Patterns in Groups 1, 2, 17 and 18

Sodium is a Group 1 alkali metal. A defining property of alkali metals is that they are highly reactive — they react readily with oxygen and moisture in the surrounding environment.

Because sodium reacts with both oxygen (from the air) and water (from atmospheric moisture), storing it exposed to air would cause it to react and be destroyed. Oil acts as a barrier, keeping air and moisture away.

The correct answer is that sodium reacts with oxygen and moisture in the air. This is a direct consequence of its very high reactivity as a Group 1 alkali metal.

The question asks why sodium is stored under oil — we need to link this to sodium's chemical properties.

The option about sodium dissolving is not quite right — sodium reacts violently with water rather than simply dissolving. This is a chemical reaction, not simple dissolving, and oil is used to prevent air contact, not just water contact.

While sodium has a relatively low melting point for a metal, it does not melt at room temperature. This option is factually incorrect.

The storage in oil is not related to toxicity concerns — it is about chemical reactivity, not poison prevention.

The remaining option — sodium reacts with oxygen and moisture in the air — is correct. This explains why oil storage is necessary as a protective barrier.

Lithium, sodium, and potassium are all Group 1 alkali metals. They appear in that order going down the group: lithium is at the top, sodium is below it, and potassium is further down.

The reactions become more vigorous going down the group: lithium fizzes gently → sodium reacts vigorously → potassium ignites. This is a clear increase in reactivity.

The observations demonstrate that reactivity increases going down Group 1. This is one of the most important trends in the Periodic Table.

The student is observing how vigorously each element reacts with water — this is a measure of reactivity, not melting point or density.

The observations describe reactions with water, not melting behaviour. Also, melting point actually decreases going down Group 1, so this is doubly incorrect.

Again, the observations are about reactivity, not density. Additionally, density generally increases going down Group 1, not decreases.

If reactivity decreased going down, potassium (lowest of the three) would react most gently — but it reacts most violently. This contradicts the observations.

Reactivity increases going down Group 1 matches the observations perfectly — each element lower in the group reacts more vigorously than the one above it.