You need to appreciate that this is also a form of ionic bonding but instead of

You need to appreciate that this is also a form of

This preview shows page 39 - 41 out of 79 pages.

You need to appreciate that this is also a form of ionic bonding but instead of pairs of atoms bonding, the electrons in the outer shells are shared amongst all the atoms in a lattice with all the atoms positively charged. These atoms are attracted to the negatively charged 'cloud' of delocalized electrons. As the name implies, a metallic bond is only formed between metallic atoms. The metal ions do not form molecules but they form large, three-dimensional, geometrical lattice structures. It is this form of ionic bonding together with the lattice structure that gives metals their special properties: o Metals can be bent and formed into complex shapes in the solid condition. o Metals readily conduct heat and electricity. The movement of the free electrons means that metallic bonded materials have good thermal and electrical conduction. o Metals have shiny surfaces when cut. Let us look at this in more detail. Fig. 5(a) below shows how the three-dimensional lattice would appear if we could see it under a powerful microscope. Fig. 5(b) shows a portion of the bonding in greater detail. The metal atoms give up one or more electrons from their valence shells to become positively charged metal ions. The electrons that have escaped from the atoms are free to move about between the metal ions.
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~ Page 40 of 79 ~ Now let us consider Fig. 6 below which, for simplicity shows forces acting between two metal ions and a free electron in a metallic bond. o The positive ions (denoted by circles containing plus signs) have like charges so they repel each other and keep apart. o The positive ions are attracted to the free electrons (denoted by the circle with the minus sign). This free electron is delocalized, meaning it is free to move about. Remember: like charges repel each other; unlike charges attract each other. o The attractive forces between the positive ions and the electrons are greater than the repulsive forces between the ions themselves. This is because the ions are nearer to the electrons than they are to each other. o The forces of attraction between the positively charged ions and the negatively charged electrons hold the ions in position in the lattice. o Remember: the lattice is three-dimensional. Let us now see how this arrangement of the ions and electrons enables metals to be bent and formed in the solid state. Fig. 8 below shows the effect of an external mechanical force. As we can see in Fig. 7(a) the metal ions are free to form bonds with any adjacent electrons in the electron ‘cloud’ that exists between the ions. Therefore, when an external mechanical force is applied to the metal as shown (in Fig. 7(b)), the ‘layers’ of ions in the lattice are free to slide over each other. Sliding can occur without the lattice being broken. When an ion moves from its old position in Fig. 7(a) to a new position in Fig. 7(b) above, it is free to form a new bond with any conveniently adjacent electrons.
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