I’m doing chemistry revision on the group one elements. My teachers taught me that the reactivity increases further down the group. When I looked on the internet it states that Potassium is more reactive than Sodium. I don’t understand why or am I just being dumb?

Yes you are correct and here’s why you are:

Thus, in potassium, the outermost electron is better shielded from the attractive force of the nucleus. It follows, therefore, that this outermost electron is more easily lost than it is in sodium, so potassium can be converted to an ionic form more readily than sodium. Hence, potassium is more reactive than sodium.

We can’t you if you only tell us that you have to show us what the answers are

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