Give the formulae of the species that will be isoelectronic with the following atoms or ions :
`Cl^-`

Explanation

To determine the species that are isoelectronic with the ion Cl⁻, we will follow these steps:

### Step 1: Determine the number of electrons in Cl⁻
Chlorine (Cl) has an atomic number of 17, which means it has 17 protons and, in its neutral state, 17 electrons. The Cl⁻ ion has gained one additional electron due to its negative charge.

**Calculation:**
- Electrons in Cl = 17
- Electrons in Cl⁻ = 17 + 1 = 18 electrons
...