Question

Which of the following properties of the elements chlorine, bromine and iodine increase with increasing atomic number?
A.Ionization energy
B.Ionic radius
C.Bond energy of the molecule $${\text{X}}_2$$
D.Enthalpy of vaporization

Answer 1

Hint: On moving down the group, atomic size increases as new shells are added to the atom. The greater the size of the atom lesser will be its electronegativity.

Complete step by step answer:
Chlorine, bromine and iodine belong to group number 17 or halogen family and belong to period 3, 4 and 5 respectively. They all have 7 valence electrons.
Ionization energy is defined as the minimum amount of energy required to remove the most loosely bounded outermost electron of a gaseous isolated atom. Removal of electrons from a neutral atom is always an endothermic process. More attractive the nucleus with the outermost electron, more will it require to release that electron, more will be ionization energy. On moving down the group, from chlorine to iodine, the number of shells increases and shielding effect increases which reduces the bond strength of the outermost electron with the nucleus causing decrease in ionization energy on moving down the group.
Radius of atom or ion on moving across the period decreases with increase of effective nuclear charge and on moving down the group, atomic or ionic radius increases with increasing number of shells in an atom. In the halogen family on moving down the group from chlorine to iodine, atomic and ionic radius increases with increasing number of shells and shielding effect in halogen atom.
Bond energy of halogens is defined as the amount of energy required to break bonds between halogen atoms. On moving down the group from chlorine to iodine, however with increase in atomic size intermolecular attraction increases causing stronger bond between halogen atoms but due to stronger overlapping of orbital; bond energy decreases along the halogens. Overlapping between 3p-3p orbital (as in chlorine) is stronger than 4p-4p (as in bromine) and 5p-5p orbital (as in iodine). A strong overlapping between the orbitals makes it difficult to break causing high bond energy.
Enthalpy of vaporisation is defined as the amount of energy required to convert the given amount of liquid into gas. Enthalpy of vaporization is also known as heat of vaporization. It is a function of pressure. On moving down the group along halogens, enthalpy of vaporization increases.

Thus, the correct options are B and D.

Note:
Halogens are coloured because of excitation between the highest occupied molecular orbital and lowest occupied molecular orbital. Halogens during excitation absorb light and appear as coloured. For example fluorine is pale yellow in colour and chlorine shows greenish colour. Reddish brown colour of bromine and dark violet colour of iodine has been observed. Halogens show a variety of colours when dissolved in different solvents.