Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 86

Ionic bonding occurs between a metal and a non‐metal. An ionic bond is an electrostatic interaction between two oppositely charged ions that are attracted to each other. Ions are formed when atoms lose or gain electrons, so they have either a positive or a negative charge. The metal ion always has a positive charge and the non‐metal ion a negative charge.

An example of this behaviour is shown by fluorine, which has the electron configuration 1s²2s²2p⁵. When a fluorine atom gains one more electron, it has the electron configuration of neon, i.e. 1s²2s²2p⁶_. It now has a charge of −1 because it has gained one negatively charged electron and becomes a fluoride ion, F⁻.

The process can be represented by the equation:

The electron arrangements in the fluorine atom and fluoride ion are shown in Figure 2.3.

Figure 2.3 Addition of an electron to a fluorine atom to generate a fluoride ion. Electrons in the fluorine atom are represented by x.

Note that the neutral F atom is called fluorine, as in the element, and the negatively charged F⁻ ion is called fluoride. This is the form of the element found in toothpaste, for example.

To form a positive ion, the atom must lose an electron. Sodium in Group 1 has just one outer electron. Sodium attains an octet of eight outer electrons by losing this single electron. It is more favourable for sodium to lose this single electron and become a Na⁺ ion than to gain an extra seven electrons. Sodium therefore loses an electron to attain the electron configuration of neon, as shown in Figure 2.4.

Figure 2.4 Arrangement of electrons in (a) a sodium atom; (b) a sodium ion.

The process can be represented by the equation:

The element to the right of sodium in the periodic table is magnesium, Mg, and an atom of magnesium has two outer electrons (1s²2s²2p⁶3s²). To attain a full outer shell, magnesium must lose both its 3s electrons. When this occurs, a Mg²⁺ ion is formed:

The process of losing or gaining an electron to form an ion is called ionisation.

Metal atoms always lose electrons on ionisation to form positive ions, whereas non‐metals gain electrons to form negative ions.

Elements in the first row of the periodic table (i.e. hydrogen and helium) cannot obtain an octet of electrons as they have a single s orbital that can only hold a maximum of two electrons. They therefore have a full outer shell when they have just two electrons in the 1s orbital. A helium atom has a full outer shell and so has no tendency to lose or gain electrons and therefore does not readily form ions. However, hydrogen has just one outer electron. It can either lose this electron and form a H⁺ ion or gain an electron to form a H⁻ ion. The H⁻ ion is called a hydride ion.

When a positively charged cation and a negatively charged anion are formed, an electrostatic interaction exists between the oppositely charged ions. This interaction is the basis of ionic bonding, and the attractive force between the ions is similar to that experienced between the north and south poles of a magnet. These attractive forces extend among all the cations and anions in a structure to give an ionic lattice. An ionic lattice is a regular three‐dimensional array of positive and negative ions extending throughout the structure. The attractive forces between the oppositely charged ions are very large; therefore, ionic bonds are very strong, and ionic lattices are difficult to break down. The bonding in sodium chloride can be used as an example of ionic bonding.

Sodium chloride is an ionic compound that contains sodium ions (Na⁺) and chloride ions (Cl⁻). Sodium is in Group 1 and therefore has one electron in its outer shell. Chlorine is in Group 7 (Group 17) and has seven outer electrons. To have a full outer shell, sodium must lose one electron and chlorine must gain one more electron, as shown in Figure 2.5.

Figure 2.5 Bonding in NaCl. Note: only outer‐shell electrons are shown for clarity.

When chlorine has a negative charge, the name changes to chloride. The same is true for fluorine (fluoride), bromine (bromide), and iodine (iodide).

In Figure 2.5, sodium's outer electrons are represented by dots and chlorine's outer electrons by crosses. By losing one electron to chlorine, sodium now has one fewer electron than protons and so has an overall positive charge. In contrast, chlorine has one more electron than protons, so it has an overall negative charge. The resulting ions are shown in square brackets, where the charge of each ion is at the top right, on the outside of the bracket.

This type of diagram, where electrons are represented by dots and crosses, is called a dot‐and‐cross diagram and sometimes a Lewis structure. Note that all electrons are identical to each other, regardless of whether they are drawn as dots or crosses.

The positive sodium ions and negative chloride ions are attracted to each other by electrostatic forces and are arranged in a regular lattice array. Figure 2.6 shows the arrangement of ions in the sodium chloride lattice. Many millions of these units are linked together in a grain of salt (Figure 2.7).

Figure 2.6 The sodium chloride lattice. Source: Based on https://www.chemguide.co.uk/atoms/structures/ionicstruct.html.

Figure 2.7 Salt, sodium chloride.

Source: Dr Philippa Cranwell.

Ionic bonding is very strong, and, as a result, ionic compounds or salts have similar physical properties. Ionic compounds usually have high melting and boiling points and are solids at room temperature. They form hard crystalline structures. When solid, they do not conduct heat or electricity; but when molten, they do, as the ions can move and carry a charge or transfer heat energy.

Ionic bonding occurs between metals and non‐metals, and there is a strong electrostatic interaction between the bonding partners.