The Foundations of Chemistry

Within a family (vertical group on the periodic table) of representative elements,

atomic radii increase from top to bottom as electrons are added to shells farther

from the nucleus.

As we move acrossthe periodic table, atoms become smaller due to increasing effective

nuclear charges. Consider the elements B (Z5, 1s^22 s^22 p^1 ) to F (Z9, 1s^22 s^22 p^5 ). In B

there are two electrons in a noble gas configuration, 1s^2 , and three electrons in the second

shell, 2s^22 p^1. The two electrons in the noble gas configuration fairly effectively screen out

the effect of two protons in the nucleus. So the electrons in the second shell of B “feel”

a greater effective nuclear charge than do those of Be. By similar arguments, we see that

in carbon (Z6, 1s^22 s^22 p^2 ) the electrons in the second shell “feel” an effective nuclear

charge greater than those of B. So we expect C atoms to be smaller than B atoms, and

they are. In nitrogen (Z7, 1s^22 s^22 p^3 ), the electrons in the second shell “feel” an even

greater effective nuclear charge, and so N atoms are smaller than C atoms.

As we move from left to right across a periodin the periodic table, atomic radii of

representative elements decreaseas a proton is added to the nucleus and an electron

is added to a particular shell.

For the transition elements, the variations are not so regular because electrons are being

added to an inner shell. All transition elements have smaller radii than the preceding

Group IA and IIA elements in the same period.

EXAMPLE 6-1 Trends in Atomic Radii

Arrange the following elements in order of increasing atomic radii. Justify your order.

Cs, F, K, Cl

Plan

Both K and Cs are Group IA metals, whereas F and Cl are halogens (VIIA nonmetals). Figure

6-1 shows that atomic radii increase as we descend a group, so KCs and FCl. Atomic

radii decrease from left to right.

Solution

The order of increasing atomic radii is FClKCs.

You should now work Exercise 18.

IONIZATION ENERGY

The first ionization energy (IE 1 ),also called first ionization potential,is

the minimum amount of energy required to remove the most loosely bound elec-

tron from an isolated gaseous atom to form an ion with a 1charge.

6-3

General trends in atomic radii of A
group elements with position in the
periodic table.

242 CHAPTER 6: Chemical Periodicity

Atomic radii

Decrease

Increase