Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 93

Carbon dioxide contains two elements: carbon and oxygen. Both are non‐metals; therefore, the type of bonding between them will be covalent, i.e. shared pairs of electrons. Carbon has four electrons in its outer shell and therefore needs to gain another four electrons to fill its outer shell. Oxygen has six electrons, so it needs to gain two more electrons. In addition, the name carbon dioxide gives a hint about the structure, with the ‘di’ showing that there are two oxygen atoms.

This information allows us to deduce that if carbon shares two electrons with each oxygen, each oxygen will have eight electrons in its outer shell and so will have a complete octet. Conversely, if oxygen shares two electrons each with carbon, carbon will have gained a share of four electrons in total, so it will also have a full outer shell. In the case of carbon dioxide, there are two pairs of electrons between each nucleus (a). Thus, there is a double bond between each carbon and oxygen atom. This is represented by a double line between the atoms (b).

For all structures containing covalent bonds, it is possible to work out the bonding and how the atoms are arranged. Some common molecules are shown in Table 2.1.

Table 2.1 The names, molecular formulae, dot‐and‐cross diagrams, and display formulae of some commonly encountered covalently bonded molecules.

Name and molecular formula	Dot‐and‐cross diagram	Display formula
Fluorine, F2
Water, H2O
Ammonia, NH3
Methane, CH4
Ethene, C2H4
Ethyne, C2H2
Oxygen, O2
Nitrogen, N2

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.