A)    Atomic mass

B)    Number of neutrons

C)    Number of valence electrons

D)    Atomic number

Correct Answer: Option D - Atomic number

The atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.

Incorrect Options

Valence electrons are the outermost electrons of an atom, which are important in determining how the atom reacts chemically with other atoms. Atoms with a complete shell of valence electrons (corresponding to an electron configuration s²p⁶) tend to be chemically inert. Atoms with one or two valence electrons more than a closed shell are highly reactive because the extra electrons are easily removed to form positive ions. Atoms with one or two valence electrons less than a closed shell are also highly reactive because of a tendency either to gain the missing electrons and form negative ions, or to share electrons and form covalent bonds.

The atomic weight is basically a measurement of the total number of particles in an atom's nucleus. The atomic weight is actually a weighted average of all of the naturally occurring isotopes of an element relative to the mass of carbon-12.

Number of Neutrons = Atomic Mass - Atomic Number