1.6.4Principles of Chemistry
Ionic Bonds: Dot and Cross Diagrams — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 37 of 50
Part 1 of 5What ionic bonding is
Electron transfer
- Electron transfer Ionic bonds form between a metal and a non-metal.
- The metal atom gives up its outer electrons and the non-metal atom accepts them.
Stable ions
- Stable ions Both atoms end up with full outer shells, as positive and negative ions.
Electrostatic attraction
- Electrostatic attraction The strong force of attraction between oppositely charged ions is the ionic bond.
- It acts in all directions, holding the ions in a giant lattice.
Part 2 of 5Drawing a dot and cross diagram
1
Draw the outer shell of each atom as a circle, using dots for the electrons of the metal and crosses for the non-metal so the transfer can be followed.
2
Show the number of outer electrons from the group number: sodium has 1, chlorine has 7.
3
Draw arrows, or just redraw the outer shells, to show the electrons moving from metal to non-metal.
4
Redraw each ion in square brackets with its charge at the top right. The metal ion usually shows no outer shell, because its next shell down is full.
5
Check that the charges balance and each ion has a complete outer shell.
Part 3 of 5Common examples
| Compound | What happens to electrons | Ions formed | Ratio |
|---|---|---|---|
| Sodium chloride, NaCl | Na gives 1 electron to Cl | Na+ (2,8) and Cl− (2,8,8) | 1 : 1 |
| Magnesium oxide, MgO | Mg gives 2 electrons to O | Mg2+ (2,8) and O2− (2,8) | 1 : 1 |
| Magnesium chloride, MgCl2 | Mg gives one electron to each of two Cl atoms | Mg2+ and two Cl− | 1 : 2 |
| Sodium oxide, Na2O | Each of two Na atoms gives 1 electron to one O atom | Two Na+ and O2− | 2 : 1 |
| Calcium fluoride, CaF2 | Ca gives one electron to each of two F atoms | Ca2+ and two F− | 1 : 2 |
Part 4 of 5Reading a diagram: example for MgCl2
1Magnesium atom
- Magnesium atom Arrangement 2,8,2 with two outer electrons, shown as dots.
2Two chlorine atoms
- Two chlorine atoms Each is 2,8,7 with seven outer electrons, shown as crosses, needing one more each.
3Transfer
- Transfer One magnesium electron goes to each chlorine atom.
4Final picture
- Final picture Mg2+ with no outer electrons shown, and two [Cl]− each with eight outer electrons, one of them a transferred dot.
Part 5 of 5What the diagram does and does not show
Strengths
- Strengths It shows clearly which atom loses electrons, which gains them, and how many.
Limitations
- Limitations It shows only one pair or small group of ions, not the giant three-dimensional lattice.
- It does not show the relative sizes of the ions or how far apart they are.
Exam detail
- Exam detail Always include the brackets and the charge on each ion, and make sure the outer shell electrons are all drawn.
Key terms
| Term | Meaning |
|---|---|
| Ionic bond | The electrostatic attraction between oppositely charged ions. |
| Dot and cross diagram | A diagram that uses dots and crosses to show the outer electrons of different atoms. |
| Electron transfer | The movement of electrons from one atom to another so that both gain full outer shells. |
| Outer shell | The shell that contains the highest-energy electrons, used in bonding. |
| Ion | A charged particle formed when an atom gains or loses electrons. |
Avoid these mistakes
1
Showing electrons shared between the atoms in an ionic compound; they are transferred, not shared.
2
Leaving out the square brackets or charge on the final ions.
3
Drawing the sodium ion with its outer electron still present. It has been lost, so the outer shell is now the full 2,8.
4
Using only one chlorine atom for magnesium chloride. Two are needed to take the two electrons.
Last updated:
\n