The ionization energy of boron is less than that of beryllium because:

Options

A. Beryllium has a higher nuclear charge than boron
B. beryllium has a lower nuclear charge than boron
C. the outermost electron in borom occupies a 2p-orbital
D. the 2s and 2p-orbitals of boron are degenerate

Answer

To answer the question regarding why the ionization energy of boron is less than that of beryllium, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy is defined as the energy required to remove an electron from an atom in its gaseous state. It is influenced by the nuclear charge and the distance of the outer electrons from the nucleus.

**Hint**: Remember that ionization energy is related to how tightly electrons are held by the nucleus.

2. **Electronic Configurations**:
- Beryllium (Be) has an atomic number of 4, and its electronic configuration is \(1s^2 2s^2\).
- Boron (B) has an atomic number of 5, and its electronic configuration is \(1s^2 2s^2 2p^1\).

**Hint**: Write down the electronic configurations to visualize the distribution of electrons.

3. **Position of Outer Electrons**:
- In beryllium, the outermost electrons are in the 2s orbital, while in boron, the outermost electron is in the 2p orbital. The 2s orbital is closer to the nucleus than the 2p orbital.

**Hint**: Consider how the distance from the nucleus affects the attraction between the nucleus and the outer electrons.

4. **Nuclear Charge Effect**:
- Beryllium has a higher effective nuclear charge acting on its outer electrons compared to boron. This is because beryllium has fewer electron shells and its outer electrons are more strongly attracted to the nucleus.

**Hint**: Recall that effective nuclear charge increases with the number of protons and decreases with electron shielding.

5. **Comparison of Ionization Energies**:
- Since beryllium has its outer electrons in the 2s orbital, they are held more tightly due to their proximity to the nucleus, resulting in a higher ionization energy. In contrast, boron's outer electron in the 2p orbital is held less tightly, leading to a lower ionization energy.

**Hint**: Think about how the energy required to remove an electron changes based on its orbital type (s vs. p).

6. **Conclusion**:
- Therefore, the correct reason for the lower ionization energy of boron compared to beryllium is that the outermost electron of boron occupies a 2p orbital, which is further from the nucleus and experiences less nuclear attraction than the 2s electrons of beryllium.

**Hint**: Summarize your findings to clearly state the main reason for the difference in ionization energies.

### Final Answer:
The ionization energy of boron is less than that of beryllium primarily because the outermost electron of boron occupies a 2p orbital, which is further from the nucleus compared to the 2s orbital of beryllium, resulting in less nuclear attraction and thus lower ionization energy.