Re: What is increasing atomic radii?

The atomic radius (radii is plural) is defined as one-half the distance between the nuclei (the center) of identical atoms that are bonded together [MODERN CHEMISTRY, p. 140, Holt, Rinehart and Winston, 2002].

Increasing atomic radii refers to the size of atomic radii getting larger. In general, the atomic radii of the main-group elements (groups 1, 2, and 13-18) increase down a group and decrease across a period. Take a look at the Group 1 elements from Li to Fr. The atomic radius of Li is 152 pm (picometers), Na 186 pm, K 227 pm, Rb 248 pm, Cs 265 pm, and Fr 270 pm. They have shown increasing atomic radii values from 152 pm to 270 pm. Now look at the values along period 3 from Na to Ar. Na has a value of 186 pm, Mg 160 pm, Al 143 pm, Si 118 pm, P 110 pm, S 103 pm, Cl 100 pm, and Ar 98 pm. Along period 3 then there is a decrease in atomic radius among the main-group elements. [MODERN CHEMISTRY, p. 141]

I hope this has helped.

Sincerely,
Charles Riner
Calvary Day School
Savannah, GA

Moderator's Note: Example -- Hydrogen gas (H2), roughly approximated

                            ---      ---
                          /     \  /     \
                         |   H   ||   H   |
                          \     /  \     /
                            ---      ---

Distance between nuclei      ----------    (10 "spaces")

Atomic radius                  -----       (5 spaces, or 1/2 above distance)