A full trend explanation has three links: the number of shells or protons, the effect on attraction to the outer electron, and the effect on behaviour. Drop any link and the marks drop with it.
This lesson is part of the periodic table section. It assumes you can already read group and period from electron arrangement.
What is the three-part template?
Use this order every time.
- State the structural difference: more shells, or more protons.
- State what that does to the pull of the nucleus on the outer electron.
- State the result: easier or harder to lose or gain an electron.
The template works for any group or period you meet.
Worked example: why is potassium more reactive than sodium?
Sodium is 2.8.1, with three shells. Potassium is 2.8.8.1, with four shells.
The outer electron of potassium is further from the nucleus, and there are more inner shells between it and the nucleus. So the attraction to the nucleus is weaker. Potassium loses its outer electron more easily, which means it reacts more vigorously.
Notice that the answer never says “potassium has more electrons”. That fact is true, but it does not explain reactivity.
Worked example: why is chlorine smaller than sodium?
Sodium is 2.8.1 and chlorine is 2.8.7. Both have three shells, so the shell count is equal.
Chlorine has 17 protons and sodium has 11. The stronger nuclear charge pulls the outer electrons in more tightly, so chlorine’s atom is smaller.
| Change | Shells | Protons | Effect on size |
|---|---|---|---|
| Down a group | Increase | Increase | Larger, because distance matters more |
| Across a period | Same | Increase | Smaller, because attraction increases |
The mistake that costs marks
The common slip is to give a trend instead of a reason, or to cite the wrong structural cause. Two answers to avoid:
- “Potassium is more reactive because it has more electrons” names a fact that does not link to reactivity.
- “Down the group, atoms get bigger” describes the trend without explaining it.
For halogens, the direction flips. Reactivity falls down group 17 because the atom needs to gain an electron, and that electron is held less strongly when the shell is farther away. Use the atomic structure and periodic position practice tool to try similar comparisons.
Check yourself
Explain why lithium is less reactive than potassium.
Answer
Lithium (2.1) has two shells, and potassium (2.8.8.1) has four. The outer electron in lithium is closer to the nucleus, with fewer inner shells between them, so it is held more strongly.
It is therefore harder for lithium to lose its outer electron, so lithium is less reactive.
What to study next
Next, test these explanations against experiment results in comparing selected groups using evidence. Then try the periodic table practice set.
If you would like a teacher to mark your written explanations, see online one-to-one Chemistry tuition.