Sec 4 Combined Science (Chemistry) Periodic Table & Reactivity notes

The modern Periodic Table arranges the elements in order of increasing proton number. The vertical columns are groups (1 to 18 in the extended numbering; the syllabus focuses on Groups 1, 17 and 18 plus the transition block you only need to recognise); the horizontal rows are periods (1 to 7).

Position is configuration. The period number equals the number of electron shells; the group number (for the main groups) equals the number of outer-shell (valence) electrons. Sodium, configuration 2.8.1, sits in period 3 and group 1. Chlorine, 2.8.7, sits in period 3, group 17.

Group chemistry follows straight from that shared outer shell: every Group 1 atom has one valence electron to lose, every Group 17 atom has seven and wants one more. Same outer shell, same chemistry — that single sentence is the reason groups exist.

Walk along period 3: sodium, magnesium, aluminium are metals — shiny, conductors, each with one to three valence electrons they lose readily. Silicon in the middle is the borderline semi-metal. Then phosphorus, sulfur, chlorine are non-metals — dull, non-conducting, with five to seven valence electrons tending to be gained, and argon ends the period with a full shell.

The link to remember: few valence electrons → metallic (easy to lose), many valence electrons → non-metallic (easier to gain or share). Going across a period, metallic character decreases; going down a group, metallic character increases (the outer shell gets further from the nucleus and is lost more easily).

The alkali metals — lithium, sodium, potassium — are soft (cuttable with a knife), low-density (lithium, sodium and potassium float on water) and low-melting compared with most metals. All have one valence electron, and all react with cold water to give an alkaline solution and hydrogen:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Lithium fizzes steadily; sodium melts into a ball and dashes around the surface; potassium ignites its own hydrogen with a lilac flame. Down the group the outer electron sits further from the nucleus, shielded by more inner shells, so it is lost more easily — which is why reactivity increases down the group.

• Predicting a new member (like rubidium): soft, low-density metal, one valence electron.
• With water: violent reaction, alkaline solution, hydrogen released.
• Compared with potassium: even more reactive, because the outer electron is even further out.

The halogens are non-metals with seven valence electrons, existing as diatomic molecules (F₂, Cl₂, Br₂, I₂). Down the group their colours deepen and their melting and boiling points rise: chlorine is a pale green gas, bromine a red-brown liquid, iodine a grey-black solid that sublimes to violet vapour.

Chemically the trend is the mirror image of Group 1: halogens react by gaining one electron, and down the group the outer shell is further out, so the pull on that extra electron weakens — reactivity decreases down the group. The proof is displacement: a more reactive halogen pushes a less reactive halogen out of its salt solution.

The equations behind the results: Cl₂ + 2KBr → 2KCl + Br₂ and Br₂ + 2KI → 2KBr + I₂. Notice the rule: a halogen displaces only halogens below it in the group. (You met this electron-grabbing in Chapter 1 — halogen displacement is redox.)

The noble gases — helium, neon, argon — have full outer shells (two for helium, eight for the rest), so they neither need to lose nor gain electrons. That is why they are monatomic and unreactive, and why argon safely fills light bulbs and shields reactive welds.

The reactivity series ranks metals by how willingly their atoms lose electrons. Three experiment families build the ranking:

The series that emerges — potassium, sodium, calcium, magnesium, (aluminium), zinc, iron, lead, (hydrogen), copper, silver. Metals above hydrogen displace hydrogen from dilute acids: Mg + 2HCl → MgCl₂ + H₂ (vigorous), Zn + 2HCl → ZnCl₂ + H₂ (steady fizzing), Fe + 2HCl → FeCl₂ + H₂ (slow, best warm). Copper and silver sit below hydrogen, so no reaction with dilute acid — that silence is data too.

• K, Na: react explosively with cold water — too reactive to try with acid.
• Ca: steady bubbles in cold water; Ca + 2H₂O → Ca(OH)₂ + H₂.
• Mg: no cold-water reaction; burns in steam → MgO + H₂.
• Zn, Fe: need steam; Zn + H₂O → ZnO + H₂.
• Pb: no visible reaction with cold dilute acid (an insoluble coat of lead chloride protects it).
• Cu, Ag: nothing with water, steam or dilute HCl — the least reactive.

The more reactive the metal, the more strongly its compounds hold together — and the harder it is to obtain the metal from its ore. The very reactive metals at the top (potassium to aluminium) are won from molten ores using electricity. The middle band (zinc to copper) can be reduced by heating with carbon, which grabs the oxygen from the ore. The least reactive — silver and gold — occur uncombined in the Earth and simply need purifying.

Rusting is iron's slow return to its ore: it happens only when iron touches both oxygen and water together (dry air alone, or air-free water alone, will not rust iron). The product is hydrated iron(III) oxide — the familiar flaky red-brown rust, which does not protect the iron underneath, so rusting eats right through.

Prevention follows from the cause: keep either oxygen or water away and rusting cannot start. Any barrier works — paint on a gate, grease or oil on tools and machinery, plastic coating on wire. The barrier must stay intact: a scratch that exposes the iron lets rust begin again.