Why do elements within a group have similar chemical properties?

Elements in the same group have similar properties because they have the same number of valence electrons.

Explanation:

What are valence electrons?

Valence electrons are the electrons on the outermost shell. For example take the element carbon or it’s symbol C. It’s has 4 valence electrons.

How do you find valence electrons?

Several ways, but one way is to draw Lewis structures, also known as Lewis dot diagrams.

Hope this covered the basics of your question :) Though there are more thorough answers for this question.