ICSE Class 9CISCE
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Mass Number

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Mass Number – denoted by ‘A’. Protons and neutrons are also called nucleons. Mass number is defined as the sum total number of protons and neutrons present in the nucleus of an atom.

In the notation for an atom, the atomic number, mass number and symbol of the element are to be written as:

Properties of mass number:

Sum of protons and neutrons provide this number of a certain element.

  • It is represented by the letter A.

  • Protons and Neutrons are together termed as nucleons.

  • Example: Atoms of a carbon consist of 6 protons and 6 neutrons. Therefore, the mass number of Carbon is 12.

  • The number of neutrons may vary in an element. However, the total number of protons is same in all atoms of an element. Therefore, the atoms of the same element with a same atomic number but a different mass number are termed as isotopes.

  • Generally, atomic mass and mass numbers are two different terms and may vary slightly. In most cases, they are not the same. However, the weight of an electron is almost negligible so we can consider the atomic mass of an atom to be almost equal to its mass number.

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