Ionization Energy - Chemistry

1. Fundamental Concepts

Ionization energy is defined as the minimum energy required for a gaseous atom in its ground state to lose one outermost electron and form a gaseous cation, with the unit of kJ/mol. It reflects the ease with which an atom loses electrons: a lower ionization energy means the atom loses electrons more easily and has stronger metallic character; conversely, a higher ionization energy indicates greater difficulty in losing electrons and stronger non-metallic character.

2. Key Concepts

First vs. Subsequent Ionization Energies
- The first ionization energy () is the energy for a gaseous ground-state atom to lose its first electron.
- The second ionization energy () is the energy for the resulting gaseous cation to lose an additional electron. For any element, , and subsequent ionization energies increase sequentially.
Periodic Trends
- Across a period (left to right): Generally increases (due to increasing nuclear charge and decreasing atomic radius, which strengthen the attraction for valence electrons). Anomalies exist (e.g., ⅡA > ⅢA, ⅤA > ⅥA) due to the extra stability of fully filled or half-filled electron subshells.
- Down a group (top to bottom): Decreases (due to increasing atomic radius, which weakens the nuclear attraction for valence electrons).
Influencing Factors
- Nuclear charge (positive correlation)
- Atomic radius (negative correlation)
- Electron configuration (fully filled, half-filled, or empty subshells lead to higher ionization energy)

3. Examples

Easy

Compare the first ionization energy of Na, Mg, and Al.
Analysis: All three elements are in Period 3, so ionization energy generally increases from left to right due to increasing nuclear charge.
Sodium has a single 3s valence electron, which is easily removed, giving it the lowest first ionization energy.
Magnesium has a fully filled 3s subshell, which is relatively stable, so its first ionization energy is higher.
Aluminum’s outer electron is in a higher-energy 3p orbital, making it easier to remove than magnesium’s 3s electron. Therefore, the order is Mg > Al > Na.
Answer: Mg > Al > Na

Medium

Explain why the first ionization energy of N is higher than that of O.
Analysis: Nitrogen has a half-filled 2p subshell, which is especially stable, so more energy is required to remove an electron. Oxygen has one paired 2p electron, and electron–electron repulsion makes it easier to remove an electron, resulting in a lower first ionization energy.
Conclusion:

Hard

The ionization energy data of an element is as follows (kJ/mol): , , , . Determine the group of this element in the periodic table.
Analysis: A significant jump occurs between and , indicating that the element reaches a stable electron configuration after losing 3 valence electrons.
Conclusion: The element belongs to Group ⅢA.

4. Problem-Solving Techniques

Comparing Ionization Energy Magnitudes
First, determine the element’s period and group position, then consider the impact of special electron configurations (fully filled/half-filled anomalies).
Analyzing Ionization Energy Jumps
The position of a sharp increase in ionization energy corresponds to the number of valence electrons (e.g., a jump at the n-th ionization energy means the atom has valence electrons).
Inferring Element Properties
Low ionization energy → strong metallic character, tendency to form cations; High ionization energy → strong non-metallic character, difficulty in losing electrons.