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.
Worked Example 2.2
Give the electronic configuration of the following atoms and the likely electron configuration and charge of the species formed by each atom upon ionisation. State the noble gas atom that has the same electron configuration as the ion formed.
1 potassium
2 chlorine
Solution
1 Potassium is an s block element in Group 1 of the periodic table. It is a metal with one outer electron. The electron configuration of potassium is 1s22s22p63s23p64s1. To gain a full outer shell, potassium tends to lose its outer electron and form a K+ ion with electron configuration 1s22s22p63s23p6. This electron configuration is the same as argon, Ar.
2 Chlorine is in Group 7 (Group 17) of the periodic table and has seven outer electrons with electron configuration 1s22s22p63s23p5. It is a non‐metal and tends to gain an electron to form the chloride, Cl−, ion with electron configuration 1s22s22p63s23p6. This electron configuration is the same as argon, Ar.
Worked Example 2.3
Draw the Lewis structure (dot‐and‐cross diagram) for calcium fluoride, CaF2.
Solution
Calcium fluoride contains two elements: calcium and fluorine. Calcium is a metal on the left of the periodic table, in Group 2. Fluorine is a non‐metal and resides in Group 7 (Group 17). Because we have a metal and a non‐metal in the compound, the type of bonding present is ionic. Therefore, the main interaction between atoms will be electrostatic: i.e. a positive ion with a negative ion.
When asked a question about the type of bonding in a compound, the first thing to check is the types of elements that are bonded and whether