CHM 501

14. Consider the melting point data for the natural forms of the second row elements. Calculate Z* using Slater's rules for the outermost electron for each of these elements. Does Z* give a reasonable accounting of the trend in the melting point? Why or why not? Should melting point be considered a periodic property of atoms? Why or why not?

Element

Melting Point (°C)

Na

98

Mg

650

Al

660

Si

1414

P

44

S

115

Cl

–101.5

Ar

–189

Answer

First, find the electron configuration to calculate Z*:

Element

Melting Point (°C)

Electron Configuration

Z*

Na

98

(1s)2(2s2p)8(3s3p)1

Z* = 11 – [2(1.00) + 8(0.85)] = 2.20

Mg

650

(1s)2(2s2p)8(3s3p)2

Z* = 12 – [2(1.00) + 8(0.85) + 1(0.35)] = 2.85

Al

660

(1s)2(2s2p)8(3s3p)3

Z* = 13 – [2(1.00) + 8(0.85) + 2(0.35)] = 3.50

Si

1414

(1s)2(2s2p)8(3s3p)4

Z* = 14 – [2(1.00) + 8(0.85) + 3(0.35)] = 4.15

P

44

(1s)2(2s2p)8(3s3p)5

Z* = 15 – [2(1.00) + 8(0.85) + 4(0.35)] = 4.80

S

115

(1s)2(2s2p)8(3s3p)6

Z* = 16 – [2(1.00) + 8(0.85) + 5(0.35)] = 5.45

Cl

–101.5

(1s)2(2s2p)8(3s3p)7

Z* = 17 – [2(1.00) + 8(0.85) + 6(0.35)] = 6.10

Ar

–189

(1s)2(2s2p)8(3s3p)8

Z* = 18 – [2(1.00) + 8(0.85) + 7(0.35)] = 6.75

There is no correlation between the melting points and Z*. This is not surprising because Z* is associated with isolated atoms (i.e., gas phase), not the solid or liquid phases. The melting point of a substance is determined by intermolecular interactions between atoms or molecules in the condensed phases. Thus, melting point might be considered a periodic property, but requires an understanding of all of the bonding attractions in place for each substance.