A metal transfers valence electrons to a non-metal, forming oppositely charged ions held by strong electrostatic attraction, for example 2Na(s) + Cl2(g) → 2NaCl(s).
An ionic compound forms when a metal reacts with a non-metal and electrons are transferred from the metal atom to the non-metal atom. The metal loses its valence electrons to become a positive ion (cation), the non-metal gains those electrons to become a negative ion (anion), and the oppositely charged ions attract one another by strong electrostatic forces. This electron transfer is one of the most heavily tested ideas in Chemical Bond, so it appears throughout 4541.
The balanced equation
The general pattern is: reactive metal + reactive non-metal → ionic compound. With correct formulae and state symbols, the standard examples are:
2Na(s) + Cl2(g) → 2NaCl(s)
2Mg(s) + O2(g) → 2MgO(s)
Mg(s) + Cl2(g) → MgCl2(s)
The chemistry behind each equation is the transfer of electrons. For sodium chloride, each sodium atom (2.8.1) loses one electron and each chlorine atom (2.8.7) gains one, which we can write as two half-processes:
Na → Na+ + e−
Cl2 + 2e− → 2Cl−
For magnesium oxide, magnesium (2.8.2) loses two electrons and oxygen (2.6) gains two: Mg → Mg2+ + 2e− and O2 + 4e− → 2O2−. Every ion ends with a stable noble-gas electron arrangement, and the ratio of ions in the formula makes the total positive and negative charge equal.
Conditions required
You need a reactive metal (typically from Group 1 or Group 2) and a reactive non-metal (typically Group 16 or Group 17). Heat is usually supplied to start the reaction, sodium is heated before it is lowered into a gas jar of chlorine, and magnesium is ignited in air or oxygen. No catalyst and no solvent are needed; the reaction is direct combination of the two elements.
What you observe
Sodium burns vigorously in chlorine with a bright yellow flame and forms a white solid, sodium chloride. Magnesium burns in oxygen with a dazzling white light and leaves a white ash, magnesium oxide. In every case the product is a solid that is white or colourless, has a high melting point, and, unlike the metallic element you started with, does not conduct electricity when solid, only when molten or dissolved in water.
Where it appears in the SPM exam
In 4541/1 you may be asked to identify the ions formed or to choose the correct formula of the ionic compound. In 4541/2 the classic question asks you to draw the dot-and-cross diagram showing electron transfer, to give the electron arrangement of each ion, and to write the balanced equation. You are also asked to link the ionic bonding to physical properties: high melting and boiling points because strong electrostatic forces must be overcome, and electrical conductivity only when the ions are free to move.
How we teach it
Our teachers make sure you separate the atom from the ion: sodium atom is 2.8.1 but the sodium ion is 2.8, and the charge sign must always be shown. The most common lost marks come from forgetting the state symbols, drawing the wrong number of electrons transferred, or writing the formula with unequal charges. Anchoring every answer to the rule “metal loses, non-metal gains, charges must balance” keeps the equation, the diagram and the properties all consistent.
Quick summary
An ionic compound is the product of complete electron transfer between a reactive metal and a reactive non-metal. Write the balanced equation with state symbols, show each ion reaching a noble-gas arrangement, and remember that the strong electrostatic attraction between the ions explains the high melting point and the conductivity only when molten or aqueous. Get those three linked answers right and the whole topic scores.
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