The Periodic Table and Groups

How it's arranged

• Elements in order of atomic number.
• Groups (columns) = same number of outer electrons = similar properties.
• Periods (rows) = same number of electron shells.
• Metals on the left/centre, non-metals top-right.

History

Mendeleev arranged elements by properties and left gaps for undiscovered elements, even predicting their properties. The modern table orders by atomic number (after protons were discovered).

Group 1 — Alkali metals (Li, Na, K…)

• 1 outer electron; very reactive.
• Reactivity increases down the group (outer electron is further from the nucleus, lost more easily).
• React with water → metal hydroxide + hydrogen, fizzing.

Group 7 — Halogens (F, Cl, Br, I)

• 7 outer electrons; reactive non-metals.
• Reactivity decreases down the group (harder to gain an electron).
• A more reactive halogen displaces a less reactive one from its salt.

Group 0 — Noble gases (He, Ne, Ar)

• Full outer shell → unreactive (inert).
• Boiling points increase down the group.

Exam tip

Link reactivity to the outer electron: Group 1 gets more reactive down (easier to lose 1 e⁻); Group 7 gets less reactive down (harder to gain 1 e⁻).