Bonding and structure, Year 10: ionic compounds, small molecules and polymers

A lot of energy is needed to break its many strong ionic bonds

提示Think about how each particle in the lattice is held, and by what number of neighbours.

为什么In the giant ionic lattice every ion is strongly attracted to the oppositely charged ions all around it. All of these attractions have to be overcome before the ions can move past one another, and that takes a large amount of energy.

Its ions are fixed in position and cannot move

提示To carry a current, charged particles have to be able to travel.

为什么The solid is full of charged particles, but each one is held in place in the lattice. Charge can flow only when the ions are free to move, which happens when the compound is melted or dissolved in water.

molten; dissolved

提示Both are ways of breaking up the rigid lattice so that the charged particles can travel.

为什么Heating an ionic solid until it becomes a liquid, or stirring it into water, frees the ions from their fixed positions. Once they can move, charge can flow.

Ions

提示Not the particles that carry current in a copper wire: here it is the charged sodium and chloride particles themselves.

为什么In the liquid, the positive sodium ions and negative chloride ions are free to move, and their movement is the flow of charge.

Ionic bonding

提示Charged particles must be present, but locked in place until the solid is broken up.

为什么A high melting point shows strong forces throughout a giant structure. Conducting only when dissolved (or molten) shows that the charge carriers are ions that are fixed in the solid. Together these point to a giant ionic lattice.

The weak forces between the molecules

提示Nothing inside each H2O unit changes; look at what holds one unit near the next.

为什么Boiling separates whole molecules from one another. The strong covalent bonds inside each molecule are not broken, which is why little energy is needed and why substances made of small molecules have low boiling points.

Intermolecular forces

提示The prefix is the one in "international": between nations.

为什么These attractions are much weaker than the covalent bonds inside a molecule. They are what has to be overcome when a substance made of molecules melts or boils.

charge

提示It is what an ion has and a neutral particle lacks.

为什么An electric current is a flow of charged particles. A molecule is neutral overall, and it has no delocalised electrons, so even when the molecules are free to move there is nothing to carry a current.

Its bigger molecules have stronger intermolecular forces

提示Compare the two formulae and ask which particle has more surface to attract its neighbours.

为什么The forces between molecules increase with the size of the molecules. More energy is then needed to separate them, so the melting and boiling points rise.

Gas or liquid

提示With so little holding one molecule to the next, they are rarely locked rigidly in place.

为什么The forces between small molecules are weak, so their melting and boiling points are low. Many have already melted, or even boiled, by the time they reach room temperature: oxygen and methane are gases, water is a liquid.

The intermolecular forces between the very large polymer molecules are relatively strong

提示Longer chains have far more places where one chain can attract the next.

为什么The attraction between molecules grows with their size, and polymer molecules are enormous. The total attraction between neighbouring chains is strong enough to hold them in place at room temperature.

Covalent

提示The atoms in the chain are non-metals, mostly carbon and hydrogen.

为什么Each atom in the chain shares pairs of electrons with its neighbours. These bonds are strong, so the chains themselves do not fall apart when the polymer softens or melts.

Melting poly(ethene) only separates the chains; melting diamond breaks covalent bonds

提示Ask what actually has to come apart in each case for the solid to flow.

为什么Poly(ethene) is made of separate long molecules held to each other by intermolecular forces, which are weaker than covalent bonds. Diamond is one giant structure, so nothing can move until covalent bonds themselves are broken.

A polymer

提示The units are very long, but they are still separate from one another.

为什么Small molecules contain only a few atoms each. A giant covalent structure is one continuous network with no separate units. Long, separate chains of covalently bonded atoms are the mark of a polymer.

intermolecular

提示It is the same kind of attraction that holds one water molecule near another.

为什么Each chain is a single huge molecule. The attractions between chains are weaker than the bonds along them, which is why many polymers soften and melt when heated without their chains breaking up.