The atomic number of an element is the number of protons found in the nucleus of every atom of that element. Atomic number and is used to distinguish one element from another . The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present).
While, Mass number (atomic mass number or nucleon number), is the total number of protons and neutrons in an atomic nucleus. It is said to be approximately equal to the atomic mass of the atom expressed in atomic mass units.
The longest hydrocarbon chain in the given compound is hexane, therefore it is the parent chain to be considered with one methyl group attached to the 3rd carbon and one chloro attached on the 2nd carbon, therefore the name of the compound is 2-chloro-3-methylhexane
