Question

Which of the following order is/are correct : (1) \(\mathrm{O}_{2}^{+}<\mathrm{O}_{2}<\mathrm{O}_{2}^{-}\) (Bond energy) (2) \(\mathrm{BF}_{3}=\mathrm{BCl}_{3}=\mathrm{BBr}_{3}\) (Bond angle) (3) \(\mathrm{N}-\mathrm{H}<\mathrm{P}-\mathrm{H}<\mathrm{As}-\mathrm{H}<\mathrm{Sb}-\mathrm{H}\) (Bond length) (4) \(\mathrm{AICl}_{3}<\mathrm{MgCl}_{2}<\mathrm{CaCl}_{2}\) ( Ionic character)

Short answer

All orders (1) through (4) are correct.

Step-by-step solution

- Bond Energy of Oxygen Species

To determine the bond energy order of oxygen species, consider their molecular orbital (MO) configuration. For \(\text{O}_2^{+}\), \(\text{O}_2\), and \(\text{O}_2^{-}\): \[ \text{O}_2^{+}: (1s^2 \text{1s}^*_2)(2s^2 \text{2s}^*_2)(2p_x^2, 2p_y^2)(\text{2p}_z^2) \] \[ \text{O}_2: (1s^2 \text{1s}^*_2)(2s^2 \text{2s}^*_2)(2p_x^2, 2p_y^2)(\text{2p}_z^2) \] \[ \text{O}_2^{-}: (1s^2 \text{1s}^*_2)(2s^2 \text{2s}^*_2)(2p_x^2, 2p_y^2)(\text{2p}_z^2)(2p_*^1) \] Where * denotes antibonding orbitals. Bond energy decreases as the number of antibonding electrons increases: \(\text{O}_2^{+} > \text{O}_2 > \text{O}_2^{-}\). So, (1) is correct.

- Bond Angle in Boron Trifluoride, Boron Trichloride, and Boron Tribromide

Boranes (\(\text{BF}_3\), \(\text{BCl}_3\), and \(\text{BBr}_3\)) all have the same planar trigonal structure (sp2 hybridization) with bond angles around 120°. Thus, bond angles are equal: \[ \text{BF}_3 = \text{BCl}_3 = \text{BBr}_3 \] So, (2) is correct.

- Bond Length in Hydrides

Bond length increases down the group due to an increasing atomic radius. Therefore, the order is: \(\text{N}-\text{H} < \text{P}-\text{H} < \text{As}-\text{H} < \text{Sb}-\text{H}\). So, (3) is correct.

- Ionic Character in Chlorides

Ionic character is dependent on the difference in electronegativity between the metal and chlorine. As we move down the group in periodic table for Group 2 elements, the ionic character increases: \(\text{AICl}_{3} < \text{MgCl}_{2} < \text{CaCl}_{2}\). So, (4) is correct.

Key concepts

Bond Energy

Bond energy is the measure of the strength of a chemical bond between two atoms. It is the amount of energy required to break one mole of bonds in a gaseous molecule. In the given exercise, we looked at the bond energy orders of different oxygen species: \( \text{O}_2^{+}, \text{O}_2, \text{O}_2^{-} \). The order is determined by their molecular orbital configurations. The more antibonding electrons present, the weaker the bond, hence lower bond energy. Thus, the correct order is \( \text{O}_2^{+} > \text{O}_2 > \text{O}_2^{-} \). Pay attention to the number of antibonding electrons when comparing bond energies among different species.

Bond Angle

Bond angle is the angle between any two bonds that include a common atom, typically measured in degrees. Consider the molecules \( \text{BF}_3, \text{BCl}_3, \text{BBr}_3 \). They all have a trigonal planar structure due to sp2 hybridization, leading to bond angles of approximately 120°. This structure means that the bond angles are equal and do not vary with the differing atoms bonded to boron. Thus, the statement \( \text{BF}_3 = \text{BCl}_3 = \text{BBr}_3 \) accurately reflects the equal bond angles found in these molecules.

Bond Length

Bond length is the average distance between the nuclei of two bonded atoms. It usually increases with the size of the atoms involved. The given order of hydrides \( \text{N}-\text{H} < \text{P}-\text{H} < \text{As}-\text{H} < \text{Sb}-\text{H} \), correctly represents an increase in bond length down the group. As you move down Group 15 in the periodic table from nitrogen to antimony, the atomic radius increases. Consequently, the bonds involving these atoms and hydrogen become longer. This relationship between atomic size and bond length is fundamental in understanding molecular structure.

Ionic Character

Ionic character refers to how much a bond between two atoms resembles an ionic bond rather than a covalent bond. It is influenced by the difference in electronegativity between the atoms. Chlorides of different metals such as \( \text{AICl}_3, \text{MgCl}_2, \text{CaCl}_2 \), have varying ionic characters. As we move down the group in the periodic table for Group 2 elements, the difference in electronegativity between the metal and chlorine typically increases, leading to more ionic character in the bonding. Therefore, the ionic character order \( \text{AICl}_3 < \text{MgCl}_2 < \text{CaCl}_2 \) is correct. Always consider electronegativity differences to determine the degree of ionic character in compounds.