Ionic bond: transfer of electrons
A metal atom transfers electrons to a non-metal atom. The positive and negative ions formed are held together by electrostatic attraction: the ionic bond.
Formation of sodium chloride
- 1Na → Na⁺ + e⁻Sodium (2, 8, 1) loses its one outer electron. Na⁺ (2, 8) has 11 protons but only 10 electrons. This step needs 496 kJ mol⁻¹
- 2Cl + e⁻ → Cl⁻Chlorine (2, 8, 7) gains that electron. Cl⁻ (2, 8, 8) is like argon. This step gives out 349 kJ mol⁻¹
- 3★Na⁺ + Cl⁻ → Na⁺Cl⁻, written NaClThe oppositely charged ions attract each other: an ionic bond
- 4Overall: 2Na(s) + Cl₂(g) → 2NaCl(s)Check: 2 Na and 2 Cl on each side
★ key step
Where does the extra energy come from?
Removing sodium's electron takes 496 kJ mol⁻¹, but chlorine gives back only 349 kJ mol⁻¹. That leaves a shortfall of 147 kJ mol⁻¹. When the ions pack together into a crystal, much more energy is released. This pays for the shortfall, so solid sodium chloride is stable. In the crystal, each Na⁺ ion has six Cl⁻ ions around it, and each Cl⁻ has six Na⁺ ions. So there is no separate NaCl molecule. NaCl is only a formula that shows one Na⁺ for every Cl⁻.
Formation of magnesium chloride, MgCl₂
- 1Mg → Mg²⁺ + 2e⁻Magnesium (2, 8, 2) loses two electrons and becomes Mg²⁺ (2, 8)
- 22Cl + 2e⁻ → 2Cl⁻Each of two chlorine atoms (2, 8, 7) gains one electron and becomes Cl⁻ (2, 8, 8)
- 3★Mg²⁺ + 2Cl⁻ → MgCl₂One Mg²⁺ ion holds two Cl⁻ ions
★ key step
Formation of magnesium oxide, MgO
- 1Mg → Mg²⁺ + 2e⁻Magnesium (2, 8, 2) loses two electrons and becomes Mg²⁺ (2, 8)
- 2O + 2e⁻ → O²⁻Oxygen (2, 6) gains both electrons and becomes the oxide ion O²⁻ (2, 8), like neon
- 3★Mg²⁺ + O²⁻ → MgOA dipositive ion and a dinegative ion attract: magnesium oxide
★ key step
How to draw electron dot pictures of ionic compounds
- Write each symbol and show only its outer-shell electrons around it.
- Use crosses for the metal's electrons and dots for the non-metal's, so the moved electron is easy to see.
- Draw a curved arrow from each outer electron of the metal to the non-metal.
- NaCl: Na with one cross, Cl with seven dots. After the arrow, write [Na]⁺ and [Cl]⁻ with eight electrons (seven dots and one cross) around Cl.
- MgO: Mg with two crosses, O with six dots, and two arrows. Then write [Mg]²⁺ and [O]²⁻ with eight electrons around O.
- Always write the charges on the ions. Missing charges lose the mark.
Ions you should know
| Atom (configuration) | Change | Ion (configuration) |
|---|---|---|
| Li (2, 1) | Loses 1 electron | Li⁺ (2) |
| K (2, 8, 8, 1) | Loses 1 electron | K⁺ (2, 8, 8) |
| Ca (2, 8, 8, 2) | Loses 2 electrons | Ca²⁺ (2, 8, 8) |
| F (2, 7) | Gains 1 electron | F⁻ (2, 8) |
| S (2, 8, 6) | Gains 2 electrons | S²⁻ (2, 8, 8) |
| N (2, 5) | Gains 3 electrons | N³⁻ (2, 8) |
The salt made in the salt pans of Gujarat and Tamil Nadu is sodium chloride, an ionic solid. Magnesium oxide is the white ash left when a magnesium ribbon burns in the school laboratory.
2 marks: “A sodium atom becomes a sodium ion. Write the change with configurations, and give the number of protons and electrons in the ion.” Na (2, 8, 1) → Na⁺ (2, 8) + e⁻. The ion has 11 protons and 10 electrons, so it carries a charge of +1.
3 marks: “Elements A (atomic number 12) and B (atomic number 8) combine. Write their configurations, the kind of bond formed and the formula of the compound, showing the electron transfer.” A is 2, 8, 2 (magnesium) and loses 2 electrons; B is 2, 6 (oxygen) and gains 2. The bond is ionic. Mg → Mg²⁺ + 2e⁻; O + 2e⁻ → O²⁻; formula AB (MgO).
5 marks: “Describe the three steps in forming an ionic bond, using sodium chloride. Why is NaCl stable even though making Na⁺ needs more energy than making Cl⁻ gives out?” Steps: Na loses 1 electron (needs 496 kJ mol⁻¹); Cl gains it (gives out 349 kJ mol⁻¹); Na⁺ and Cl⁻ attract. The shortfall of 147 kJ mol⁻¹ is paid by the larger energy released when the ions pack into a crystal, so solid NaCl is stable.
Metals lose and turn plus; non-metals gain and turn minus; opposite charges hold on.