The periodic table as a map of atomic structure
The periodic law states that the properties of elements and their compounds change periodically with atomic number. This happens because similar outer-electron arrangements repeat.
Learning goals: connect atomic number with electron arrangement; identify valence electrons from a main-group position; explain trends across periods and down groups; predict simple properties of the first three periods, potassium and calcium.
1. Position and electron structure
Atomic number $Z$ equals the number of protons and, in a neutral atom, the number of electrons. Electrons occupy energy levels or shells. For the first 20 elements a useful school model fills shells as 2, 8, 8, 2. The period number shows the number of occupied shells. For main-group elements, the group helps identify the number of valence electrons. Elements in one group therefore often form similar ions and compounds.