Question

From the given elements Na P Ca and Br which one has the smallest atomic size ?

Answer 1

Answer: Phosphorous has the smallest atomic size.

[ NOTE: I am assuming that you are in Gr 11 so this is the advanced approach. Do msg back if thats not the case ]

Let's look at the atomic numbers and electronic configuration of the elements:

1) Na ->  Z = 11 [Ne] 3s1

2) P -> Z = 15 [Ne] 3s2 3p3

3) Ca -> Z = 20 [Ar] 4s2

4) Br -> Z = 35 [Ar] 4s2 4p5

Now since we know the atomic size increases with increase in n (Principal Quantum number),which means more shells get added to the atom (down a group), thus increasing atomic radii.

Also, as we move across a period (Left to Right), more number of electrons get added, thus increasing the attraction by the positively charged nucleus and decreasing radii by pulling the shells closer.

And according to the electronic configurations, Phosphorus  satisfies the criterion of smallest n in the list (n = 3) with p-orbital electrons, increasing attraction and decreasing radii.