1. Isotopes are atoms of the same element with different masses

Atoms of the same element can have different mass. We say that they are isotopes of the same element.

For example, hydrogen has three isotopes with different relative mass, which is indicated by the number behind their name. So hydrogen-1 has a relative mass of 1, while hydrogen-2 has twice the mass. Hydrogen-3 is the heaviest isotope, with a relative mass of 3.

While their relative mass is different, isotopes are like peas in a pod in terms of chemical properties. They react in an almost identical way, making it impossible to distinguish them through reactions. For example, all three isotopes of hydrogen react explosively with oxygen in a 2:1 ratio.

Isotopes of the same element have different physical properties but similar chemical properties.

2. The core difference lies in the nucleus: neutrons

To get to the core of why isotopes have varying mass but similar chemical properties, we have to examine their atomic structure.

Isotopes have the same number of protons but different number of neutrons packed into the nucleus. Since the neutron has a relative mass of 1, having more neutrons will increase the relative mass of an isotope. For example, hydrogen-3 is the heaviest isotope of hydrogen as it has the most number of neutrons.

But neutrons are electrically neutral. Having more of them does not change the overall positive charge of the nucleus, which will attract the same number of electrons. With the same electronic configuration, isotopes form the same type of bond and have similar chemical properties. This is evident in the isotopes of hydrogen, as they all have a single electron each.

Isotopes are defined as atoms of the same element with the same number of protons but different number of neutrons. Consequently, they have the same atomic number but different mass number.

3. Abundance: some isotopes are more common than others

isotope of hydrogen mass number natural abundance/%
hydrogen-1 1 99.97
hydrogen-2 2 0.03
hydrogen-3 3 trace

Hydrogen-1 is the most common isotope, with a relative abundance of 99.97%. This means that out of 10000 hydrogen atoms in a sample of hydrogen gas, 9997 are hydrogen-1.

On the other hand, hydrogen-3 is highly unstable and can barely exist. It only forms temporarily under very special circumstances, like in a nuclear reactor or when a nuclear weapon explodes. Therefore, it has a relative abundance of close to zero.

Abundance is the percentage of the number of atoms of a particular isotope out of the total number of atoms in a sample of an element.

4. Relative atomic mass of an element is an average value

Since a sample of an element is a mixture of different isotopes, its relative atomic mass (Aᵣ) is in fact the average relative mass of all the atoms in the mixture. To calculate it, we have to take into account the abundance of each isotope, as shown in the formula below:

Aᵣ = relative mass of isotope A × abundance of isotope A + relative mass of isotope B × abundance of isotope B

Let’s apply this formula on hydrogen.

  • 1. Identify the relative abundance of each isotope

    hydrogen-1 = 99.97%
    hydrogen-2 = 0.03%
    hydrogen-3 ≈ 0%

    We will ignore hydrogen-3 as its relative abundance is close to zero.

  • 2. Calculate the relative atomic mass using the formula

    Aᵣ = 1×99.97% + 2×0.03%
    Aᵣ = 1.0003 ≈ 1

When we round off the relative atomic mass of hydrogen to the nearest whole number, it is approximately 1. This corroborates with the data booklet, which shows the relative atomic mass of an element below its chemical symbol.

Likewise, the relative atomic mass of chlorine is 35.5 because it consists two isotopes: chlorine-35 and chlorine-37. The lighter isotope is more common, with a relative abundance of 75%. This explains why the average value of 35.5 is closer to 35 than to 37.