Sec 4 Chemistry The Periodic Table notes

Elements are listed in the modern table in order of increasing proton number (atomic number). The vertical groups contain elements with the same number of valence electrons — which is why they behave alike. The horizontal periods count occupied electron shells: sodium in period 3 has three shells; chlorine in period 3 has three shells too.

A thick staircase of metals fills the left and centre; non-metals cluster top-right; the divide runs diagonally past silicon. Hydrogen is the awkward guest: placed at the top of Group I by shell count, but a non-metal that forms H⁺, not the alkali pattern — examiners like asking why.

Lithium, sodium and potassium are soft (cut with a knife), low-density (float on water), and melt at surprisingly low temperatures. Each has one valence electron, which it loses readily — so they are powerful reducing agents and never found uncombined in nature. Because they tarnish instantly in air, they are stored under oil.

On water they fizz around forming an alkaline solution: 2Na + 2H₂O → 2NaOH + H₂ (the gas pops with a lighted splint; phenolphthalein flushes pink). Down the group the outer electron sits further from the nucleus, held more loosely — so reactivity increases Li < Na < K: lithium floats and fizzes, sodium whizzes, and potassium ignites its own hydrogen with a lilac flame.

Fluorine, chlorine, bromine and iodine exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) and are toxic. At room temperature the state and colour walk down the group: chlorine pale green gas, bromine red-brown liquid (and its vapour!), iodine grey-black solid with purple vapour — melting and boiling points rise as molecules get heavier.

Reactivity runs downhill: each halogen has 7 valence electrons and wants one more, but the incoming electron meets a more shielded pull further down the group. So fluorine > chlorine > bromine > iodine. The proof is displacement: Cl₂ + 2KBr → 2KCl + Br₂ (solution turns orange); Br₂ + 2KI → 2KBr + I₂ (brown-black iodine). Chlorine displaces both; iodine displaces neither.

Helium, neon, argon, krypton and xenon have completely full valence shells (2 for helium, 8 for the rest), so they have no urge to gain, lose or share electrons: monatomic, colourless, chemically unreactive. That laziness is exactly their industrial value — a gas that refuses to react is perfect where reactivity would ruin things.

The block between Groups II and III holds the transition elements: iron, copper, nickel, chromium and friends. They are hard, strong, dense metals with high melting points (iron melts at 1538 °C), and — unlike the fixed Group I metals — they show variable oxidation states: iron in Fe²⁺ and Fe³⁺, copper in Cu⁺ and Cu²⁺, manganese from +2 all the way to +7 in KMnO₄.

Their compounds are coloured — blue Cu²⁺, green Fe²⁺, red-brown Fe³⁺, purple MnO₄⁻ — a built-in qualitative-analysis palette (see Sec 3 Chapter 7). And they are the industrial catalysts: iron in the Haber process, nickel in the manufacture of margarine, and so on.