Question

How do you find the number of protons and neutrons and electrons of a particular atom, if all you are given is the atomic number and the mass number?

Answer 1

Hint In an atom the number of protons will be equal to the number of electrons present. The nucleus of the atom accounts for the atomic mass of an atom.

Complete answer:
In the question, we are asked to find the number of protons, neutrons and electrons if only provided with atomic number and atomic mass.
From the lower classes, we are studying about the subatomic particles present in the atom. The main subatomic particles present are protons, electrons and neutrons. Let’s see some basic data about the three subatomic particles and then discuss how these subatomic particles are related to the atomic number and atomic mass number.
The protons are positively charged species that possess a charge +1 and are represented as ${{p}^{+}}$ and electrons are negatively charged species that possess a charge of -1 and are represented as ${{e}^{-}}$.
The neutrons are neutral species which do not have any charge but possess mass and are represented as ${{n}^{0}}$.
Now let’s discuss the relation between atomic number and mass number with the number of electrons, protons and neutrons and for that we have to know what does atomic number and mass number means for an element.
Atomic number of an element is equal to the number of protons present in an atom or number of electrons present. Atomic number is the unique number given for an element for its identification.
Hence, $\text{Atomic}\,\text{number=No}\,\text{.}\,\text{of}\,{{\text{p}}^{\text{+}}}\text{s=No}\text{.}\,\text{of}\,{{\text{e}}^{\text{-}}}\text{s}$.
So if we know the atomic number of an element, we can calculate the number of protons and electrons.
Then next comes the relation of atomic mass with these atomic species. We know that the atomic nucleus accounts for the atomic mass of an element. And we are very familiar that the atomic nucleus consists of neutrons and protons.
Hence, mass number is the summation of the number of protons and neutrons present in the atom.
$\text{Mass}\,\text{number=No}\text{.}\,\text{of}\,{{\text{p}}^{\text{+}}}\text{s+No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{s}$
Now let’s see an example for better understanding.
We are provided with the data that the atomic number of Na is 11 and mass number is 23.
Let’s find the number of electrons, protons and neutrons in Na.
We are given that the atomic number of Na is 11 so the number of protons will be 11 and also the number of electrons will be 11.
Now we have to calculate the number of neutrons, for that it is given that the atomic mass is 23.$\text{No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{s=Mass}\,\text{number-No}\text{.of}\,{{\text{p}}^{+}}s$
$\text{No}\text{.}\,\text{of}\,{{\text{n}}^{\text{0}}}\text{s=23-11=12}$

So there are 11 electrons in the sodium atom.

Note: It is not necessary that the number of protons and neutrons will be equal in an atom, in many cases it is true but cases like Na have a number of neutrons which is not equal to the number of protons.