The Foundations of Chemistry

Figure 6-2 A plot of first

ionization energies for the first 38

elements versus atomic number.

The noble gases have very high first

ionization energies, and the IA

metals have low first ionization

energies. Note the similarities in the

variations for the Period 2 elements,

3 through 10, to those for the

Period 3 elements, 11 through 18,

as well as for the later A group

elements. Variations for B group

elements are not nearly so

pronounced as those for A group

elements.

6-3 Ionization Energy 243

For calcium, for example, the first ionization energy, IE 1 , is 590 kJ/mol:

Ca(g)590 kJ88nCa(g)e

The second ionization energy (IE 2 )is the amount of energy required to remove the
second electron. For calcium, it may be represented as

Ca(g)1145 kJ88nCa^2 (g)e

For a given element, IE 2 is always greater than IE 1 because it is always more difficult to
remove an electron from a positively charged ion than from the corresponding neutral
atom. Table 6-1 gives first ionization energies.
Ionization energies measure how tightly electrons are bound to atoms. Ionization always
requires energy to remove an electron from the attractive force of the nucleus. Low ioniza-
tion energies indicate ease of removal of electrons, and hence ease of positive ion (cation)
formation. Figure 6-2 shows a plot of first ionization energy versus atomic number for
several elements.

H

1312

Li

520

Na

496

K

419

Rb

403

Cs

377

Be

899

Mg

738

Ca

599

Sr

550

Ba

503

Sc

631

Y

617

La

538

Ti

658

Zr

661

Hf

681

V

650

Nb

664

Ta

761

Cr

652

Mo

685

W

770

Mn

717

Tc

702

Re

760

Fe

759

Ru

711

Os

840

Co

758

Rh

720

Ir

880

Ni

757

Pd

804

Pt

870

Cu

745

Ag

731

Au

890

Zn

906

Cd

868

Hg

1007

B

801

Al

578

Ga

579

In

558

Tl

589

C

1086

Si

786

Ge

762

Sn

709

Pb

715

N

1402

P

1012

As

947

Sb

834

Bi

703

O

1314

S

1000

Se

941

Te

869

Po

812

F

1681

Cl

1251

Br

1140

I

1008

At

890

He

2372

Ne

2081

Ar

1521

Kr

1351

Xe

1170

Rn

1037

TABLE 6-1 First Ionization Energies (kJ/mol of atoms) of Some Elements

2500

2000

1500

1000

500

0

5 1015 20 253035

Atomic number

First ionization energy (kJ/mol)

Period 2 Period 3 Period 4

H

He

Li

Be

B

C

N

O

F

Ne

Na

Mg

Al

Si

P

S

Cl

Ar

K

Ca

Sc

Ti

V

Cr

Mn

Fe

CoCu

Zn

Ga

Ge

As

Br

Kr

Sr

Rb

Ni Se