Fill in the blanksQuestion 1 · Recall
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By the end you can place an unfamiliar element in its group and period, then predict its characteristic physical and chemical properties — and give a reason for each prediction.
Periodicity is the repeating pattern of properties across the Periodic Table. Elements in the same group have the same number of outer-shell electrons, so they form the same kinds of ions and compounds.
That is why you can say sensible things about an element you have never seen in a bottle. You do not memorise its properties; you read them off its position.
There are two directions to think in:
Chemistry questions usually say predict. They want a property plus the trend it came from, not a guess.
| Property (down a group) | Group 1 and Group 2 metals | Group 17 non-metals |
|---|---|---|
| Atomic radius | increases | increases |
| First ionisation energy | decreases | decreases |
| Attraction for the outer electrons | weaker | weaker |
| Reactivity | increases (electrons lost more easily) | decreases (electrons gained less easily) |
| Melting point of the element | Group 1 falls; Group 2 has no simple trend | rises |
| Bond strength in the element | — | falls from Cl₂ to I₂ (F₂ is the odd one out) |
| Oxidising power | — | decreases |
| Thermal stability of the hydride | — | decreases |
Two rows surprise people, so read them again: metals get more reactive down a group, non-metals get less.
Almost every trend in that table comes from a single change: the outer electrons are further from the nucleus and more shielded as you go down.
Weaker attraction means:
Analogy: passing a bucket along a row of people. The further it has to travel and the more people standing in between, the weaker the grip at the far end.
Where the analogy breaks: shielding is not people blocking a bucket. Inner electrons repel the outer electron, cancelling part of the nucleus's pull. The nuclear charge really does rise down a group — distance and shielding simply win.