packed into a tiny centre, the nucleus (about 10⁻¹⁵ m across, in an atom about 10⁻¹⁰ m across); and the electrons move around the nucleus. If the atom were the size of a cricket stadium, its nucleus would be about the size of a pea at the centre.

Two numbers describe every atom:

• Atomic number, Z = the number of protons.
• Mass number, A = protons + neutrons (together called nucleons).

So neutrons = A – Z. A neutral atom has Z electrons; a cation (positive ion) has fewer, an anion (negative ion) has more. Example: sodium, ²³₁₁Na, has 11 protons, 23 – 11 = 12 neutrons, and 11 electrons; the ion Na⁺ has only 10 electrons.

Isotopes are atoms with the same Z but different A: hydrogen has three, ¹H (protium), ²H (deuterium, D) and ³H (tritium, T). Isobars have the same A but different Z, such as ¹⁴C and ¹⁴N.

Rutherford's model had a serious problem. An electron moving in a circle is accelerating, and by the laws of physics an accelerating charge should radiate energy, lose speed and spiral into the nucleus. Real atoms are stable, so the model could not explain this.