We know that a neutral atom of bromine is larger than a neutral atom of magnesium because:

a. group number tells us how many electron orbitals surround a nucleus; bromine is in group 7a while magnesium is in group 2a.

b. period number 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.

b. Period number tells us how many electron orbitals surround a nucleus; bromine is in period 4 while magnesium is in period 3.

Explanation:

As you go down the Periodic Table from one Period to the next, you are going to the next electron shell that is further out.

The valence electrons of bromine are in a larger shell than those in magnesium.

a. Wrong. Atomic size decreases as you go from left to right in a Period.

c. Wrong. The fact that two elements have different physical and chemical properties tells you nothing about their relative sizes.