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Updated on November 6, 2011

Beyond the simplicity of the atomic number, neutron number and the mass number of the atoms of an element is the existence of Isotopes. Though the perception of elements that exist in Isotopes is usually the most abundant Isotope, the actual atomic mass is derived from the contribution of all the Isotopes of a specific element. Now, let's begin by defining Isotopes.

What is an Isotope?

An Element that exists in two or more different forms with the same atomic number, but different atomic masses is said to exhibit Isotopy. Each of the element is regarded as an Isotope. But the disparity in the atomic masses should pose a question of diverse chemical properties. The truth is that Isotopes of a defined element share similar chemical properties because they contain the same atomic number (number of Protons)

The next question should be how the presence of similar number of protons could be instrumental in the chemical properties of an element. This is put to rest considering the fact that the electron is primarily responsible for the chemical properties of an element. And since the number of proton is equal to the number of electrons in a neutral atom, it becomes decisive that Isotopes shares similar chemical properties. The only difference is the nuclear properties.

Two terms are important to the understanding of Isotopes and Isotopy: Mass Number and Atomic number.

Mass Number

Mass number targets the atoms of an element. It is the number of protons and neutrons in a single atom of an element.

Atomic Number

Atomic mass considers the individual masses of all the atoms in an element. It is the average mass of all the atoms in an element.

The atomic mass of Carbon, for example, is not specific of the most common Isotope: Carbon-12, but an average of the percentage contribution of each member of the carbon Isotope.

Examples of Isotopes

Examples of elements that exhibits Isotopy includes Hydrogen, Chlorine, Oxygen, Boron, and Carbon. Each of them has Isotopes with specific Atomic masses. Hydrogen for instance, has two Isotopes: Hydrogen-1 and Hydrogen-2. Though the percentage abundance of Hydrogen-2 is so minute: 0.015%; compared to Hydrogen-1, it multiplied by its atomic mass and added to the product of the percentage abundance of Hydrogen-1 to arrive at the actual atomic mass of Hydrogen.

The Percentage abundance and the atomic mass of specific Isotopes of the elements listed above are as tabulated below.

Isotopes, % Abundance and Atomic Mass

% Abundance
Atomic Mass

Calculation of Atomic Mass with Isotopes

Considering that the atomic mass of Carbon-12 is 12.00 with percentage abundance as 98.90, and Carbon-13: 1.10 and 13.003354826; the atomic mass of Carbon will be,

(Percentage abundance of carbon-12 × atomic mass of Carbon-12) + (Percentage abundance of Carbon-13 × atomic mass of Carbon-13)

= (98.90/100×12) + (1.10/100×13.003354826) = 11.868+0.1430 = 12.011

This is the reason why Carbon is said to have an atomic mass of 12.011amu even though each of the Isotopes of Carbon has their own Atomic masses that are different from the general atomic mass.

You might be required to predict an unknown element, or to find the atomic mass of an element with its percentage abundance and the atomic masses of its Isotopes. All that is required is to simply adopt the formula in the calculation above.


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    • profile image

      fauza 21 months ago

      I didn't get what I asked for

    • profile image

      Dale Tillett 2 years ago

      it good, but didn't give me exactly what i asked for

    • profile image

      dale tillett 2 years ago

      its good,but didn't give exactly what i asked for.

    • James Agbogun profile image

      James Agbogun 5 years ago

      Thank you CWanamaker. The role of C-14 in Dating and Uranium in Radioactivity cannot be ignored. It is observe that the abundance of Carbon-14 is in 1ppm hence, it's contribution to the calculation of the atomic mass of Carbon is rather insignificant. Once more, I thank you for ensuring that we do not assume that carbon is made up of only two Isotopes.

    • CWanamaker profile image

      CWanamaker 5 years ago from Arizona

      I enjoy reading about chemistry. Lets not forget our favorite isotopes: Carbon 14 which is important for dating ancient objects and Uranium 235 which is important in nuclear power plants. Voted Up!