Account for the decrease in first ionization energy between phosphorous and sulphur.

Explanation

To account for the decrease in first ionization energy between phosphorus (P) and sulfur (S), we can follow these steps:

### Step 1: Understand Ionization Energy
First, we need to define what ionization energy is. The first ionization energy is the energy required to remove the outermost electron from a neutral atom in its gaseous state.

### Step 2: Examine Electronic Configurations
Next, we look at the electronic configurations of phosphorus and sulfur:
- Phosphorus (atomic number 15) has the electronic configuration: \(1s^2 2s^2 2p^6 3s^2 3p^3\).
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