Chemical Bonding
Senior 2 · Chemistry
Chemical Bonding
Lesson 1 of 4
25 min
Sodium is a soft metal that fizzes and can catch fire in water. Chlorine is a yellow-green, poisonous gas. Yet when the two react, they make the white crystals of sodium chloride that you sprinkle on your food every day. The new substance has completely different properties from the elements that made it, because the atoms are now held together in a new way: by chemical bonds.
A chemical bond is a strong force of attraction that holds atoms or ions together. There are three main types: ionic, covalent and metallic. The type of bond explains why salt is hard and dissolves in water, why candle wax melts in your hand, and why copper wire bends.
By the end of this lesson you will be able to explain how ions form, describe ionic bonding with dot-and-cross diagrams, write the formulae of ionic compounds and explain their properties.
The noble gases (group 18) hardly react at all. Their atoms have a full outer shell: 2 electrons for helium (a duplet) and 8 electrons for the others (an octet). Atoms of other elements react so as to reach the same stable arrangement as the nearest noble gas. They can do this in three ways: by losing electrons, by gaining electrons, or by sharing electrons.
An ion is an atom or group of atoms that carries an electric charge because it has lost or gained electrons.
| Atom | Electron arrangement | Change | Ion | Ion's arrangement | Same as |
|---|---|---|---|---|---|
| Sodium, Na | 2,8,1 | loses 1 electron | Na+ | 2,8 | neon |
| Magnesium, Mg | 2,8,2 | loses 2 electrons | Mg2+ | 2,8 | neon |
| Chlorine, Cl | 2,8,7 | gains 1 electron | Cl− | 2,8,8 | argon |
| Oxygen, O | 2,6 | gains 2 electrons | O2− | 2,8 | neon |
A sodium ion has 11 protons but only 10 electrons, so its charge is +1. A chloride ion has 17 protons and 18 electrons, so its charge is −1. Notice the name change: chlorine becomes chloride, and oxygen becomes oxide.
The charge on an ion can be predicted from the group: group 1 forms 1+ ions, group 2 forms 2+, group 13 forms 3+, group 15 forms 3−, group 16 forms 2− and group 17 forms 1−.
Try it: open Build an Atom (version 1.8.4) and build a lithium atom (3 protons, 4 neutrons, 3 electrons). Remove one electron and watch the net charge change to +1. Then build a fluorine atom (9 protons, 10 neutrons, 9 electrons) and add one electron: you have made a fluoride ion, F−, with the same 2,8 arrangement as neon.
When a metal reacts with a non-metal, electrons are transferred from the metal atoms to the non-metal atoms. The oppositely charged ions that form attract each other strongly. An ionic bond is the electrostatic attraction between oppositely charged ions.
Dot-and-cross diagrams. We show the electrons of one atom as dots (•) and those of the other as crosses (×). For sodium chloride:
Describe the formation of magnesium oxide and write its formula.
Solution. Magnesium (2,8,2) loses its 2 outer electrons to form Mg2+ (2,8). Oxygen (2,6) gains those 2 electrons to form O2− (2,8). One Mg2+ balances one O2−, so the formula is MgO. Because oxygen exists as O2 molecules, the balanced equation is:
2Mg(s) + O2(g) → 2MgO(s)
Explain why the formula of calcium chloride is CaCl2.
Solution. Calcium (2,8,8,2) must lose 2 electrons to reach the argon arrangement. Each chlorine atom (2,8,7) can accept only 1 electron. So one calcium atom gives one electron to each of two chlorine atoms, forming one Ca2+ and two Cl− ions. The charges balance: (+2) + 2 × ( − 1) = 0.
Ca(s) + Cl2(g) → CaCl2(s)
Work out the formula of aluminium oxide.
Solution. The ions are Al3+ and O2−. The lowest common multiple of 3 and 2 is 6, so we need 2 aluminium ions (2 × +3 = +6) and 3 oxide ions (3 × −2 = −6). The formula is Al2O3.
An ionic compound is not made of separate molecules. In solid sodium chloride, each Na+ ion is surrounded by six Cl− ions, and each Cl− ion by six Na+ ions, repeated in every direction millions of times. This regular arrangement is a giant ionic lattice. The formula NaCl tells you only the ratio of ions (1 : 1), not a molecule.
Tip
Formulae in one step: swap the charge numbers. For Al3+ and O2−, the 3 becomes the number of oxygens and the 2 becomes the number of aluminiums: Al2O3. Then check that the total charge is zero: 2 × (+3) + 3 × ( − 2) = 0. Cancel if you can: Mg2+ and O2− give Mg2O2, which simplifies to MgO.
Exam technique: in a dot-and-cross diagram of an ionic compound, always show the charges and the square brackets, and show only the outer shells.
Try each item before you read the solution.
Use electron arrangements to describe how potassium chloride forms from potassium (atomic number 19) and chlorine (atomic number 17).
Solution: Potassium, 2,8,8,1, loses its single outer electron to become K+, 2,8,8. Chlorine, 2,8,7, gains that electron to become Cl−, 2,8,8. Both ions have the argon arrangement. The oppositely charged ions attract: this is the ionic bond. The formula is KCl.
Write the formulae of (a) lithium oxide, (b) calcium fluoride, (c) magnesium nitride (nitride is N3−).
Solution: (a) Li+ and O2− give Li2O. (b) Ca2+ and F− give CaF2. (c) Mg2+ and N3−: swap the numbers to get Mg3N2; check 3 × (+2) + 2 × ( − 3) = 0.
Magnesium oxide melts at about 2850 °C but sodium chloride melts at 801 °C. Suggest why.
Solution: Both are giant ionic lattices, but Mg2+ and O2− carry double charges, while Na+ and Cl− carry single charges. The attraction between doubly charged ions is much stronger, so far more energy is needed to break up the magnesium oxide lattice.
Keza connects a bulb to two carbon rods in solid salt, and the bulb stays off. When she dissolves the salt in water, the bulb lights. Explain.
Solution: In solid sodium chloride the ions are held in fixed positions and cannot move, so no current flows. In solution the Na+ and Cl− ions are free to move, so they carry charge through the liquid and the bulb lights.
Summary
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