Question

What is the atomic mass (u) of calcium?
A. $10$
B. $20$
C. $30$
D. $40$

Answer 1

Hint: Atomic number of calcium is $20$and mass number is $40$. It is obvious you know that atomic mass actually represents the total weight or mass of an atom or an element and it can be calculated if you know the number of protons and number of neutrons and then by adding both the numbers, in short if you know the atomic mass number.

Complete step by step solution:
First of all, let us know about some terms.
- Atomic number: The atomic number of an element refers to the number of protons in the nucleus of each atom of that element. And an atom is classified as an element in the periodic table based exclusively on its atomic number. It is usually denoted by the letter “Z”.
- Mass number: An element’s mass number is the total number of protons and the number of neutrons present in the nucleus of that atom. It is generally denoted by the letter “A”. So, if you know the mass number or atomic number (which equals the number of protons), the number of neutrons can be calculated by simply deducting the atomic number from the mass number.
- Atomic mass: The atomic mass of an element is the mass of one atom of that element and it is generally expressed in the unit “amu” or “u”.
So, here in the question, given to find out the atomic mass of calcium.
So, we know that the atomic number of calcium is $20$.
Therefore, the number of protons will be $20$.
And the mass number is $40$.
So, the number of neutrons can be calculated by deducting the mass number and atomic number.
So, the number of neutrons will be $40-20=20$.
Now, as we know atomic mass refers to the total mass of the atom. So, by adding the number of protons and neutrons, we will get the atomic number.
Thus, atomic mass of calcium will be $20+20=40$.
Therefore, the atomic mass (u) of calcium is $40$.

Hence, the correct option is D.

Note: The number of protons will determine the atomic mass of an atom while the number of nucleons (i.e. sum of protons and neutrons) will determine the mass number of an atom. Generally, mass number is a function of atomic mass. So, if you know the mass number you can easily calculate the atomic mass.