Question

The first ionisation energy of oxygen is less than that of nitrogen. Which of the following is the correct reason for this observation?

• A. Lesser effective nuclear charge of oxygen than nitrogen
• B. Lesser atomic size of oxygen than nitrogen
• C. Due to stability of half-filled electronic configurations
• D. Greater effective nuclear charge of oxygen than nitrogen

Answer 1

The correct option is C Due to stability of half-filled electronic configurations

As we move from left to right along the period, ionization energy increases due to increase in effective nuclear charge. Hence, ionization energy generally increases as we move from left to right. However, stability of half and completely filled electronic configuration can affect the trend.
Nitrogen has half-filled electronic configuration and hence its ionization energy is greater than oxygen. In oxygen after it loses its electron, it goes to half-filled electronic configuration and hence, it has less ionization energy than nitrogen.