CC1.3 — The periodic table (Edexcel 1SC0)

Arrangement

Elements are arranged in order of increasing atomic number. Elements with similar properties are in the same group (vertical column). The period (horizontal row) number indicates the number of electron shells.

• Groups 1–7 (and 0): number of electrons in outer shell = group number (Group 1 = 1 outer electron; Group 7 = 7 outer electrons; Group 0 = full outer shell).
• Period number = number of occupied electron shells.

Mendeleev's contribution

Mendeleev (1869) arranged elements by atomic mass and left gaps for undiscovered elements (e.g. predicted properties of gallium and germanium). Ordered by atomic mass mostly, but corrected some by properties. Modern table uses atomic number (Moseley, 1913).

Metals vs non-metals

• Metals: left and centre of table (all d-block, groups 1 and 2, and most others).
• Non-metals: right of table (groups 14–17 and group 0).
• Metalloids: borderline elements (Si, Ge, As, Sb).

Trends across a period (e.g. period 3: Na → Cl)

• Atomic radius decreases (more protons → greater nuclear attraction on electrons).
• Ionisation energy generally increases.
• Metallic character decreases; non-metallic character increases.

Trends down a group

• Atomic radius increases (more shells).
• Ionisation energy decreases (outermost electrons further from nucleus, more shielded).
• Reactivity increases in Group 1 (easier to lose outer electron); decreases in Group 7 (harder to gain electron).