An element’s group tells you how many electrons it loses or gains. From that you can write the ion, the compound formula and a balanced equation for a reaction you have not seen.
This lesson is part of the periodic table section. It follows comparing selected groups using evidence.
What is the prediction chain?
Work through the same four links each time.
- Find the group from the electron arrangement.
- Decide whether the atom loses or gains electrons, and the ion charge.
- Write the compound formula so the charges cancel.
- Balance the equation, showing diatomic molecules such as Cl₂ and O₂.
Worked example: rubidium and chlorine
Rubidium is in group 1, directly below potassium. It has one outer electron, so it forms Rb⁺. Chlorine is in group 17 and forms Cl⁻.
The charges cancel with one of each, so the compound is RbCl. The equation is 2Rb + Cl₂ → 2RbCl. Because rubidium is below potassium, the prediction is that it reacts even more vigorously.
Worked example: calcium and chlorine
Calcium is in group 2. It has two outer electrons, so it forms Ca²⁺. Two Cl⁻ ions are needed to cancel the charge, so the formula is CaCl₂.
The equation is Ca + Cl₂ → CaCl₂, which is already balanced with one calcium and two chlorine atoms on each side.
| Element | Group | Ion | Compound with chlorine |
|---|---|---|---|
| Rb | 1 | Rb⁺ | RbCl |
| Ca | 2 | Ca²⁺ | CaCl₂ |
| Br | 17 | Br⁻ | (with K) KBr |
The mistake that costs marks
The slip is to write Cl instead of Cl₂ in the equation, or to write the compound as RbCl₂ by copying the pattern from calcium. Both come from skipping the charge step.
| Wrong | Why | Correct |
|---|---|---|
| Rb + Cl → RbCl | Chlorine is diatomic | 2Rb + Cl₂ → 2RbCl |
| RbCl₂ | Rb⁺ needs only one Cl⁻ | RbCl |
| CaCl | Ca²⁺ needs two Cl⁻ | CaCl₂ |
Always write the ions and check that the total charge is zero before writing the formula. The atomic structure and periodic position practice tool has more positions to try.
Check yourself
Predict the product when potassium burns in oxygen, and write a balanced equation.
Answer
Potassium is in group 1, so it forms K⁺. Oxygen is in group 16 and forms O²⁻. Two K⁺ cancel one O²⁻, so the oxide is K₂O.
Balanced equation: 4K + O₂ → 2K₂O. Check: four K and two O on each side.
What to study next
Practise the whole chapter together in the periodic table practice set. If the charge step felt weak, go back to relating electron arrangement to group and period.
To work through unfamiliar elements with a teacher, see online one-to-one Chemistry tuition.