Question

Atomic masses of isotopes of carbon $\left( _{6}{{C}^{12}} \right)$ having neutrons $7$ and $8$ respectively are:
A.$14,{ }15$
B.$15,{ }16$
C.$16,{ }17$
D.$13,{ }14$

Answer 1

Isotopes of an element are those which have the same number of protons but different neutrons. The atomic mass of an element can be calculated by simply adding the number of protons and neutrons present in that isotope of the element.

Complete step by step answer:
Atoms of every element contain a set of characteristic numbers of protons. In fact, those number of protons actually gives us an idea of the atom that we are looking at (e.g., all the atoms with six protons must be carbon atoms); the number of protons present inside an atom is known as the atomic number. Unlike the number of neutrons which can vary for a given specific element. When the same atom differs only in their number of neutrons are termed as isotopes. Addition of the number of neutrons and number of protons determine mass number of an element:
$mass{ }number~=protons~+neutrons.$
We can easily calculate how many neutrons an atom has, by simply subtracting the number of protons, from the mass number of that atom.
A property which is closely related to the mass number of an atom is its atomic mass. The atomic mass of a single atom can be defined as its total mass and is generally expressed in atomic mass units or amu. Going by the definition, an atom of carbon having six neutrons, $carbon-12$ , has the value of atomic mass of $12{ }amu$ .
Since isotopes of elements have distinct atomic masses, scientists may also find out the relative atomic mass which is sometimes termed as the atomic weight regarding an element. The relative atomic mass can be defined as an average of the atomic masses of all the different isotopes in a sample, with each contribution of isotope, to the average which is determined by size of the sample we take.
So in the question, atomic mass of carbon when the number of neutrons is $7$ is:
$6+7=13$
Atomic mass of carbon when the number of neutrons is $8$ ,
$6+8=14$
So the correct answer is option D. Since it matches the atomic masses calculated by us.

Note:
-The sum of the number of protons and neutrons present in an atom makes up the atomic mass of an element. Each isotope of an element has a different number of neutrons, so the value of atomic mass also changes. So in the case of carbon the number of neutrons was seven and eight, hence the atomic masses became thirteen and fourteen respectively.
-Some specific isotopes of elements are radioactive in nature, which means they are highly unstable and tend to lose their neutrons until they become stable.