Question

How do you find out the number of protons, electrons, neutrons there are in an atom when you know the atomic mass?

Answer 1

In an atom we know that the number of protons and number of electrons are equal and is equal to the atomic number of an atom. The nucleus of the atom accounts for the atomic mass of the element and it contains proton and neutron.

Complete step-by-step answer:
We know that the skeletal representation of an atom is depicted as $X_{Z}^{A}$
Here A= atomic number and Z= mass number
We have already studied in the junior classes that an atom consists of protons, electrons and neutrons. Protons are positively charged species that possess a charge of $+1$. Electrons are the negatively charged species with charge of $-1$. Neutrons are the neutral charged particles.

Now we know that in an atom the number of protons and electrons are equal and the atom as whole is neutral. Also, we know that the number of protons and number of electrons are also equal to the atomic number. Therefore, we can say that number of protons= number of electrons = Atomic number $\left( A \right)$

Also, it is clear to us that nucleus constitutes the atomic mass of an atom. We also know the fact that nuclei in an atom consist of protons and neutrons. And we have already seen above that the no. of proton = atomic number. so, if we need to find out the number of neutrons then we can simply subtract the mass number and atomic number.

Therefore, we can say that number of neutrons in an atom= Mass number- Atomic number
$\therefore number\,of\,neutrons=\,Z-A$

Hence, we can say that the number of protons and electrons are equal to the atomic number. atomic number can be identified by looking at the atom and one should memorize it. Also, the number of neutrons is simply the subtraction of atomic number and mass number.

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 sodium have a number of neutrons which are not equal to the protons.