Atomic structure and electron shells

The idea

Atoms have a radius of about 0.1 nm (1 × 10⁻¹⁰ m); the nucleus is about 1/10 000 of that. The atomic number is the number of protons; the mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass is a mean weighted by the abundance of each isotope. Electrons fill shells: 2 in the first, then 8, then 8 (for the first 20 elements).

1. Protons = atomic number; neutrons = mass number − atomic number.
2. Weighted mean: (mass × abundance + ...) ÷ 100.
3. Fill shells 2, 8, 8, then the fourth: potassium is 2,8,8,1.

Watch out Electrons equal protons only in a neutral atom, not in an ion.