The Periodic Table (C1.2)

Development of the periodic table

• John Newlands (1864): arranged elements in order of relative atomic mass; every eighth element had similar properties (Law of Octaves) — limited by known elements.
• Dmitri Mendeleev (1869): arranged by atomic mass; left gaps for undiscovered elements and predicted their properties. Genius move — predictions later confirmed.
• Modern table: arranged by atomic number (proton number); reveals periodic trends in properties.

Structure

• Periods (horizontal rows): elements with the same number of electron shells.
• Groups (vertical columns): elements with the same number of outer electrons → similar chemical properties.
• Elements to the left of the zigzag line = metals; to the right = non-metals.

Metals vs non-metals

Group 1 — Alkali metals (Li, Na, K, Rb, Cs, Fr)

• One outer electron → highly reactive (lose it easily).
• React with water → metal hydroxide + hydrogen gas:
  2Na + 2H₂O → 2NaOH + H₂
• Reactivity increases down the group — outer electron further from nucleus → less attraction → easier to lose.
• Stored under oil (prevents reaction with air/water).

Group 7 — Halogens (F, Cl, Br, I, At)

• Seven outer electrons → reactive non-metals (gain one electron).
• Exist as diatomic molecules: F₂, Cl₂, Br₂, I₂.
• Reactivity decreases down the group — outer shell further from nucleus → less able to attract an electron.
• Displacement reactions: more reactive halogen displaces less reactive one from solution.
  Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromine)

Group 0 — Noble gases (He, Ne, Ar, Kr, Xe, Rn)

• Full outer shells → very stable → virtually unreactive/inert.
• Monatomic (single atoms).
• Boiling points increase down the group (stronger intermolecular forces as atoms get larger).
• Used where unreactivity is needed: He in balloons, Ar in light bulbs, Xe in flash photography.

Common exam errors

1. Saying Mendeleev arranged elements by mass number — he used relative atomic mass (before proton concept).
2. Forgetting Group 1 reactivity increases down (not decreases) — the opposite of Group 7.
3. Saying noble gases have no electrons in their outer shell — they have full outer shells (8, or 2 for He).