Periodic table and periodicity
Chemistry · WAEC and JAMB · SS2 and SS3
A topic about trends rather than facts. If you can explain WHY a trend exists, you can answer any version of the question.
What you need to know
- Groups are vertical columns sharing the same number of outer electrons and therefore similar chemistry.
- Periods are horizontal rows; across a period the number of protons rises while the shell stays the same.
- Atomic radius decreases across a period because greater nuclear charge pulls the same shell inward.
- Atomic radius increases down a group because a new shell is added each time.
- Ionisation energy increases across a period and decreases down a group.
- Metallic character decreases across a period and increases down a group.
- Group 1 alkali metals are highly reactive and reactivity increases downward.
- Group 7 halogens are reactive non-metals and reactivity decreases downward.
Key terms
- Ionisation energy
- The energy needed to remove one mole of electrons from one mole of gaseous atoms.
- Transition element
- An element with an incomplete d sub-shell; these form coloured ions and act as catalysts.
Worked example
Explain why sodium is more reactive than magnesium.
- Sodium has one outer electron, magnesium has two.
- Removing one electron needs less energy than removing two.
- Sodium also has a slightly larger radius, so its outer electron is held less tightly.
Answer: Sodium loses its single outer electron more easily, so it reacts more readily.
The mistake to avoid
Reactivity increases down Group 1 but decreases down Group 7. Candidates who learn one trend and assume the other follows it lose both marks.
In the exam
Trend questions want a reason, not just a direction. "Decreases across the period because nuclear charge increases while shielding stays the same" is the full answer.