To identify the correct order of the size of the given species (Ca²⁺, K⁺, Ar, Cl⁻, and S²⁻), we will follow these steps:
### Step 1: Determine the number of electrons in each species.
- **Ca²⁺**: Calcium has an atomic number of 20, so it has 20 protons and, after losing 2 electrons, it has 18 electrons.
- **K⁺**: Potassium has an atomic number of 19, so it has 19 protons and, after losing 1 electron, it has 18 electrons.
- **Ar**: Argon has an atomic number of 18, so it has 18 protons and 18 electrons.
- **Cl⁻**: Chlorine has an atomic number of 17, so it has 17 protons and, after gaining 1 electron, it has 18 electrons.
- **S²⁻**: Sulfur has an atomic number of 16, so it has 16 protons and, after gaining 2 electrons, it has 18 electrons.
### Step 2: Identify the isoelectronic species.
All the species (Ca²⁺, K⁺, Ar, Cl⁻, and S²⁻) have the same number of electrons (18), making them isoelectronic.
### Step 3: Analyze the effect of nuclear charge on size.
The size of isoelectronic species is influenced by the number of protons (nuclear charge):
- **Ca²⁺**: 20 protons
- **K⁺**: 19 protons
- **Ar**: 18 protons
- **Cl⁻**: 17 protons
- **S²⁻**: 16 protons
As the number of protons increases, the effective nuclear charge increases, which pulls the electrons closer to the nucleus and results in a smaller ionic radius.
### Step 4: Arrange the species in order of size.
Based on the number of protons:
- **Smallest size**: Ca²⁺ (20 protons)
- **Next smallest**: K⁺ (19 protons)
- **Next**: Ar (18 protons)
- **Next**: Cl⁻ (17 protons)
- **Largest size**: S²⁻ (16 protons)
### Final Order of Size:
The correct order of size from smallest to largest is:
**Ca²⁺ < K⁺ < Ar < Cl⁻ < S²⁻**
