Chemistry, Third edition

72 5 · MORE ABOUT BONDING

Metallic bonding

As we have seen, metals have very distinctive properties. Any successful model of the

bonding in metals must account for these properties. Metals have a crystallinestruc-

ture – a crystal can be regarded as a rigid substance in which particles are arranged in a

repeating pattern. We can regard a metallic crystal as consisting of a lattice of positive

ions, surrounded by a cloud of electrons which are free to move. The electrostatic

attraction between the positive ions and the negatively charged cloud of electron

density ‘glues’ the whole structure together. Each atom of the metal has given up its

outer (i.e. valence) electrons to become a positive ion. These valence electrons form

part of the electron cloud and, because they are free to move, they no longer ‘belong’ to

the particular atom from which they have been released. This is illustrated in Fig. 5.6.

The model explains the properties of metals:

1.Metals are shiny because the free electrons absorb light that falls on

the metal and then re-emit the light back. Most of the light that falls

on the metal is re-emitted, so the metal appears shiny. This is why

metals are used in mirrors. Most metals are a silvery colour – the

red/gold colours of copper and gold occur because they absorb some

wavelengths of visible light more than others.

2.The free electrons in the structure allow metals to conduct electricity

5.5

Polar and non-polar molecules

1. Which of the following molecules have an
   overall dipole moment?
   (i) CS 2 (ii)NH 3 (iii)SiCl 4
   (iv)H 2 S (v)CHCl 3.


2. An electron density map for hydrogen fluoride,
   HF, is shown below in Fig. 5.4.
   (i)iDoes the molecule have a dipole?
   (ii)In which direction is the dipole?

Exercise 5G

Fig. 5.4Electron density map for

hydrogen fluoride. (Note that the

electron density increases going from

the outer to the inner contours.)

BOX 5.2

Demonstrating the polarity of

molecules

You can easily demonstrate that water

molecules are polar by rubbing a plastic ruler

with a dry duster, then bringing it close to a

narrow stream of water flowing from a burette

(do not allow the ruler to touch the water!).

The stream of water will bend because the

polar water molecules are attracted to the

charged rod. Do the same experiment with

hexane, instead of water. Hexane molecules

are non-polar and the stream of hexane

should not be deflected (Fig. 5.5).

Fig. 5.5Demonstration of the polarity of

hexane and water molecules.

Fig. 5.6Metallic bonding.