Which of the following are true for an element?

(i) Atomic number = number of protons + number of electrons

(ii) Mass number = number of protons + number of neutrons

(iii) Atomic mass = number of protons = number of neutrons

(iv) Atomic number = number of protons = number of electrons

All the atoms of an element have the same atomic number. Atomic number is defined by the number of protons present in an element. Also, a number of protons is the same as the number of electrons in an element. Therefore, statement 4 is correct. Mass of an atom is due to the protons and neutrons alone. Hence, mass number = number of protons +number of neutrons.