Valency and Ion Formation
In 1807, Humphry Davy passed electricity through molten salt and isolated pure sodium, a metal so reactive it bursts into orange flame on contact with water.
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Q1 · When you dissolve table salt in water, the salt seems to disappear, where do you think the sodium and chlorine actually go, and do they stay as they were?
Q2 · Why do you think atoms ever bother gaining or losing electrons, what might they be trying to achieve by doing so?
● Know
- The definition of valency and ion
- How cations and anions form
- Common ions and their charges (Na⁺, Cl⁻, Ca²⁺, O²⁻, Mg²⁺)
● Understand
- Why losing electrons makes an atom positive (not negative)
- How to determine an ion's charge from its position in the periodic table
- How ionic charges determine the formula of a compound
● Can do
- Determine ion charges from periodic table position
- Write ionic formulas using valency (cross-and-swap method)
- Explain why losing electrons makes atoms positive, not negative
An atom is normally neutral because it has equal numbers of protons and electrons. An ion forms when an atom gains or loses electrons, so that the positive and negative charges no longer balance. When a metal atom loses one or more electrons, it has more protons than electrons and becomes a positively charged cation. When a non-metal atom gains electrons, it has more electrons than protons and becomes a negatively charged anion. The driving force is the same for both: each atom is trying to reach a noble gas electron arrangement (a full outer shell), which is the most stable state an atom can have.
For sodium chloride: sodium (2,8,1) loses its single outer electron to become Na⁺ (2,8), matching neon's configuration. Chlorine (2,8,7) gains that electron to become Cl⁻ (2,8,8), matching argon's configuration. Both ions are now stable. The process is exothermic, energy is released when the ions form, which is why ionic compounds are energetically stable and have high melting points: a great deal of energy must be put back in to separate them again.
Magnesium oxide (MgO): magnesium (2,8,2) loses 2 electrons → Mg²⁺ (2,8). Oxygen (2,6) gains 2 electrons → O²⁻ (2,8). Both achieve neon configuration. MgO has a melting point of 2852 °C, used as a refractory lining in steel furnaces at BlueScope's Port Kembla plant in NSW.
The Australian Institute of Marine Science in Townsville studies how dissolved sodium chloride (NaCl) ions in seawater affect the Great Barrier Reef's chemical environment. The ionic composition of seawater, Na⁺, Cl⁻, Mg²⁺, SO₄²⁻, directly controls which minerals reef corals can extract to build their calcium carbonate skeletons.
The electrons in the outermost shell of an atom are called its valence electrons, and they are the only electrons involved in bonding. The number of valence electrons follows the group an element sits in on the periodic table: Group 1 metals have 1, Group 2 metals have 2, Group 17 non-metals have 7, and the noble gases in Group 18 have a full outer shell (8, or 2 for helium). A full outer shell is the most stable arrangement an atom can have, which is why the noble gases almost never react. The tendency of other atoms to gain or lose electrons until they too have a full outer shell of eight is called the octet rule.
This single idea explains why atoms form the ions they do. Metals have only a few valence electrons, so the easiest way to reach a full shell is to lose them, becoming positive cations. Sodium (1 valence electron) loses 1 to form Na⁺; magnesium (2) loses 2 to form Mg²⁺. Non-metals have nearly full outer shells, so the easiest path is to gain the few electrons they are missing, becoming negative anions. Chlorine (7 valence electrons) gains 1 to form Cl⁻; oxygen (6) gains 2 to form O²⁻. The valency of an element is simply the number of electrons it gains or loses to obey the octet rule, which is also the size of the charge on the ion it forms.
Aluminium sits in Group 13 and has 3 valence electrons. Losing all 3 is the quickest way to reach a full outer shell, so aluminium forms the Al³⁺ ion with a valency of 3. This is why aluminium foil, cans, and window frames are all built from atoms that readily give up three electrons.
The Royal Australian Mint in Canberra chooses metals partly for how their valence electrons behave. The gold-coloured $1 and $2 coins are made from an alloy of copper, aluminium, and nickel, metals whose loosely held valence electrons give the coins their conductivity, shine, and resistance to corrosion.
Every ionic compound is electrically neutral overall, the total positive charge from the cations must exactly balance the total negative charge from the anions. The cross-and-swap method is a quick way to work out the correct formula. Step 1: write the cation and anion with their charges, for example Mg²⁺ and Cl⁻. Step 2: take the size of each ion's charge (ignore the + and −) and write it as the subscript on the other ion. The 2 from magnesium becomes the subscript on chlorine, and the 1 from chlorine becomes the subscript on magnesium. Step 3: this gives Mg₁Cl₂, which we write as MgCl₂ (a subscript of 1 is never written). Step 4: simplify the subscripts to the smallest whole numbers if they share a common factor.
Always check your answer by adding up the charges: in MgCl₂ the magnesium contributes +2 and the two chloride ions contribute 2 × (−1) = −2, so the compound is neutral. The same method works for any ionic compound. For sodium oxide, Na⁺ and O²⁻ cross over to give Na₂O. For aluminium oxide, Al³⁺ and O²⁻ cross over to give Al₂O₃. With practice you will be able to write the formula of any ionic compound straight from the charges on its ions.
Calcium chloride: the ions are Ca²⁺ and Cl⁻. Cross-and-swap puts the 2 from calcium onto chlorine and the 1 from chlorine onto calcium, giving CaCl₂. Check: (+2) + 2 × (−1) = 0, so the formula is balanced. Calcium chloride spread on icy roads in alpine NSW works because this neutral ionic compound dissolves and lowers the freezing point of water.
Chemists and engineers at fertiliser producers like Incitec Pivot in NSW write ionic formulas constantly. Knowing that ammonium and phosphate ions combine in fixed ratios lets them calculate exactly how much of each ingredient is needed to make balanced, neutral compounds for Australian farms.
Every ionic compound is electrically overall, so the positive and negative charges must balance. In the cross-and-swap method, the size of each ion's becomes the subscript on the other ion. For Mg²⁺ and Cl⁻ this gives the formula . A subscript of is never written. You can check the formula by making sure the total positive and negative charges add up to .
At the start of this lesson, you heard something surprising: table salt is made from sodium, which explodes in water, and chlorine, a toxic gas used as a weapon in World War I, yet NaCl is perfectly safe to eat. That total transformation in properties happens through electron transfer, which creates ions with completely new characteristics.
Now that you've worked through the lesson, can you explain why sodium and chloride ions are so different from the elements they came from? How does understanding valency help you predict which ions an element will form?
Q1. Explain why a sodium atom (Na) becomes positively charged when it forms an ion. Use the terms 'valence electron' and 'cation' in your answer.
Q2. Using the cross-and-swap method, determine the correct ionic formula for: (a) calcium chloride, (b) aluminium oxide. Show your working.
Q3. A student says 'atoms gain electrons when they want to become negative.' Evaluate this statement. What is actually driving ion formation?
Model answers (click to reveal)
SAQ 1 (2 marks)
Model answer: A neutral sodium atom has 11 protons (+) and 11 electrons (-), so its charges cancel. To reach a stable, full outer shell, the sodium atom loses its single valence electron to another atom. It now has 11 protons but only 10 electrons, so there is one more positive proton than negative electron. This gives the atom an overall charge of +1, making it a positively charged ion called a cation, written Na⁺.
SAQ 2 (3 marks)
Model answer: (a) Calcium chloride. The ions are Ca²⁺ and Cl⁻. Cross-and-swap: the 2 from calcium becomes the subscript on chlorine, and the 1 from chlorine becomes the subscript on calcium. This gives Ca₁Cl₂, written as CaCl₂. Check: total positive charge = +2, total negative charge = 2 × (−1) = −2, so the compound is neutral. (b) Aluminium oxide. The ions are Al³⁺ and O²⁻. Cross-and-swap: the 3 from aluminium becomes the subscript on oxygen, and the 2 from oxygen becomes the subscript on aluminium. This gives Al₂O₃. Check: total positive charge = 2 × (+3) = +6, total negative charge = 3 × (−2) = −6, so the compound is neutral.
SAQ 3 (3 marks)
Model answer: The statement is incorrect. Atoms do not 'want' anything, and they do not gain electrons in order to become negative. The real driving force behind ion formation is the tendency of atoms to reach a stable electron arrangement, a full outer shell that matches the nearest noble gas, which is the lowest-energy, most stable state. A non-metal such as chlorine is only one electron short of a full outer shell, so it is energetically favourable for it to gain one electron. The negative charge is simply a consequence of having more electrons than protons after the gain, not the aim. Other atoms (metals) instead lose electrons and become positive, which shows that becoming negative is not a universal goal but just one outcome of atoms seeking stability.