What are isotopes? Give an example also.

Hint: The atomic mass of an element depends upon the number of protons and neutrons present in its nucleus. Sometimes, the nucleus of the same element may contain one or more extra neutrons. This causes the atomic mass of the element to change while maintaining its chemical properties. This results in formation of an isotope.

Complete step by step answer:
An isotope is a variant of an element which differs in the number of neutrons present in its nucleus. The number of protons and electrons remain the same, thus not affecting its atomic number. Isotopes are not shown in the periodic table, this is because it is not identified as a different element. For example, hydrogen has three naturally occurring isotopes. They are:
Protium: It has only 1 proton in its nucleus and is represented as \(_{\rm{1}}^{\rm{1}}{\rm{H}}\) It has an atomic mass of 1 u. Normally, the compounds of hydrogen we see and study in everyday life are made up of protium.
Deuterium: It has 1 proton and 1 neutron in its nucleus. It has an atomic mass of 2 u. It is represented as \(_{\rm{1}}^2{\rm{H}}\) It can be found in ocean water. The water molecule made up of deuterium is also known as heavy water and is used in nuclear reactors to control the fission speed.
Tritium: It has 1 proton and 2 neutrons. The mass of tritium is 3 u and is represented as \(_{\rm{1}}^3{\rm{H}}\) It is extremely rare on Earth most of which is found in trace amounts in the atmosphere. Tritium is radioactive and is sometimes used as fuel in nuclear reactors.

Note:
Many elements in the periodic table have isotopes. Some isotopes are radioactive or have a smaller half-life than the original element. This is why they are not as common as the original elements, i.e. the ones present in the periodic table. As isotopes have different atomic masses, the reduced masses of the isotopes are also different. This changes the centre of gravity of the molecule.