Question

Find the mass number of sodium having 12 neutrons.
A. 22
B. 23
C. 11
D. 12

Answer 1

Neutrons consist of neither positive nor negative charge so it is neutral. We can calculate the mass number by adding the number of protons and neutrons of that element. Also, a number of protons are equal to the no. of electrons for all electrically neutral atoms.

Complete step by step answer:
- In the given question we have to find the mass number of sodium.
- The mass number is the total weight or mass of the atom or the molecule. It depends on the natural abundance of the isotopes also.
- It is given that the number of neutrons of the sodium is 12.
- As we know that the mass number for an atom is equal to the addition of a number of protons and number of neutrons.
- Now, as we know that the number of protons or positive charge is equal to the no. of electrons or negative charge for an electrically neutral atom.

- So, we know that the atomic no. of sodium is 11 so the no. of protons will also be 11.
- So, the mass number will be:
$\text{Mass number = No}\text{. of protons + no}\text{. of neutrons}$
$\text{Mass number = 11 + 12 = 23g}$
- The representation of the sodium atom will be $\text{Na}_{11}^{23}$.
So, the correct answer is “Option B”.

Note: For those elements who have either positive or negative charge, have a different number of electrons whereas they will have the same number of the proton. For example, ${{\text{O}}^{-2}}\text{ and O}$ they have the same number of protons that are 8 but have different no. of electrons that is 6 and 8 respectively.