How does atomic size evolve with respect to the Periodic Table?

Atomic size decreases across a Period, from left to right as we face the Table, BUT INCREASES down a Group.

We compare atomic neon and atomic chlorine, #Z=10(Ar)#, versus #Z=17(Cl)#. Because chlorine is third-row rather than second row, we must immediately conclude that chlorine is LARGER. But we are not finished.

As chemists, as physical scientists, we should seek out the data that inform our argument. This site gives an atomic radius of #42xx10^-12*m# for neon, and #79xx10^-12*m# for chlorine. The increase in radius on adding a new valence shell to the atom is clear.

• From Periodic table, we can see chlorine belongs to group no. 17 whereas neon belongs to group no. 18.

• For fluorine family:
  F
  Cl
  Br
  I
  At

• For noble gases family:
  He
  Ne
  Ar
  Kr
  Xe
  Rn

• Now , we know as we go down the group the no. of shells increases because electron add into new shell results in screening effect . Due to which the atomic size increases down the group.

• Also, as we go from left to right , the nuclear charge increases and electrons add into same shell, results in no screening effect. Due to increase in nuclear charge , the atomic size decreases from left to right.

So, it is clear that atomic size of chlorine is greater than neon.