Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 94

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.