We know that a neutral atom of bromine (br) is larger than a neutral atom of magnesium (mg) because a.) group numbers tells us how many electrons orbitals surround a nucleus, bromine is in group 7a while magnesium is in group 2a b.)period numbers tells us how many electron orbitals surround a nucleus, bromine is in period 4 while magnesium is in period 3 c.)bromine and magnesium have very different physical and chemical properties.

Based on the periodic table, atomic size increases from top to bottom within a group. This is because, electrons are added to new outer shells. These outer electrons are shielded by the inner electrons and therefore experience a lower nuclear attraction which results in an increase in the atomic size (radius).

Now, Mg precedes Br in the periodic table, i.e. Mg belongs to the 3rd period while Br belongs to the 4th. Thus, Br will have more orbitals that are occupied by electrons and hence a larger size.

Ans: B)

4d^55s^1 ( third choice)