Chemical bonds, ionic, covalent and metallic

There are three types of strong chemical bonds: ionic, covalent and metallic.

For ionic bonding the particles are oppositely charged ions. For covalent bonding the particles are atoms which share pairs of electrons. For metallic bonding the particles are atoms which share delocalised electrons.

You should be able to explain chemical bonding in terms of electrostatic forces and the transfer or sharing of electrons.

All three types of bond come down to the same thing: negative electrons attracted to positive nuclei. What differs is what happens to the outer electrons. In ionic bonding they are transferred from one atom to another, and the oppositely charged ions then attract each other. In covalent bonding they are shared between two nuclei, both of which attract the shared pair. In metallic bonding they are given up to the whole structure, and every positive ion is attracted to the sea of electrons around it. Every one of those attractions is electrostatic — that is the word an examiner is looking for.

When a metal atom reacts with a non-metal atom, electrons in the outer shell of the metal atom are transferred. Metal atoms lose electrons to become positively charged ions. Non-metal atoms gain electrons to become negatively charged ions.

The ions produced by metals in Groups 1 and 2, and by non-metals in Groups 6 and 7, have the electronic structure of a noble gas (Group 0). That is why the transfer happens at all: it is the shortest route to a full outer shell for both atoms.

The charge on the ions produced by metals in Groups 1 and 2, and by non-metals in Groups 6 and 7, relates to the group number of the element in the periodic table.

You should be able to work out the charge on the ions of metals and non-metals from the group number, limited to metals in Groups 1 and 2 and non-metals in Groups 6 and 7.

You should be able to draw dot and cross diagrams for ionic compounds formed by metals in Groups 1 and 2 with non-metals in Groups 6 and 7. Two conventions matter: use dots for one element's electrons and crosses for the other's, so the transfer can be seen; and put square brackets with the charge around each ion, because an ion is not an atom.

An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice, and this is called ionic bonding.

"In all directions" is the phrase that does the work. A sodium ion is not bonded to one particular chloride ion — it is attracted to every chloride ion around it, and the structure repeats until the crystal ends.

You should be able to:

• deduce that a compound is ionic from a diagram of its structure in one of the specified forms • describe the limitations of using dot and cross, ball and stick, two-dimensional and three-dimensional diagrams to represent a giant ionic structure • work out the empirical formula of an ionic compound from a given model or diagram that shows the ions in the structure.

You should be familiar with the structure of sodium chloride, but you do not need to know the structures of other ionic compounds.

When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong.

Covalently bonded substances may consist of small molecules. You should be able to recognise common substances that consist of small molecules from their chemical formula — a formula built from a few non-metal atoms, such as H₂O, CO₂ or NH₃, is a small molecule.

Some covalently bonded substances have very large molecules, such as polymers. Some have giant covalent structures, such as diamond and silicon dioxide.

Polymers can be represented in a form where a repeating unit is written in brackets with a subscript n, where n is a large number. That notation says the same short unit repeats along a very long chain, rather than listing thousands of atoms.

You should be able to recognise substances as small molecules, polymers or giant structures from diagrams showing their bonding. The test is what the diagram shows at its edges: a small molecule is complete, a polymer has a bracketed unit with bonds running out of both sides, and a giant covalent structure has bonds continuing off the edge of the picture in every direction.

You should be able to:

• draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane • represent the covalent bonds in small molecules, in the repeating units of polymers, and in part of giant covalent structures, using a line to represent a single bond • describe the limitations of using dot and cross, ball and stick, two-dimensional and three-dimensional diagrams to represent molecules or giant structures • deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.

Metals consist of giant structures of atoms arranged in a regular pattern.

The electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure. The sharing of delocalised electrons gives rise to strong metallic bonds.

You should be able to recognise substances as metallic giant structures from diagrams showing their bonding. The give-away is a regular grid of identical positive circles with loose electrons drawn between them — no negative ions, and no separate molecules.

Because the delocalised electrons can move, metallic bonding will explain, in the next sub-topic, why metals conduct electricity and heat, why they have high melting points, and why they can be bent and shaped.