Chemistry of Representative Elements

The representative (main-group) elements are the s-block (Groups IA & IIA) and the p-block (Groups IIIA–VIIIA). Their valence shells are being filled, so their chemistry is governed directly by the number of valence electrons.

• Modern periodic law — properties are a periodic function of atomic number (Moseley). Mendeleev's earlier law used atomic mass.
• Period — a horizontal row; across it the atomic number rises one at a time and electrons fill the same outer shell.
• Group — a vertical column; members share the same number of valence electrons, hence similar properties.

Five trends are decided by a tug-of-war: the nuclear charge pulling electrons in, versus the shielding of inner electrons and the number of shells pushing them out. Use the toggles on the right to compare radius, ionization energy and electronegativity across Period 3.

• Across a period → radius decreases. Nuclear charge rises but electrons enter the same shell, so the stronger pull draws the shell inward.
• Down a group ↓ radius increases. A new shell is added each period, so the outer electrons sit further out.

On the right the glowing spheres carry their real pm values: across Period 3 they collapse from Na (186 pm) to Cl (99 pm), then down Group IA they balloon outward — the two directions in one picture.

• Across a period → IE increases — smaller radius and higher nuclear charge hold electrons tightly.
• Down a group ↓ IE decreases — the valence electron is farther out and well shielded.

The overall climb is broken by two famous dips. Al sits below Mg because its lone 3p¹ electron is easier to pull off than Mg's filled 3s². S sits below P because S has a paired 3p electron whose extra repulsion makes it leave more easily than the half-filled 3p³ of phosphorus.

• Across a period → EN increases — a smaller, higher-charge atom pulls bonding electrons harder. F is the most electronegative element.
• Down a group ↓ EN decreases as atoms grow and shielding rises.

Electronegativity decides how a bond's electrons are shared. A big difference (e.g. Na–Cl) gives an ionic bond; a small one (e.g. C–H) gives a covalent bond. On the right, the periodic table is painted as a heat-map — the top-right corner near fluorine runs hottest, the bottom-left metals stay coolest.

s-block in brief

Group IA (ns¹) form M⁺ and are the most reactive metals (kept under kerosene): 2Na + 2H₂O → 2NaOH + H₂↑; flame colours Na yellow, K lilac. Group IIA (ns²) form M²⁺, are harder and less reactive; flame colours Ca brick-red, Sr crimson, Ba apple-green — the colours of fireworks.

p-block families

From boron (IIIA) through carbon, nitrogen, oxygen, the halogens to the noble gases, character grades from metals to non-metals. The first member of each group (B, C, N, O, F) is anomalous: small, very electronegative, with no available d-orbitals.

Hydrogen (1s¹) fits no group neatly — it resembles the alkali metals (one valence electron, forms H⁺, a reducing agent) and the halogens (needs one electron, forms H⁻, is diatomic H₂). So it is given a unique, anomalous position.

• Inert-pair effect — in heavy p-block elements the outer ns² pair resists bonding, so the lower (group−2) oxidation state becomes more stable down the group.

It explains why Pb²⁺ is more stable than Pb⁴⁺ and why Tl⁺ beats Tl³⁺. The cause is poor shielding by intervening d- and f-electrons, which holds the ns² pair tightly to the nucleus — the panel shows that pair refusing to leave.

• Diagonal relationship — some second-period elements resemble the element one place right and one period down more than their own group.

The classic pairs are Li & Mg, Be & Al and B & Si. The cause is a near-equal charge-to-radius ratio (polarising power): moving right raises charge while moving down raises size, so the two changes cancel along the diagonal.

The Contact process makes sulphuric acid in three stages: S→SO₂ (burn sulphur), SO₂→SO₃ (the reversible catalytic step), then SO₃→oleum→H₂SO₄. Conditions from Le Chatelier: ~450 °C (a compromise — low T favours SO₃ but is too slow), 1–2 atm (fewer gas moles forward), excess air and a V₂O₅ catalyst. Its biggest use is fertiliser; it is also the electrolyte in your car battery.

1. Periodic classification & the five trends (with reasons).
2. Radius, ionization energy & the Al/S dips; electronegativity.
3. s-block reactions & flame colours; p-block families & the oxide trend.
4. Hydrogen's position; inert-pair effect; diagonal relationship.
5. The Contact process: stages, conditions & uses of H₂SO₄.