Describe ionic bonding as the transfer of electrons, work out the formula of an ionic compound, and relate the giant ionic structure to its properties

What this dot point is asking

The syllabus wants you to describe ionic bonding as the transfer of electrons from a metal to a non-metal, to work out the formula of an ionic compound from the charges on its ions, and to link the giant ionic lattice to the properties of ionic compounds, high melting points, brittleness, and conducting electricity only when molten or dissolved. The central idea is that atoms transfer electrons to reach full outer shells, forming charged ions that attract each other strongly.

The answer

How ionic bonds form

Atoms are most stable with a full outer shell of electrons. A metal atom has a few outer electrons and loses them to become a positive ion. A non-metal atom is short of a full shell and gains electrons to become a negative ion. The electrons are transferred from the metal to the non-metal.

The oppositely charged ions then attract each other strongly. This strong electrostatic attraction between positive and negative ions is the ionic bond.

Working out the charge on an ion

The charge usually matches the group:

• Group I metals form $1+$ ions (such as $\text{Na}^{+}$).
• Group II metals form $2+$ ions (such as $\text{Mg}^{2+}$).
• Group VII non-metals form $1-$ ions (such as $\text{Cl}^{-}$).
• Group VI non-metals form $2-$ ions (such as $\text{O}^{2-}$).

Working out the formula

The compound must be neutral overall, so the total positive charge balances the total negative charge. Combine the ions so the charges cancel:

• $\text{Na}^{+}$ with $\text{Cl}^{-}$ gives $\text{NaCl}$ (one each).
• $\text{Mg}^{2+}$ with $\text{Cl}^{-}$ needs two chlorides: $\text{MgCl}_2$.
• $\text{Na}^{+}$ with $\text{O}^{2-}$ needs two sodiums: $\text{Na}_2\text{O}$.

The giant ionic lattice and its properties

An ionic compound is not made of separate molecules. The ions are arranged in a huge, regular three-dimensional pattern called a giant ionic lattice, held together by strong forces throughout. This explains:

• High melting and boiling points: a lot of energy is needed to overcome the strong attractions between many ions.
• Conducts only when molten or dissolved: the ions can move and carry charge only when free; in the solid they are fixed.
• Brittle: a knock can line up like charges, which repel and split the crystal.

Examples in context

Example 1. Table salt as a giant lattice. Common salt, sodium chloride, is held in a giant lattice of $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions. The many strong attractions are why it is a hard, high-melting solid, and why it only conducts electricity once it is melted or dissolved in water, the basis of electrolysis later in the course.

Example 2. Why magnesium oxide lines furnaces. Magnesium oxide, $\text{MgO}$, has $2+$ and $2-$ ions, so the attractions are even stronger than in sodium chloride and its melting point is extremely high. This is why it is used to line furnaces that must withstand great heat.

Try this

Q1. Write the formula of the ion formed by a potassium atom (Group I) and by an oxygen atom (Group VI). [2 marks]

• Cue. Potassium loses one electron to form $\text{K}^{+}$; oxygen gains two electrons to form $\text{O}^{2-}$.

Q2. Work out the formula of potassium oxide. [2 marks]

• Cue. Two $\text{K}^{+}$ ions are needed to balance one $\text{O}^{2-}$ ion, so the formula is $\text{K}_2\text{O}$.

Q3. Explain why molten sodium chloride conducts electricity but solid sodium chloride does not. [2 marks]

• Cue. When molten the ions are free to move and carry charge; when solid the ions are held in fixed positions in the lattice and cannot move, so it does not conduct.