Which of these pairs of elements have the same number of valence electrons? phosphorus (p) and chlorine (cl) berylium (be) and calcium (ca) chlorine (cl) and helium (he) oxygen (o) and carbon (c)

Option (b) is the correct answer.

Explanation:

It is known that elements of same group tend to possess same number of valence electrons. As a result, they show same chemical properties.

Phosphorous (P) and chlorine (Cl) belongs to third period. Valence electrons of phosphorous are 5 and chlorine has 7 valence electrons.

Beryllium (Be) and Calcium (Ca) are group 2 elements and both of them have two valence electrons.

Helium (He) is a noble gas and it does not have any valence electrons.

Oxygen (O) has six valence electrons and carbon (C) has 4 valence electrons.

Therefore, we can conclude that berylium (Be) and calcium (Ca) is the pairs of elements have the same number of valence electrons.

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.