Ionic, Covalent & Metallic Bonding (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about ionic, covalent and metallic bonding. All three are attractions involving the outer-shell electrons of the atoms. What separates them is what those electrons do: they are either transferred, shared, or delocalised.

Every one of these bonds comes down to the outer-shell electrons of the atoms involved. In ionic bonding those electrons are transferred from one atom to another. In covalent bonding they are shared between two atoms. In metallic bonding they leave their atoms altogether and move freely through the structure. Three different things happening to the same electrons, and three different kinds of attraction as a result.

We take them in that order. Ionic bonding first, then covalent bonding, including dative covalent bonding, which is a covalent bond where both of the shared electrons come from one atom. After that, dot and cross diagrams, which are how the outer electrons in ionic and covalent substances get drawn. And finally, metallic bonding last.

Ionic bonding involves the transfer of electrons from a metallic element to a non-metallic element. Metals sit on the left-hand side of the periodic table and non-metals sit on the right. Transferring electrons usually leaves both of them with a full outer shell. Metals lose electrons from their valence shell and form positively charged cations. Non-metal atoms gain electrons and form negatively charged anions.

Cations and anions are oppositely charged, so they attract each other. The ionic bond is that electrostatic force of attraction between the oppositely charged ion. It occurs in all directions, which is why it is described as non-directional. It is very strong and requires a lot of energy to overcome, which causes high melting points in ionic compounds. The ions are arranged into a lattice, a regular repeating pattern in which the positive charges cancel out the negative charges, so the lattice overall is electrically neutral. This is the first of the three things outer electrons can do: leave one atom completely and join another.

Covalent bonding occurs between non-metal atoms. Here the electrons are not transferred, they are shared. A covalent bond is the electrostatic force of attraction between the positively charged nuclei of two atoms and a shared pair of electrons in their outer shells. By sharing, each atom achieves a full outer shell similar to the electron configuration of a noble gas, and that makes the atoms more stable.

Atoms can share more than one pair of electrons. A single bond is one shared pair, so two electrons in total. A double bond is two pairs, four electrons, as in oxygen and carbon dioxide. A triple bond is three pairs, six electrons, as in nitrogen. This is the second thing outer electrons can do: be shared and belong to both atoms at once.

A dative covalent bond is still a shared pair of electrons, but both of those electrons come from the same atom. Some molecules contain a lone pair that can be donated to an electron-deficient atom, meaning an atom with an unfilled outer orbital that can accept a pair of electrons. It is also known as coordinate bonding.

The ammonium ion is the example the notes use. A hydrogen ion is electron-deficient and has space for two electrons in its outer shell. The nitrogen atom in ammonia has a lone pair, and it donates that pair to the hydrogen ion to form the dative covalent bond. The electrons are still shared, they are just supplied by one side.

Dot and cross diagrams show the arrangement of the outer-shell electrons in an ionic or covalent compound or element. Only the outer electrons are shown. Dots represent the electrons from one atom and crosses represent the electrons from the other, so the diagram shows where each electron came from. Shared electrons are drawn as pairs in the overlap between the atoms.

For an ionic compound, the diagram shows the metal atom transferring electrons to the non-metal atom, with both achieving a noble gas configuration. Brackets are used to show that the charge on the ion is spread evenly, and the charge itself is written at the top right-hand corner. For a covalent compound, the atoms share their outer valence electrons instead. It is the same set of outer electrons in both cases, drawn differently depending on whether they were transferred or shared.

Metal atoms are tightly packed together in lattice structures, and when they are, the electrons in their outer shells are free to move throughout the whole structure. Those free-moving electrons are called delocalised electrons, as they are not bound to the atom they came from.

Once the electrons are delocalised, the metal atoms become positively charged ions. The positive charges repel each other and keep the neatly arranged lattice in place. There are also very strong forces of attraction between the positive metal centres and the sea of delocalised electrons. This is the third thing outer electrons can do: leave their atoms without joining any particular partner.

Metals usually lose all the electrons from their outer valence shell to become cations, and you can use the group an atom belongs to on the periodic table to work out how many electrons it is likely to lose or gain. A dative covalent bond is drawn using an arrow, running in the direction they are going, from the donated pair of electrons to the electron-deficient atom.

In ionic bonding, electrons are transferred from a metal to a non-metal, forming oppositely charged ions that attract each other in a lattice. In covalent bonding, electrons are shared between non-metal atoms, and in a dative covalent bond both of the shared electrons come from the same atom. In metallic bonding, the outer electrons are delocalised, and the positive metal ions sit in a sea of them. Dot and cross diagrams show where those outer electrons sit in the ionic and covalent cases. Three types of bonding, and one set of outer-shell electrons doing three different things.

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