The Foundations of Chemistry

EXAMPLE 5-10 Electron Configurations

Use Table 5-5 to determine the electron configurations of (a) magnesium, Mg; (b) germanium,

Ge; and (c) molybdenum, Mo.

Plan

We will use the electron configurations indicated in Table 5-5 for each group. Each period(row)

begins filling a new shell (new value of n). Elements to the right of the dorbital block have

the d orbitals in the (n
1) shell already filled. We often find it convenient to collect all sets

of orbitals with the same value of ntogether, to emphasize the number of electrons in the outer-

mostshell, that is, the shell with the highest value of n.

Solution

(a) Magnesium, Mg, is in Group IIA, which has the general configuration s^2 ; it is in Period

3 (third row). The last filled noble gas configuration is that of neon, or [Ne]. The electron

configuration of Mg is [Ne] 3s^2.

(b) Germanium, Ge, is in Group IVA, for which Table 5-5 shows the general configuration

s^2 p^2. It is in Period 4 (the fourth row), so we interpret this as 4s^24 p^2. The last filled noble gas

configuration is that of argon, Ar, accounting for 18 electrons. In addition, Ge lies beyond the

224 CHAPTER 5: The Structure of Atoms

11

Na

12

Mg

90

Th

91

Pa

92

U

93

Np

94

Pu

95

Am

96

Cm

58

Ce

59

Pr

60

Nd

61

Pm

62

Sm

63

Eu

64

Gd

19

K

20

Ca

21

Sc

22

Ti

23

V

24

Cr

25

Mn

26

Fe

27

Co

28

Ni

29

Cu

30

Zn

37

Rb

38

Sr

39

Y

40

Zr

41

Nb

42

Mo

43

Tc

44

Ru

45

Rh

46

Pd

47

Ag

48

Cd

49

In

50

Sn

51

Sb

52

Te

53

I

54

Xe

55

Cs

56

Ba

57

La

72

Hf

73

Ta

74

W

75

Re

76

Os

77

Ir

78

Pt

79

Au

80

Hg

81

Tl

82

Pb

83

Bi

84

Po

85

At

86

Rn

87

Fr

88

Ra

89

Ac

104

Rf

105

Db

106

Sg

107

Bh Hs Mt

108 109 110 111 112

2

He

5

B

6

C

7

N

8

O

9

F

10

Ne

13

Al

14

Si

15

P

16

S

17

Cl

18

Ar

1

H

3

Li

4

Be

66

Dy

67

Ho

68

Er

69

Tm

70

Yb

71

Lu

65

Tb

98

Cf

99

Es

100

Fm

101

Md

102

No

103

Lr

97

Bk

LANTHANIDE

SERIES

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)

IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA VIIIA

31

Ga

32

Ge

33

As

34

Se

35

Br

36

Kr

58

Ce

71

Lu

90

Th

103

Lr

111 112

f

orbital

block

d orbital block

d^2 s^2 d^3 s^2 d^5 s^1 d^5 s^2 d^6 s^2 d^7 s^2 d^8 s^2 d^10 s^1 d^10 s^2

s^2 p^1 s^2 p^2 s^2 p^3 s^2 p^4 s^2 p^5 s^2 p^6

p orbital block

d^4 s^1

d^4 s^2

d^7 s^1 d^8 s^1

d^9 s^1

d^10 s^0

ACTINIDE

SERIES

4 f subshell

being filled

5 f subshell

n = 7 being filled

n = 6

n = 7

n = 6

n = 5

n = 4

n = 3

n = 2

n = 1

d^1 s^2

GROUPS

s orbital block

s^1 s^2

s^2

TABLE 5-5 The s, p, d, and f Blocks of the Periodic Table*

*n is the principal quantum number. The d^1 s^2 , d^2 s^2 ,...designations represent knownconfigurations. They refer to (n
1)d

and ns orbitals. Several exceptions to the configurations indicated above each group are shown in gray.