Predict the formulae of the stable binary compounds that would be formed by the following pairs of elements
(a) calcium and oxygen (b) aluminium and carbon.

Explanation

To predict the formulae of the stable binary compounds formed by the given pairs of elements, we need to follow these steps:

### Step 1: Determine the Valency of Each Element
- **Calcium (Ca)**: Calcium is in Group 2 of the periodic table, and it has a valency of +2 because it tends to lose two electrons to achieve a stable electronic configuration.
- **Oxygen (O)**: Oxygen is in Group 16 and has a valency of -2 because it tends to gain two electrons to achieve a stable electronic configuration.

### Step 2: Combine the Valencies to Form the Compound
- When forming a binary compound, the total positive charge must balance the total negative charge.
- For calcium and oxygen, the charges are +2 (from Ca) and -2 (from O).
- Since the charges are equal and opposite, they combine in a 1:1 ratio.

### Step 3: Write the Formula
- The formula for the compound formed by calcium and oxygen is **CaO**.

### Step 4: Repeat for the Second Pair of Elements
- **Aluminium (Al)**: Aluminium is in Group 13 and has a valency of +3 because it tends to lose three electrons.
- **Carbon (C)**: Carbon is in Group 14 and has a valency of -4 because it can either gain or lose four electrons, but typically it shares electrons in covalent bonding.

### Step 5: Combine the Valencies for Aluminium and Carbon
- To balance the charges, we can use the least common multiple of the valencies.
- The least common multiple of +3 (from Al) and -4 (from C) is 12.
- Therefore, we need 4 Aluminium ions (+3 each) and 3 Carbon ions (-4 each) to balance the charges.

### Step 6: Write the Formula for Aluminium and Carbon
- The formula for the compound formed by aluminium and carbon is **Al4C3**.

### Final Answers:
- (a) The formula for the compound formed by calcium and oxygen is **CaO**.
- (b) The formula for the compound formed by aluminium and carbon is **Al4C3**.

---