Question 9
Lithium, sodium and potassium have largely similar chemical properties. Why?
The right answer is the one marked above.
Because each of them has exactly one electron in its last energy level. That is what puts all three in the same vertical column — group I.
They certainly do not have the same number of energy levels: lithium has 2, sodium 3 and potassium 4, which is why they are in three different periods. Being in the same group is about the last level only.