Question

What is the mass number of an element?
A) Total number of protons in an atom
B) Total number of neutrons in an atom
C) Total number of protons and neutrons in an atom
D) Total number of electrons in an atom

Answer 1

We need to know that the mass number is approximately twice the atomic number of an element. Mass number is not the atomic weight so don’t get confused between mass number and atomic weight they are two different things.

Complete step by step answer:
Before explaining what mass is. We will learn what an element is made up of?
As we know that elements can be metals, non metals or metalloids which are periodically arranged in a periodic table according to their atomic numbers. It can be also said that elements are made up of only one kind of atom.
Considering Beryllium $\left( {Be} \right)$ as an example,
$^9B{e_4}$

Element	Symbol	Atomic Number	No. of Protons	No. of Neutrons	No. of Electrons	Mass Number
Beryllium	$Be$	$4$	$4$	$5$	$4$	$9$

No. of protons= Atomic number
No. of electrons= Atomic number
No. of neutrons = Mass number- Atomic number
Since, we know the atomic number for a particular element, so it’s easy to calculate its respective protons and electrons.

Option A)
Since option A) says that mass number is the total number of protons in an atom which is incorrect, as the total number of protons signifies the atomic number of an atom.
Option B)
This option is also incorrect as it says that Mass number is the total number of neutrons in an atom, considering the example shown above and the formula used, No. of neutrons is the difference between mass number and atomic number thus this option is not right.
Option C)
Considering the above example. It is clearly seen that in case of $Be$, there are
No. of protons $= 4$
No. of neutrons $= 5$
Thus, the mass number comes out to be $9$ which is simply done by adding no. of protons and no. of neutrons.
So from here it is said that mass number is the sum of no. of protons and neutrons in an atom.
Thus option C) is correct.
Option D)
This option is incorrect since no. of electrons signifies the atomic number of an element.

Hence, Option (C) is the correct answer.

Note: We have to remember that the mass number is the sum of the total number of neutrons and protons present in an atom. Atomic mass is the average mass of an atom in an element based on its relative abundance. Atomic number can be the total no. of protons or total no. of electrons.