Chemistry Foundations

4.5.1 - Introduction to Metallic Bonding

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Introduction to Metallic Bonding

So far we have learnt about two types of bonding: covalent bonding and ionic bonding. We will now learn about the third type of bonding, which is called metallic bonding.

Metallic bonding happens when many metal atoms become metal ions together

Metal atoms can become ions by losing electrons. For example, the diagram below shows a magnesium atom losing two electrons to become a magnesium ion:

On the left a magnesium ion is shown. It has three shells, with two electrons in the outer shell. An arrow from this atom point to the right. Another smaller arrow comes off the main arrow, showing two electrons leaving. The main arrow points to a magnesium ion with a charge of 2+.

The formation of a magnesium ion from a magnesium atom.

Metallic bonding happens when many metal atoms come together and form ions at the same time. For example, if lots of magnesium atoms come together at the same time and form magnesium ions (Mg2+) they become metallically bonded together.

The electrons which leave the atoms become delocalised

The electrons which leave the atoms become what are called delocalised electrons (or free electrons). This means that they are able to move around freely between the ions, rather than being part of any ion.

Note that it is only the electrons which are lost that become delocalised - the other electrons are still within their individual ions.

The electrons released as the metal atoms become metal ions form what is described as a sea of delocalised electrons. This means that the metal ions are surrounded by delocalised electrons, flowing past them in random directions.

The metal ions themselves are not free to move around like the delocalised electrons. They are arranged in rows and stay in their fixed positions while the delocalised electrons flow around them.

Diagram showing metallically bonded magnesium ions. There are 9 ions arranged in a 3 by 3 grid. Between the ions there are delocalised electrons (there 18 delocalised electrons).

Metallically bonded magnesium ions

The delocalised electrons hold the structure together

The metal ions are positively charged (they are cations). The delocalised electrons are negatively charged. Therefore, there are forces of electrostatic attraction between the metal ions and the delocalised electrons.

As the delocalised electrons move around, they attract the metal ions which are near them. This means that each metal ion constantly has delocalised electrons around it attracting it. It is these attractions between the metal ions and the delocalised electrons which hold the whole structure together.

The sea of delocalised electrons is like a glue which holds the metal ions together. Without it, the metal ions would repel each other and fly apart.

There are no individual metallic bonds

With covalent and ionic bonding we can point to individual bonds. Within a molecule, there are individual covalent bonds between pairs of atoms, and within an ionic compound, there are individual covalent bonds between pairs of ions.

However, with metallic bonding there are no individual metallic bonds. The whole structure is held together by the total effect of all of the attractions between the metal ions and the sea of delocalised electrons, and this overall effect is called metallic bonding. Therefore, while it is common to talk about metallic bonding, people do not usually talk about metallic bonds.

Flashcards

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What is metallic bonding?

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