The periodic table: arrangement and group trends

The periodic table organises every known element by proton number (atomic number). Elements in the same vertical column (group) have the same number of electrons in their outer shell — that is why they behave similarly chemically.

Reading the table

• Period = horizontal row. The period number tells you how many electron shells the atom has.
• Group = vertical column. The group number tells you how many electrons are in the outer shell (for Groups 1, 2, 3-7 and 0).
• Metals sit on the left and middle; non-metals on the right of the staircase line.

Group 1 — the alkali metals (Li, Na, K, Rb, Cs)

• Soft, low density, low melting point compared with most metals.
• React with water to form a metal hydroxide + hydrogen.
• Reactivity increases down the group because the outer electron is further from the nucleus and more easily lost.

Example: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Sodium fizzes and melts into a ball; potassium catches fire with a lilac flame.

Group 7 — the halogens (F, Cl, Br, I)

• Diatomic non-metals (Cl₂, Br₂, I₂).
• Form salts with metals (e.g. NaCl).
• Reactivity decreases down the group because the outer shell is further from the nucleus and gains an electron less easily.

Displacement: a more reactive halogen displaces a less reactive halogen from a salt solution. Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) — solution turns orange.

Group 0 — the noble gases (He, Ne, Ar, Kr, Xe)

• Full outer shell → unreactive (inert).
• Used where chemical inertness matters: helium in balloons, argon in filament lamps, neon in signs.

CCEA tip

For "explain why Group 1 reactivity increases down the group" you must mention BOTH that the outer electron is further from the nucleus AND that there is more shielding from inner shells. One point alone scores B1 of B2.