The atomic number of an element is the total number of which particles in the nucleus?
neutrons
protons
electrons
protons and electrons
4. all atoms of the same element have the same
a. number of neutrons
b. number of protons
c. mass numbers
d. mass
5. isotopes of the same element have different
a. positions on the periodic table
b. chemical behavior
c. atomic numbers
d. mass numbers

An atom of element has there subatomic particles namely, proton, electron and neutron. Here, for a neutral atom, number of proton is equal to number of electron (this is not in the case of ions), this is equal to atomic number of an atom. In an atom, nucleus contains protons and neutrons which is responsible for mass of the atom and electrons move around nucleus in fixed orbits. Thus, atomic mass of an atom is equal to sum of number of protons and neutrons.

Option (b): Proton is the particle in nucleus of an atom, whose total number is equal to atomic number of that atom.

(4) Option (b): Atoms of same element have same atomic number because mass number can be different for different isotopes of atom. Since, atomic number is equal to number of protons, thus, number of protons are same for all atoms of the same element.

(5) Option (d): Isotopes are defined as atoms of same element with same atomic number but different mass number. Thus, correct option is (d) mass numbers.

the answer is definitely c.