Isotopes

Atoms of an element that have the same number of protons, but different numbers of neutrons in their nuclei are called isotopes. Isotopes of an element can be easily identified because:

Three isotopes of hydrogen

All hydrogen atoms contain one proton and one electron, but they can contain different numbers of neutrons. Hydrogen-1 is the most abundant isotope of hydrogen.

Name of isotopeSymbolProtonsNeutronsElectrons
Hydrogen-1 _{1}^{1}\textrm{H}11 - 1 = 01
Hydrogen-2 _{1}^{2}\textrm{H}12 - 1 = 11
Hydrogen-3 _{1}^{3}\textrm{H}13 - 1 = 21

An isotope is named after the element and the mass number of its atoms. For example, carbon-12 is an isotope of carbon with a mass number of 12.

All three isotopes of hydrogen have identical chemical properties. This is because the number of electrons determines chemical properties, and all three isotopes have one electron in their atoms.

Question

The atomic number of carbon is 6. How many neutrons are there in the nucleus of a carbon-12 atom?

Number of neutrons = mass number - atomic number

= 12 - 6

= 6

Question

How many neutrons are there in the nucleus of a carbon-13 atom?

Number of neutrons = mass number - atomic number

= 13 - 6

= 7

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