8 min readConcept sheet2 or 3-mark question on electron dot structures; 1-mark MCQ on properties; sometimes a 5-mark answer

Ionic compounds: how metals and non-metals bond

A metal loses electrons and a non-metal gains them, so both get a full outer shell. The oppositely charged ions attract and form an ionic compound.

Ionic (electrovalent) bondThe attraction between oppositely charged ions

Noble gas configurationA full outer shell, e.g. neon 2, 8 and argon 2, 8, 8

Sodium chlorideNa (2, 8, 1) gives one electron to Cl (2, 8, 7)

Melting point of NaCl1074 K

Why atoms react

Noble gases such as helium, neon and argon have a full outer shell, and they hardly react. Other atoms react to reach the same stable arrangement, called the noble gas configuration. Metals have 1 to 3 electrons in their outer shell, and lose them to form positive ions (cations). Non-metals have 5 to 7 outer electrons, and gain electrons to form negative ions (anions).

Formation of sodium chloride

1
Na → Na⁺ + e⁻
Sodium (2, 8, 1) loses one electron and becomes Na⁺ (2, 8), like neon
2
Cl + e⁻ → Cl⁻
Chlorine (2, 8, 7) gains that electron and becomes Cl⁻ (2, 8, 8), like argon
3★
Na⁺ + Cl⁻ → NaCl
The oppositely charged ions attract each other: an ionic bond

★ key step

Formation of magnesium chloride

1
Mg → Mg²⁺ + 2e⁻
Magnesium (2, 8, 2) loses two electrons and becomes Mg²⁺ (2, 8)
2
Cl + e⁻ → Cl⁻, for each of two chlorine atoms
Each chlorine (2, 8, 7) gains one electron and becomes Cl⁻ (2, 8, 8)
3★
Mg²⁺ + 2Cl⁻ → MgCl₂
One Mg²⁺ ion holds two Cl⁻ ions

★ key step

How to draw electron dot structures

Write the symbol of each atom, and show only its outer-shell electrons around it.
Use crosses for the metal's electrons and dots for the non-metal's, so you can see which electron moved.
Draw a curved arrow from each outer electron of the metal to the non-metal.
After the arrow, write the ions in square brackets with their charges: [Na]⁺ and [Cl]⁻, with 8 electrons shown around Cl.
For MgCl₂, draw one Mg with two crosses and two Cl atoms. Draw two arrows, then write [Mg]²⁺ and two [Cl]⁻ ions.

Properties of ionic compounds

Property	Reason
Hard solids that are brittle	Strong attraction between ions; under pressure, the layers break apart
High melting and boiling points (NaCl melts at 1074 K)	A lot of energy is needed to overcome the strong attraction between ions
Soluble in water, but insoluble in kerosene and petrol	Water pulls the ions apart; kerosene cannot
Conduct electricity when molten or dissolved	The ions are free to move and carry charge
Do not conduct as solids	The ions are held in fixed positions

In real life

Table salt does not melt even on a hot tawa, because its ions hold together very strongly. Salt water conducts electricity, so wet, sweaty hands near switches are dangerous.

Exam angle

2 marks: “Common salt stays solid on a hot tawa, but a piece of wax melts. Explain this using the kind of bonding in salt.” Salt (NaCl) is an ionic compound. Its oppositely charged ions attract each other strongly, so a lot of energy is needed to separate them. It melts only at 1074 K.

2 marks: “Why do ionic compounds conduct electricity when molten, but not as solids?” In the solid, the ions are held in fixed places. When molten or dissolved, the ions move freely and carry the current.

3 marks: “Show the formation of potassium chloride (KCl) and calcium oxide (CaO) by the transfer of electrons.” KCl: K (2, 8, 8, 1) gives 1 electron to Cl (2, 8, 7), giving K⁺ (2, 8, 8) and Cl⁻ (2, 8, 8). CaO: Ca (2, 8, 8, 2) gives 2 electrons to O (2, 6), giving Ca²⁺ (2, 8, 8) and O²⁻ (2, 8).

Remember it

Metals lose, non-metals gain; opposite charges hold the chain.