An ionic bond forms when a metal atom transfers electrons to a non-metal atom. The two atoms become oppositely charged ions, and the attraction between them is the bond.
This lesson is part of SPM Chemistry chemical bonding. You need electron arrangements first, so revise relating electron arrangement to group and period if writing 2.8.2 still feels slow.
What must an ionic bonding diagram show?
A complete diagram answers five checks, in this order.
- The electron arrangement of each atom before the change.
- An arrow or label showing which electrons move, and how many.
- Each ion drawn with its new outer shell, inside square brackets.
- The charge of each ion at the top right of its bracket.
- The number of each ion, so that the total charge is zero.
If any check is missing, a marker has nothing to credit for that step.
Worked example: magnesium and fluorine
Magnesium has proton number 12, so its atom is 2.8.2. Fluorine has proton number 9, so its atom is 2.7.
Magnesium needs to lose two electrons to reach a full second shell. Fluorine needs to gain only one, so one fluorine atom cannot take both. Two fluorine atoms are needed for each magnesium atom.
| Magnesium | Fluorine | |
|---|---|---|
| Atom | 2.8.2 | 2.7 |
| Change | Loses 2 electrons | Each atom gains 1 electron |
| Ion | Mg²⁺ with 2.8 | F⁻ with 2.8 |
| Number needed | 1 | 2 |
Charge check: (+2) + 2 × (−1) = 0. The formula is MgF₂.
In the drawing, put one magnesium atom in the middle and a fluorine atom on each side. Draw one arrow from the magnesium outer shell to each fluorine atom. Then redraw all three as ions in brackets with their charges.
The mistakes that lose marks
These two errors cost the most marks in ionic diagrams.
| Mistake | What goes wrong | Correct version |
|---|---|---|
| Sharing electrons | The student draws overlapping shells as in a covalent bond | Electrons move from one atom to the other, so the shells do not overlap |
| Missing brackets and charges | The ion is drawn as a bare atom with a full shell | Write [Mg]²⁺ and [F]⁻ with the charge outside the bracket |
A third slip is giving magnesium the arrangement 2.8.2 after it has become an ion. The ion has lost its third shell, so it is 2.8.
Check yourself
Show how lithium (proton number 3) and oxygen (proton number 8) form lithium oxide. State the formula.
Answer
Lithium atom: 2.1. Oxygen atom: 2.6.
Each lithium atom loses one electron to become Li⁺ with 2, a full first shell. Oxygen needs two electrons to reach 2.8, so it forms O²⁻.
Two lithium atoms give two electrons, which one oxygen atom accepts. The charge is 2 × (+1) + (−2) = 0, so the formula is Li₂O.
What to study next
Next, look at explaining properties from bonding and structure, which uses the ions you just drew. Then test yourself with the chemical bonding practice set.
If you want a teacher to watch you draw and mark each check, see online one-to-one Chemistry tuition.