Question

An atom of an element has 26 electrons and has a mass number 56. The nucleus of this atom contains neutrons.
(A) 26
(B) 36
(C) 30
(D) 56

Answer 1

The mass number ‘A’ of an atom is equal to the total number of protons and neutrons present in the nucleus of an atom of an element. In the neutral state, the number of protons of an atom is always equal to the number of an electron revolving around the nucleus of an atom. It is denoted by ‘z’

Complete step by step solution:
The atomic number of an atom is the number of protons that are found in the nucleus of the atom of an element. It is denoted by the ‘Z’.
$\text{ Atomic number (Z) = number of proton in nucleus }$
The positively charged nucleus or the charge on the proton is neutralized by the negatively charged electrons revolving around the nucleus. Thus, in zero charge state (neutral state) the number of electrons in the atom of an element is equal to the number of the proton.
$\text{ Atomic number (Z) = number of proton in nucleus = number of electron }$ (1)
The atomic mass of an atom is defined as the total mass which an atom of any element can exhibit. It is generally expressed in terms of the atomic mass unit. We know that the atoms are made up of the proton, neutrons, and electrons.
The nucleus of the atom holds the mass of the atom. The nucleus of an atom is made of the neutrons and protons. The electrons which are negatively charged atomic particles revolve around the nucleus.
The atomic mass of the atom is the combined mass of the neutrons and the protons in the nucleus. The mass of electrons is considered negligible concerning the mass of protons and neutrons.
Thus, the atomic mass ‘A’ is equal to the sum of neutrons and protons in the atom.
$\text{ Atomic mass (A) = No}\text{.of proton + No}\text{.of neutron }$ (2)
Now let us have a look at the data given in the question. We have,
The number of electrons in the atom equals 26.
The mass number of an atom of an element is equal to the 56.
We have to find the number of neutrons.
From the atomic number relation (1), we know that the number of an electron is equal to the number of the proton. Thus,
$\text{ Number of proton = 26}$
Now, put the value of the mass number and the number of proton in the equation (2) we have,
$\text{ Atomic mass (A) = No}\text{.of proton + No}\text{.of neutron}$
That is ,
$\begin{aligned} &\text{ }56\text{ = 26 + No}\text{.of neutron} \\\ &\therefore \text{No}\text{.of neutron = 56 }-26\text{ = 30} \\\ \end{aligned}$
Thus, here we get the total number of neutrons in the atom with the number of electron 26 and mass number 56 is equal to 30.

Hence, (C) is the correct option.

Note: The neutrons are charged with less atomic sub-particles which minimizes the repulsion between the positively charged protons. The number of neutrons can vary in the nucleus this leads to the different mass number for an element. Such atoms which have the same atomic number but a different mass number are known as the isotopes. for example, carbon has two isotopes carbon -12 and carbon -14 (here the atomic number i.e. Number of protons for the carbon is the same which is 6 but the number of neutrons is found to be 6 and 8 respectively).