Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 119

Consider a covalent bond formed between two atoms A and B. The single bond is composed of a pair of electrons. If atoms A and B have the same electronegativity, then on average, the pair of electrons will be located evenly between the two atoms, as shown in Figure 2.21. This type of bond is called a pure covalent bond and is formed when the elements at the end of the bond are the same: for example, Cl₂ or H₂.

Figure 2.21 (a) A pair of electrons shared evenly between two atoms with the same electronegativity; (b) a representation of the cloud of electrons.

However, if one of the atoms (say, atom B) is more electronegative than the other, the electrons will be pulled towards that atom, and the distribution will no longer be evenly spread. Atom B will have a greater share of electrons than atom A, as shown in Figure 2.22. The result of this is that atom B becomes slightly negatively charged compared to atom A. We represent this charge by a δ− sign next to atom B. Because atom B takes up a slight negative charge, atom A must become slightly positively charged to balance the overall charge, and this is represented by the symbol δ+.

Figure 2.22 (a) A pair of electrons shared unevenly between two atoms with different electronegativities; (b) a representation of the cloud of electrons, showing the charge distribution.

The Greek letter δ is pronounced ‘delta’ and in maths and chemistry means ‘a little bit’ or ‘slightly’. So δ− means slightly negative, and δ+ means slightly positively charged.

A bond where there is an uneven distribution of charge, such as that shown in Figure 2.23, is called a polar covalent bond. It occurs between atoms where there is a reasonable difference in electronegativity. Examples of polar covalent bonds are found in hydrogen chloride, HCl, where the chlorine atom is more electronegative than hydrogen; and carbon dioxide, CO₂, where oxygen is more electronegative than carbon.

Figure 2.23 Polar covalent bonds in (a) hydrogen chloride, HCl and (b) carbon dioxide, CO₂, showing the charge distribution.

If there is a large difference in electronegativity between the atoms in a bond, then the electrons are concentrated on the more electronegative element. In the extreme case, the more electronegative element pulls the electrons completely towards itself and becomes a negatively charged ion (an anion), and the bonding is ionic. This generally occurs when one of the atoms in the bond is a metal and the other a non‐metal. The non‐metal becomes an anion and the metal a cation, and now electrostatic interactions hold the ions together in the lattice.

 Pure covalent bonds are formed when the atoms in a bond have the same electronegativity values.

 Polar covalent bonds are formed when the atoms in a bond have a small difference in electronegativity values.

 Ionic bonds are formed between metals and non‐metals that typically have a large electronegativity difference.