Taking into account the definition of isotopes, carbon-14 and nitrogen-14 are not considered isotopes because they have have different atomic numbers.
First of all, all atoms are made up of subatomic particles: protons and neutrons, which are part of their nucleus, and electrons, which revolve around them. Protons are positively charged, neutrons are neutrally charged, and electrons are negatively charged (electrons).
On the other side, the mass number tells us the total number of particles in the nucleus. That is, the mass number is the sum of the number of protons and the number of neutrons in the atomic nucleus:
Mass number = (number of protons) + (number of neutrons)
Then, atomic number is defined as the number of protons or number in the atomic nucleus.
The same chemical element can be made up of different atoms, that is, their atomic numbers are the same, but the number of neutrons is different. These atoms are called isotopes of the element.
In this case, the elements carbon and nitrogen have different atomic numbers (this is different numbers of protons), and the same mass number. So, carbon-14 and nitrogen-14 are not considered isotopes.
In summary, carbon-14 and nitrogen-14 are not considered isotopes because they have have different atomic numbers.
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