What is the correct relationship of these elements regarding their ionization energy?

Sr<Ca<Be

Explanation:

Br, Ca, and Sr are all elements of same group as they all have 2 valence electrons. As we move down in a group, a new shell is being added that increases the size of the atom. As the size of an atom increases, it is easy to remove the electron from it's outer most shell in it's gaseous state. Since, Be is above Ca and Ca is above Sr. The size increases in the order, Be < Ca < Sr. It means Be is the smallest one of them and so it's hard to remove from it's outer most shell and hence it's ionization energy is higher.

Sr is largest one in these three elements and so it's easy to remove outer most electron from it that means the ionization energy is least. Ca is between Be and Sr.

So, an increasing order of ionization energy is Sr<Ca<Be

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