The Periodic Table: IGCSE Chemistry Exam Guide

The periodic table is both a topic in its own right and a tool that underlies every other topic in 0620. Questions range from simple recall (name a noble gas, state the trend in Group I reactivity) to extended explanations requiring atomic structure reasoning. The periodic table is provided in the exam, but the trends and explanations are not — those must come from your preparation. The full topic is covered under the periodic table.

The three question categories

1. Group trends (the bulk of the marks)

The exam repeatedly tests two groups: Group I (alkali metals) and Group VII (halogens). For each, you need the trend and the reason.

Group I — alkali metals:

• Reactivity increases down the group.
• Melting and boiling points decrease down the group.
• Reactions with water become more vigorous: lithium fizzes gently, sodium melts into a ball and fizzes vigorously, potassium ignites with a lilac flame.
• Products with water: metal hydroxide + hydrogen. 2Na + 2H2O -> 2NaOH + H2.

Why reactivity increases: Going down the group, each element has more electron shells. The outer electron is further from the nucleus and more shielded by inner electrons, so it is lost more easily. Easier loss of the outer electron means faster, more vigorous reaction.

Group VII — halogens:

• Reactivity decreases down the group.
• Melting and boiling points increase down the group (F2 and Cl2 are gases, Br2 is a liquid, I2 is a solid at room temperature).
• Colour darkens: pale yellow (F2), yellow-green (Cl2), red-brown (Br2), grey-black solid / purple vapour (I2).

Why reactivity decreases: Going down the group, each element has more electron shells. The outer shell is further from the nucleus and more shielded, so attracting an extra electron into the outer shell is harder. Harder gain of an electron means lower reactivity.

2. Displacement reactions (halogens)

A more reactive halogen displaces a less reactive one from a solution of its salt:

Cl2 + 2KBr -> 2KCl + Br2

Chlorine displaces bromide and iodide. Bromine displaces iodide only. Iodine displaces nothing.

The exam tests this with a table of observations: you add halogen water to halide solutions and record the colour change. Chlorine water added to potassium iodide solution turns the solution brown (iodine is released).

3. Noble gases

Noble gases (Group 0/VIII) are unreactive because they have a full outer electron shell — a stable electronic configuration. Their uses follow from their properties: helium in balloons (low density, non-flammable), neon in advertising signs (glows when electricity passes through), argon in welding and light bulbs (inert atmosphere).

The explain question: atomic structure reasoning

The highest-value periodic table questions on Paper 4 ask you to explain a trend. The mark scheme always wants the same three elements:

1. Name the trend. “Reactivity increases down Group I.”
2. State the structural change. “Going down the group, the number of electron shells increases / the outer electron is further from the nucleus.”
3. Link to the property. “So the outer electron is more easily lost / less energy is needed to remove it, making the element more reactive.”

Missing any one of these three elements loses a mark. The most common error is stating the trend without explaining it, or explaining it without naming the structural change. Phrasing advice for these questions is in answering describe and explain questions.

Across a period: the patterns Paper 2 tests

Moving left to right across Period 3 (Na to Ar):

• Elements change from metals to non-metals.
• Oxides change from basic (Na2O, MgO) to amphoteric (Al2O3) to acidic (SiO2, P4O10, SO3).
• Electrical conductivity is high on the left (metallic bonding), low on the right (covalent molecules), with silicon as a semiconductor.

Paper 2 MCQs often present a property and ask which element in a period fits. Knowing the metal-to-non-metal transition answers most of these.

Transition metals: what the syllabus actually requires

The syllabus requires four properties of transition metals, and the exam rarely goes beyond them:

1. They form coloured compounds (copper sulfate is blue, iron(III) oxide is red-brown).
2. They have variable oxidation states (iron can be Fe2+ or Fe3+).
3. They or their compounds act as catalysts (iron in the Haber process, manganese(IV) oxide in decomposing hydrogen peroxide).
4. They have high melting points and high density compared to Group I metals.

Do not confuse these with Group I properties. The exam sometimes asks you to compare a transition metal (iron) with a Group I metal (sodium) — the answer involves reactivity, hardness, melting point and density.

Worked exam question

Q (Paper 4): Explain why chlorine is more reactive than bromine. (3 marks)

Model answer: Both chlorine and bromine are in Group VII and react by gaining one electron to form a negative ion (1). Chlorine has fewer electron shells than bromine, so the outer shell is closer to the nucleus / less shielded (1). Therefore chlorine attracts an incoming electron more strongly, gaining it more easily, making it more reactive (1).

Three marks for three ideas: how halogens react, the structural difference, and the link to reactivity. Writing “chlorine is higher up the group so it is more reactive” earns zero because it restates the trend without explaining it.

If periodic table trends are a weak area, the fix is usually about learning to structure the explanation rather than learning more content. A trial lesson can identify whether the difficulty is in the atomic structure reasoning or in the exam phrasing, and target the right one.