When calculating total valence electrons, extra electron pairs are added to which atom?
a. central
b. inside
c. outside
d. upper

When calculating total valence electrons, extra electron pairs are added to the Outside of an atom. The answer is letter C. Suppose you have a compound of CCl4. You know that chlorine can only share 1 electron because 7 of its electrons are filled. Also, in carbon, it can only share 4 electrons because 4 of it are already filled. That is why carbon needs four chlorine to form CCl4.

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.