Question

The single covalent bond energy of carbon with A, B, C and D atoms are $240,380,270,480{\text{kcalmo}}{{\text{l}}^{ - 1}}$. Thus the smallest atom among the A, B, C and D is:
A. A
B. B
C. C
D. D

Answer 1

Bond making and breaking involves energy changes. This energy is called bond energy. More the bond energy, smaller is an atom. Bond energy, also called the mean bond enthalpy or average bond enthalpy is the measure of bond strength in a chemical bond.

Complete step by step answer:
All chemical reactions involve breaking the old bonds and making new bonds. The heat absorbed or released in a reaction comes from the chemical bonds being broken or made respectively. Greater the bond energy, stronger is the bond. Bond dissociation energy is also a measure of bond strength in a chemical bond. It is the change in enthalpy of a homolysis reaction at $0{\text{K}}$ . It is not the same as average bond energy. Electron density between atoms increases the nuclear attraction.
Size of the atoms makes a difference. When the atomic radii of bonding atoms increases, bond length increases and bond energy decreases. For example, bond energy between carbon and chlorine is higher than that with bromine or iodine. This is because chlorine is smaller in size than bromine and iodine. Bond energy decreases with increase in the atomic size.
Here atom D has higher energy than A, B and C. Therefore D is the smallest atom.
Therefore option D is correct.

Additional information- The energy required to break the bond between two covalently bonded atoms is called bond dissociation energy. Bond length is also related with bond energy. Single bond is longest while triple bond is shortest. Single bond has lower energy and a triple bond has higher energy.

Note:
Bond length is measured using X-ray crystallography. Bond energy is experimentally determined by measuring the heat required to break a bond. Enthalpy of a compound’s formation is determined by bond energy. At a large distance between the atoms there is no interaction. Thus there is no energy.