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