If there are two elements in the same group, what is the same about them? Atomic mass, or they have the same number of valence electrons? (There are other options but i seriously doubt that they are correct)

Same number of valence electrons

Explanation:

The way that the periodic table is designed is so that all elements in the same group will have the same number of valence electrons. This is because elements can have the same number of valence electrons despite having different totals of electrons, because the valence electrons can reside in any valence shell.

Atomic mass changes for every element, and the elements get heavier the further down the table you go, so it is not atomic mass.

Hope this helped!