Question

Boron differs from the other clements of its own group in several properties. The anomalous behaviour of boron may be attributed to (1) its small size and high ionization energy. (2) its high electronegativity. (3) the presence of only two electrons in the penultimate shell and the absence of $d$ orbitals in the valence shell. (4) All of the above.

Short answer

The anomalous behavior of boron is due to its small size, high ionization energy, high electronegativity, and unique electron configuration. Hence, the answer is (4) All of the above.

Step-by-step solution

- Understanding Anomalous Behavior

Consider why boron exhibits different properties compared to other elements in its group. This is often due to differences in atomic structure and electron configuration.

- Examining the Options

Review each given option and see if it can explain the anomalous behavior of boron:(1) Small size and high ionization energy(2) High electronegativity(3) Electrons in the penultimate shell and absence of d orbitals(4) All of the above

- Small Size and High Ionization Energy

Boron has a smaller atomic radius and higher ionization energy compared to other elements in its group, which affects its reactivity and bonding.

- High Electronegativity

Boron also has a higher electronegativity compared to its group counterparts, influencing its chemical properties and behavior.

- Electron Configuration

The presence of only two electrons in the penultimate shell and the absence of d orbitals in the valence shell makes boron's electron configuration unique within its group.

- Conclusion

Summarize that since all the provided factors (small size, high ionization energy, high electronegativity, and unique electron configuration) contribute to the anomalous behavior of boron, the most appropriate answer is (4) All of the above.

Key concepts

Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom or ion in its gaseous state. For boron, this energy is relatively high compared to other elements in its group. This high ionization energy is partly due to boron's small atomic size.
Because the outer electrons are close to the nucleus, they are held more tightly and thus require more energy to be removed.
The high ionization energy affects boron's chemical reactivity, making it less reactive compared to its group counterparts.
Therefore, compounds of boron are generally more stable, and boron itself does not easily lose electrons to form positive ions.

Electronegativity

Electronegativity refers to the tendency of an atom to attract electrons towards itself when it forms a chemical bond. Boron has a significantly higher electronegativity than other elements in its group.
This high electronegativity makes boron more likely to attract electrons in bonding situations, which influences its chemical properties.
It can lead to the formation of covalent bonds rather than ionic bonds. In compounds, boron tends to share electrons rather than transferring them, which can significantly impact the nature and stability of these compounds.

Electron Configuration

The electron configuration of an atom describes the distribution of electrons in its atomic orbitals. Boron has an electron configuration of 1s² 2s² 2p¹. One unique aspect of boron's electron configuration is the presence of only two electrons in the penultimate shell and the absence of d orbitals in the valence shell.
This makes it very different from other elements in Group 13 that have more complex electron configurations involving d orbitals.
The fewer electrons in higher energy levels mean that boron does not follow some of the same trends in chemical properties that are observed with heavier elements.

Atomic Radius

The atomic radius is a measure of the size of an atom, typically the distance from the center of the nucleus to the outer boundary of the surrounding electrons. Boron has a remarkably small atomic radius.
This small size can be attributed to the fewer number of electron shells compared to other elements in the same group.
The small atomic radius means that the nuclear charge is more effective in pulling the electrons closer. This contributes to boron's high ionization energy and electronegativity.
Also, the compact size allows boron to form stable, strong bonds with other elements.

Chemical Properties

Chemical properties define how a substance interacts with other substances. Because of its high ionization energy, high electronegativity, unique electron configuration, and small atomic radius, boron exhibits several chemical behaviors that are quite different from its group members.
For instance, boron typically forms covalent bonds, not ionic bonds, which is unusual for elements in Group 13.
Additionally, boron compounds like boranes and borates display unique chemistry that is not mirrored by aluminum or gallium compounds.
These distinct chemical properties also contribute to its usage in various industries, from glass-making to electronics.