When we are looking at the atomic number of an element in the periodic table, we may not know it, but these elements may have isotopes. This depends on the number of their neutrons. In this lesson, we will learn about the three isotopes of hydrogen.

What Are Isotopes?

Let’s imagine identical twins or identical triplets – they all look the same on the outside, but when we look more closely, we notice small physical differences, like for instance, their fingerprints. Also, as we get to know them more individually, we will notice subtle differences in their preferences and personalities.

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Let’s compare these identical twins and triplets to isotopes of an element. Just like twins and triplets, we can think of isotopes as different versions of an element. Isotopes are different versions of the same element that all have the same atomic number but different number of neutrons. Because the number of neutrons are different, they also have different atomic masses, the total number of protons and neutrons combined.

As an example, let’s take a look at the isotopes of hydrogen. Hydrogen has three isotopes: hydrogen-1 (protium), hydrogen-2 (deuterium) and hydrogen-3 (tritium). In the following illustration, we can see subscripts and superscripts. The superscripts 1, 2 and 3 written before H are the atomic masses of the isotopes of hydrogen and the subscript 1 is the atomic number. We can see here that the atomic numbers (or number of protons) of the isotopes of hydrogen are the same, but their neutrons and atomic masses are different.

__Isotopes of Hydrogen__

Three Isotopes of Hydrogen

Earlier, we have shown the three isotopes of hydrogen: protium, deuterium and tritium. Protium is also known as hydrogen-1, deuterium is also known as hydrogen-2 and tritium is also known as hydrogen-3.

Let’s compare how these hydrogen atoms are different in the following table. We can see that for the symbols, the superscripts before H are the atomic mass and the subscripts are the number of protons or the atomic number. Protium is also called hydrogen-1. The same goes for the other two isotopes of hydrogen.

Three Isotopes of Hydrogen

The three isotopes of hydrogen are illustrated here:

Illustration: Isotopes of Hydrogen

Besides the number of neutrons and the atomic number, these isotopes of hydrogen also differ in terms of their natural abundance. When we say natural abundance, for isotopes, this refers to the abundance of that isotope found on the planet. The natural abundances of the isotopes of hydrogen are shown in the following table.

Isotopes of Hydrogen: Natural Abundance

Protium (hydrogen-1) has an atomic mass of 1.00782504, and is a stable isotope. It has one proton and no neutrons. Protium is also known as ordinary hydrogen. Looking at its natural abundance, it is the most common of all the hydrogen isotopes.

Deuterium (hydrogen-2) is the second most abundant isotope of hydrogen and it makes up 0.0026 to 0.0184% of the hydrogen that is naturally found on the Earth. Its atomic mass is 2.01410178, and it has one proton and one neutron. Because the nucleus of deuterium is two times heavier than protium, deuterium is also known as ‘heavy hydrogen.’ This was discovered in 1931 by an American chemist named Harold C. Urey.

The main difference between protium and deuterium is, while deuterium can also react like protium, deuterium reacts more slowly than protium. For that reason, deuterium is used as a tracer when investigating different chemical reactions. When deuterium undergoes nuclear fusion with tritium, it releases a huge amount of energy at high temperature, so this nuclear fusion has been utilized for thermonuclear weapons. Later on, in place of deuterium and tritium, a more stable substance called lithium deuteride (LiD) is used as fuel for thermonuclear weapons.

Tritium (hydrogen-3), has an atomic mass of 3.0160492, and it consists of one proton and two neutrons. Its nucleus is triple the mass of protium. What distinguishes tritium from protium and deuterium is that it is radioactive, which means that its nucleus is unstable so its nucleus decays and emits radiation.

Tritium was discovered in 1934 by three physicists: Ernest Rutherford, M.L. Oliphant and Paul Harteck. Tritium can be a gas, but it most commonly occurs in water in very small percentages that is not harmful. However, if it occurs in higher concentrations in water, it may result in higher cancer risk.

Lesson Summary

Let’s review! Isotopes of an element have the same number of protons and electrons but different number of neutrons and different atomic masses. Hydrogen, in particular, has three isotopes: protium or hydrogen-1, deuterium or hydrogen-2 and tritium or hydrogen-3.

The most naturally abundant isotope of hydrogen on earth is protium, followed by deuterium, while tritium only has trace and amounts. Protium, also known as ordinary hydrogen, is the lightest isotope of hydrogen, followed by deuterium, also known as heavy hydrogen. The heaviest one is tritium. It is also important to note that tritium is the only radioactive isotope of hydrogen, so high exposure to tritium may cause cancer.

Below is a table showing the summary of information about the three isotopes of hydrogen.

Three Isotopes of Hydrogen: Protium, Deuterium and Tritium