Question

For the isotope $_{6}{{C}^{13}}$, the number of neutrons is:
(A)- 6
(B)- 7
(C)- 19
(D)- 13

Answer 1

Isotopes are the atoms of an element having identical atomic numbers but different mass numbers. The isotope of an atom is distinguished or characterized by its atomic number, which is represented as Z and its mass number, generally represented by A.
$_{Z}{{X}^{A}}$
Atomic number is equal to the number of protons. Mass number is equal to the total number of protons and neutrons.

Complete step by step answer:
We know that atomic number gives the number of protons in the nucleus of an isotope. Thus, we can write
Number of protons = atomic number (Z)
We further know that mass number gives the total number of protons and neutrons (also called nucleons) in the nucleus, i.e.
Number of protons + number of neutrons = mass number (A)

Therefore, number of protons = mass number (A) – number of protons
But we already know that
Number of protons = atomic number (Z)
Therefore, we can find the number of protons as
Number of protons = mass number (A) – atomic number (Z)
The given isotope of carbon is $_{6}{{C}^{13}}$.
Since the atomic number (Z) is written as a subscript and mass number (A) is written as a superscript of the atomic isotope symbol. Therefore, we can deduce that,
Atomic number of $_{6}{{C}^{13}}$ isotope, Z= 6
Mass number of $_{6}{{C}^{13}}$, A = 13
Therefore, using the above relation for calculating the number of neutrons, we obtain
Number of neutrons in $_{6}{{C}^{13}}$ = A – Z = 13 – 6 = 7.
So, the correct answer is “Option B”.

Additional Information:
The most common isotopes of carbon are $_{6}{{C}^{12}}$, $_{6}{{C}^{13}}$ and $_{6}{{C}^{14}}$. are non-radioactive whereas $_{6}{{C}^{14}}$ is a radioactive isotope.

Note: Do not confuse mass number with atomic number. Mass number is written on the top and the atomic number is written at the foot of the atomic symbol. Mass number is always greater than atomic number of an isotope of the atom as it is equal to the number of protons and neutrons.