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
Drop a grain of table salt into a glass of water and watch it disappear, the crystal lattice breaks apart into charged particles that scatter invisibly through the liquid and allow electricity to flow where solid salt could not. Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom. The metal atom loses one or more electrons, becoming a positively charged cation. The non-metal atom gains those electrons, becoming a negatively charged anion. The driving force is the same for both: each atom achieves a noble gas electron configuration (full outer shell), reaching minimum energy and maximum stability. The opposite charges of the resulting ions attract each other strongly, this electrostatic attraction is the ionic bond.
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.
Ionic compounds don't exist as individual pairs of ions, they form a giant three-dimensional ionic lattice. In sodium chloride, each Na⁺ ion is surrounded by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions. This alternating arrangement maximises attractive forces between opposite charges while minimising repulsive forces between like charges. Millions of ions stack into this regular, repeating pattern, a crystal. The regularity is why salt crystals are always cubic in shape.
The lattice is very strong because every ion is held by multiple electrostatic forces simultaneously, it takes a great deal of energy to pull all those ions apart. This gives ionic compounds their characteristic high melting points (NaCl: 801 °C). However, the lattice is also brittle: if you hit a salt crystal with a hammer, the layers shift and like charges align, creating a line of repulsion that shatters the crystal along a cleavage plane. This brittleness is a key limitation of ionic materials in engineering.
Calcium fluoride (CaF₂, fluorite) forms beautiful cubic crystals. Each Ca²⁺ is surrounded by 8 F⁻ ions; each F⁻ by 4 Ca²⁺ ions. The crystal is so regular that it cleaves perfectly along planes when struck, producing flat triangular faces, a direct consequence of the regular lattice geometry.
Alumina (Al₂O₃) in its corundum crystal form has an ionic lattice strong enough to be used as grinding wheels and sandpaper at Australian manufacturing plants. Ruby and sapphire are corundum with trace impurity ions, precious gemstones and industrial abrasives both arise from the same ionic lattice structure.
The ionic lattice structure directly explains every bulk property of ionic compounds. They are hard but brittle because the lattice resists deformation but shatters along cleavage planes. They have high melting points because many strong electrostatic bonds must be broken simultaneously. They do not conduct electricity as solids because the ions are locked in fixed positions, charge cannot flow. However, when ionic compounds are dissolved in water or melted, the ions are free to move, making them good conductors of electricity, a property crucial to batteries, electrolysis, and nerve cells.
Many ionic compounds are soluble in water because water molecules are polar, their slightly positive hydrogen ends attract anions, and their slightly negative oxygen end attracts cations, pulling the lattice apart ion by ion. This is why NaCl dissolves readily in water but MgO does not, the higher charge density of Mg²⁺ and O²⁻ creates a stronger lattice that water molecules cannot easily disrupt. Each property follows logically from the structure, understanding the structure means you can predict all the properties.
Sodium chloride dissolved in water produces a conducting solution: 1 mol/L NaCl solution has a conductivity of about 10 S/m. Solid NaCl has near-zero conductivity. This switching behaviour, insulator when solid, conductor when dissolved, is exploited in electrolysis cells used by Orica (Newcastle, NSW) to produce chlorine for water treatment.
Orica's Kooragang Island facility near Newcastle uses the chlor-alkali process, electrolysis of dissolved NaCl (brine), to produce chlorine gas and sodium hydroxide. These two ionic compounds are feedstocks for PVC plastic, paper bleaching, and water purification across NSW and Queensland.
Ionic compounds are hard but because the lattice shatters along cleavage planes. They have high points because many strong electrostatic bonds must be broken at once. As solids they do not conduct electricity because the ions are locked in fixed . However, when dissolved or , the ions become free to move and can carry charge. Many ionic compounds dissolve in water because water molecules are .
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?