To ensure that you understand this topic, try to draw each of the molecules in Table 2.1 using dot‐and‐cross diagrams and starting from the component atoms, accounting for all of the electrons shown.
Dative covalent bonding
In some of the molecules shown in Table 2.1, a pair of electrons is ‘left over’ or not involved in covalent bond formation. Examples of molecules where this occurs include ammonia (one pair of electrons on the nitrogen), water (two pairs of electrons on the oxygen), and nitrogen (one pair of electrons on each nitrogen). This pair of electrons is called a lone pair of electrons or, more colloquially, a lone pair. Such pairs of electrons can also undergo bonding to other species that can accept a pair of electrons, such as a positive hydrogen ion, H+. When both electrons in a covalent bond originate from the same atom, a dative covalent bond is formed. An example of this is ammonia (NH3), which can form a dative covalent bond to a proton (H+) to form an ammonium ion (NH4+). The dot‐and‐cross diagrams showing the formation of a dative covalent bond in the ammonium ion are given in Figure a2.10. A dative covalent bond can be shown either as a line or as an arrow. The arrow is drawn such that it starts on the atom that has donated the electrons and finishes on the atom that has gained the pair of electrons, as shown in Figure 2.10b. Once formed, the dative covalent bond between the nitrogen and hydrogen atoms is chemically no different to the other nitrogen—hydrogen bonds in the ammonium ion.
Figure 2.10 (a) Bonding in the ammonium ion, NH4+. Note: only outer‐shell electrons are shown for clarity. (b) An alternative representation of bonding in the ammonium ion.
A dative covalent bond is formed when both electrons in a covalent bond are provided by the same atom.
Simple molecular covalent bonding and giant covalent bonding
Covalent bonding can lead to two different types of structures. These are called simple covalent (or simple molecular) or giant covalent (or giant molecular) structures. The main difference between simple molecular covalent bonding and giant covalent bonding is that a simple molecule is just that: a small molecule that contains covalent bonds. Examples of molecules that have simple molecular bonding are oxygen (O2), methane (CH4), chlorine (Cl2), and ethanol (C2H6O). Giant covalent bonding leads to extended repeated units such as in silicon dioxide (SiO2) and graphite and diamond (both elemental forms of carbon). A substance that has a giant covalent structure is much more complex and contains an extended array of bonds. This type of bonding does not result in discrete molecules because the bonding is repeated throughout the structure. The structures formed are not molecules as the bonding continues repeatedly in three dimensions. The structures of diamond, silicon dioxide, and graphite are shown in Figure 2.11.
Figure 2.11 (a) Bonding in diamond; (b) bonding in silicon dioxide; (c) two‐dimensional bonding in graphite, looking down from above; (d) three‐dimensional bonding in graphite, looking through the layers.
2.2 Valence Shell Electron Pair Repulsion Theory (VSEPR)
The shape of a covalently bonded molecule or ion is determined by the number and arrangement of the pairs of electrons around the central atom. Electron pairs are negatively charged centres and are repelled by neighbouring centres of electron density. The structures formed are most stable when the centres of electron density are as far apart as possible from each other. If you remember this, you will be able to tackle any problems involving the shapes of simple molecules. The process