Trends in Ionization Energy
Down a Group
The trend in first ionization energy down group 2 and other groups is generally decreasing. This is due to:
- Increasing atomic radius
- Enhanced shielding effect from additional electron shells
Highlight: Although nuclear charge increases down a group, the effect of larger atomic radius and increased shielding outweighs this, resulting in lower ionization energies.
Across a Period
The ionization energy across period 3 and other periods generally increases. This trend is explained by:
- Increasing nuclear charge
- Relatively constant atomic radius
- Minimal change in shielding effect
Highlight: The increasing nuclear charge across a period leads to stronger attraction between the nucleus and outer electrons, making it harder to remove electrons.
Anomalies in Periodic Trends
There are two notable anomalies in the trend of ionization energy across period 2 and other periods:
- Small drop between groups 2 and 3 (e.g., Mg to Al)
- Small drop between groups 5 and 6 (e.g., P to S)
These anomalies are explained by subtle differences in electron configurations and orbital energies.
Example: The drop from Mg to Al is due to Al's outer electron being in the slightly higher energy 3p orbital, which is further from the nucleus and experiences additional shielding from 3s electrons.
Understanding these trends and anomalies is crucial for OCR Chemistry A Level students preparing for exams and OCR Chemistry A Level past papers.
