Question

Which statement is not correct.
A. Dipole moment: $ C{H_3}F > C{H_3}Cl > C{H_3}Br > C{H_3}I $
B. Bond energy: $ {N_2} > {O_2} > {H_2} > C{l_2} $
C. Bond polarity: $ H - F > H - Cl > H - Br > H - I $
D. Covalent character: $ AlC{l_3} > BeC{l_2} > CaC{l_2} $

Answer 1

Hint :A covalent bond can be categorized on the basis of various bond parameters like bond length, bond strength, bond enthalpy (also known as bond energy), bond polarity and bond order. These bond parameters give us a perception about stability of a chemical compound and the strength by which these chemical bonds hold the atoms together in a compound.

Complete Step By Step Answer:
Let us discuss each property or bond parameter given in options separately.
Dipole moment: It is a measure of polarity of chemical bond among two atoms in a molecule. In covalent compounds, it arises due to the electronegativity difference between two covalently bonded atoms. As it has both magnitude and direction, so it is a vector quantity.
Now, the given decreasing order for the dipole moment of primary alkyl halides is as follows:
 $ C{H_3}F > C{H_3}Cl > C{H_3}Br > C{H_3}I $
Although fluorine is more electronegative than chlorine but $ C{H_3}Cl $ has a larger dipole moment than $ C{H_3}F $ because the dipole moment depends upon two factors i.e., distance between the atoms and charge. As the carbon-fluorine bond is much shorter than carbon-chlorine bond. So, chloromethane has greater dipole moment than fluoromethane. Therefore, the correct decreasing order of dipole moment is as follows:
 $ C{H_3}Cl > C{H_3}F > C{H_3}Br > C{H_3}I $
Bond energy: It is the amount of energy required to break a chemical compound into its individual elements. It is a measure of the strength of a chemical bond. Greater the bond energy of a compound, the more is the strength of its chemical bond.
Now, the given decreasing order of bond energy for diatomic molecules is as follows:
 $ {N_2} > {O_2} > {H_2} > C{l_2} $
Dinitrogen will have the highest bond energy due to the presence of a triple bond, then dioxygen consists of a double bond so it will be placed at second position. Among $ {H_2} $ and $ C{l_2} $ , dihydrogen will have greater bond energy because $ 1s - 1s $ overlapping of orbital takes place which is much stable than overlapping in chlorine i.e., $ 3p - 3p $ overlapping. Hence, the given order of bond energy is correct.
Bond polarity: It is the separation of charge along a chemical bond. Greater the electronegativity difference of atoms, the greater will be its bond polarity. The decreasing order of electronegativity of halogens is as follows:
 $ F > Cl > Br > I $
Hence, the order for bond polarity in hydrogen halides will be as follows:
 $ H - F > H - Cl > H - Br > H - I $
Covalent character: According to Fajan’s rule, smaller the size of a cation, greater will be its covalent character. The order for the size of cations in given molecules is as follows:
 $ A{l^{3 + }} < B{e^{2 + }} < C{a^{2 + }} $
Hence the order of covalent character is as follows:
 $ AlC{l_3} > BeC{l_2} > CaC{l_2} $
Therefore, the incorrect statement is Dipole moment: $ C{H_3}F > C{H_3}Cl > C{H_3}Br > C{H_3}I $
Thus, option (A) is the correct answer.

Note :
It is important to note that the size of ions can be compared on the basis of the charge present on the atom. For cations, greater the charge on the atom, the smaller will be the size of cations whereas for anions, greater the charge on the atom, the larger will be the size of anions.