Bonding and structure, Year 10: giant covalent structures and metals

diamond; graphite; silicon dioxide

提示Two are forms of carbon; the third is the main substance in sand.

为什么In each of them every atom is covalently bonded to its neighbours throughout the solid, so there are no separate molecules. These are the three examples named in the specification.

Many strong covalent bonds must be broken

提示There are no separate molecules to slide apart, so ask what has to give way instead.

为什么Every atom is held to its neighbours by covalent bonds, all the way through the solid. Before the atoms can move, large numbers of these strong bonds have to be overcome, which takes a great deal of energy.

Silicon dioxide is a giant structure; carbon dioxide is small molecules

提示Similar names and formulae hide a big difference in how far the bonding extends.

为什么To melt silicon dioxide, strong covalent bonds throughout the structure have to be broken. Carbon dioxide molecules only have to be separated from one another, against weak intermolecular forces.

A giant covalent structure

提示Its name starts with a word for something enormous.

为什么Because the bonding carries on in every direction, a whole crystal is in effect one piece. Such solids have very high melting points.

silica

提示It is the name of the element with its ending changed to "-a".

为什么Silicon dioxide has a giant covalent structure of silicon and oxygen atoms. Like diamond, it is hard and has a very high melting point.

Its atoms are arranged in layers that can slide over each other

提示Think of a pack of playing cards being pushed sideways.

为什么In a pure metal all the atoms are the same size, so they pack in regular layers. A force can push one layer along over the next, and the delocalised electrons keep holding the structure together in its new shape.

Atoms of different sizes distort the layers, so they cannot slide easily

提示Imagine a few marbles of the wrong size dropped into neat rows of identical ones.

为什么The added atoms are bigger or smaller than the atoms of the main metal. They break up the regular layers, so a greater force is needed to make one layer move over another.

An alloy

提示Brass and bronze are two examples.

为什么Pure metals are often too soft for the job. Mixing in atoms of another element distorts the regular layers and makes the material harder. The specification describes alloys as metals mixed with other metals; steel, which is iron with a little carbon, shows that the added element is not always a metal.

Strong metallic bonds throughout the giant structure take a lot of energy to break

提示Think about what holds every atom in place, and how far that extends.

为什么The sharing of delocalised electrons bonds every atom strongly into the whole structure. A large amount of energy is needed to overcome this bonding, so most metals are solids with high melting points.

soft

提示It is the opposite of the property that alloying is meant to improve.

为什么The layers of atoms in a pure metal slide over each other easily, so the metal bends and wears too readily. Adding atoms of a different size makes sliding more difficult.

delocalised

提示The word describes electrons that belong to no single atom.

为什么An electric current is a flow of charge. In a metal the outer electrons are free to move through the whole structure, so when a potential difference is applied they drift along and carry the charge.

Thermal energy

提示It is why a metal spoon left in hot soup soon burns your fingers.

为什么The free electrons gain energy at the hot end and move quickly through the structure, passing energy on as they go. This makes metals much better conductors of thermal energy than most non-metals.

Electrons in copper; ions in sodium chloride

提示One has charged particles that are already free in the solid; the other has to be broken up first.

为什么A metal has delocalised electrons that can move even in the solid. An ionic compound has no free electrons; its charge carriers are its ions, and they can move only when the lattice has been melted or dissolved.