Question

The atomic number of an element is determined by:
(A) The number of electrons in an atom
(B) The number of neutrons in one atom
(C) The valency of the element
(D) The number of protons in one atom

Answer 1

Hint: In order to answer this question, try to proceed by defining an atom. Then, describe the subatomic particles of which the tom is made. And, then you will get an idea about the atomic number and mass number of an atom. Try to remember what is the formula used to calculate the atomic number and mass number and you will get your answer.

Complete step by step solution:
An atom is the smallest unit of matter which is composed of three subatomic particles- the proton, the neutron, and the electron. Protons are referred to as the positively charged particles present in the nucleus while electrons are the negatively charged particles that are present in the around the nucleus and neutrons are the neutrally charged particles present in the nucleus along with the protons.
So, protons and neutrons make up the nucleus of the atom, a dense and positively charged core whereas the electrons can be found around the nucleus.
We know that the number of protons equals the number of electrons giving the atom a neutral charge.
The number of protons within the nucleus of an atom is equal to the atomic number of the corresponding element while the sum of the number of protons and neutrons is equal to the mass number of that element.
Thus, Atomic number is equal to the number of protons whereas the mass number is equal to the number of nucleons (the sum of protons and neutrons).
So, from here we can conclude that, if the number of protons is given the atomic number can be determined.

Hence, the correct option is (D).

Note: Atoms having the same number of electrons are referred to as isoelectronic atoms. Atoms of the same element having different numbers of neutrons are referred to as isotopes whereas atoms of different chemical elements having the same number of nucleons (i.e. same number of protons and neutrons) are referred to as isobars.