Question

Which of the following is equal to the mass number of a nucleus?
${\text{A}}{\text{.}}$ Number of electrons it contains
${\text{B}}{\text{.}}$ Number of protons it contains
${\text{C}}{\text{.}}$ Number of neutrons it contains
${\text{D}}{\text{.}}$ Number of nucleons it contains

Answer 1

Hint: Here, we will proceed by defining an atom. Then, we will discuss the subatomic particles of which the atom is made. Then, we will discuss the atomic number and atomic mass number of an atom.

Complete Step-by-Step solution:
The atom is the smallest unit of matter that is composed of three subatomic particles: the proton, the neutron, and the electron. Protons and neutrons make up the nucleus of the atom, a dense and positively charged core, whereas the negatively charged electrons can be found around the nucleus in an electron cloud.
An atom consists of a positively charged nucleus surrounded by one or more negatively charged particles called electrons. The number of protons found in the nucleus equals the number of electrons that surround it, giving the atom a neutral charge (neutrons have zero charge).
When two atoms are close enough to combine chemically and form chemical bonds with one another, each atom “sees” the outermost electrons of the other atom. These outer electrons, called valence electrons, are the most important factors in the chemical behaviour of atoms. Both the neutron and the proton have key roles in the characteristics of an atom. The number of neutrons determines the type of isotope the element could be and the proton determines the number of electrons that surround its nucleus.
The nucleus of an atom is composed of protons and neutrons; it is therefore positively charged. The number of protons within the nucleus of a given atom is equal to the atomic number of the corresponding element, which can be found on the periodic table. The number of neutrons within the nucleus of a given atom can be found by subtracting the atomic number from the atomic mass. The mass number is the sum of protons and neutrons.
i.e., Atomic Mass number = Number of protons + Number of neutrons
Together, the protons and neutrons are termed as nucleons
Therefore, Atomic mass number = Number of nucleons
Therefore, the mass number of a nucleus of an atom is equal to the number of nucleons (i.e., sum of the number of neutrons and protons) present in the nucleus of that atom.

Hence, option D is correct.

Note- Atoms of the same element that have a different number of neutrons are known as isotopes. Most elements have several naturally occurring isotopes. Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons (i.e., same number of protons and neutrons).