3. why do elements in a group have similar properties?

Elements belonging to the same periodic group reflects similar chemical properties because of the same number of valence electrons residing in their outermost orbits that reflect the same energy level.

Explanation:

Elements of a periodic group reflecting similar properties

We often observe that a specific group of elements reflect common characteristics towards other elements. This is because these elements have their valence electrons in the same energy levels.

This is the prime factor according to which elements are kept in a specific group in the periodic table. All elements in the group have the same number of valence electrons. These elements may have different number of electrons over all. Elements belonging to the same group shows the same chemical properties such as bonding with other elements, reacting to other substances etc.

b would be the answer