Question

Explain why atoms have different isotopes. In other words, how is it that helium can exist in three different forms?

Answer 1

Hint: Isotopes are atoms of the same element which differs in the number of neutrons present inside the nucleus. The presence of more or less number of neutrons makes the atom unstable and so these isotopes are found in low amounts in nature.

Complete step by step answer:
An atom is composed of electrons, protons and neutrons of which the proton and neutron reside inside the nucleus and the electron revolves around the nucleus in several orbits. The protons and the neutrons are collectively called nucleons.
The protons are positively charged species and the electrons are negatively charged species. A neutral atom contains an equal number of protons and electrons. The number of protons is also referred to as the atomic number of an element.
The neutrons are the neutral species which is responsible for the mass of an element. The number of neutrons varies for an element. An element may have the same number of neutrons or may have more number of neutrons than the number of protons and electrons. Such atoms of the same elements are called as isotopes of the element.
For example, hydrogen is an element in the periodic table with atomic number $$1$$ and mass number $$1$$. Some hydrogen nuclei are found to contain $$1$$ neutron known as the deuterium isotope with mass number $$2$$ and some contain $$2$$ neutrons known as the tritium isotope with mass number$$3$$. Most atoms contain a number of isotopes in different proportions. Thus hydrogen has three isotopes of which hydrogen and deuterium are stable but tritium is unstable.
Helium is an element in the periodic table with atomic number $$2$$ and mass number $$4$$. So helium contains two protons and two neutrons. However several other isotopes of helium are found to exist with very small half lives. They contain fewer or more neutrons as $$1$$ and $$3$$. Thus the mass number of helium atoms in these isotopes is $${}^{3}He$$ and $${}^{5}He$$ respectively.

Note: The number of neutrons is calculated using the atomic number and mass number of an atom. It is obtained by subtracting the atomic number from the mass number. The higher elements in the periodic table which have a higher number of neutrons exhibit a phenomenon called radioactivity.