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Chemistry > Structure of Atom > Mass Number
Structure of Atom

Mass Number

Did you know that protons and neutrons together are termed as nucleons? Nucleons reside inside the nucleus and are denoted by the mass number is denoted as “A.” All atoms of a particular chemical element will have the same atomic number, number of protons present in the nucleus of an atom, but different mass numbers. But what exactly are these mass numbers? Let’s know more about it.

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Mass Number

It is defined as the sum of protons and neutrons. The mass number is almost equal to the atomic mass of a particular atom. Therefore it can be written as

Mass no. of an atom = No. of protons + No. of neutrons

Thus, it represents the total number of neutrons present in the nucleus of an atom.

Use of Mass Number

Mass number helps to give an idea of the isotopic mass. Isotopic mass is measured in atomic mass units or “u.” An isotope of an element will have the same atomic number but a different mass number. Isotope mainly differs in the number of neutrons. Different isotopes of the same element will have a different mass number. However, isotopes of different elements can have the same mass number such as carbon-14 (6 protons + 8 neutrons) and nitrogen-14 (7 protons + 7 neutrons).

mass number

Properties of Mass Number

The various properties of mass number are enumerated here. Let us have a look,

  • 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.

Mass number illustration

X= Chemical Symbol of a particular element
N= Neutron number
Z= Atomic Number= Number of protons
A= Mass number= Z+N

Solved Examples for You

Q: The atomic numb er of aluminum is 13 and atomic mass is 27u. Calculate the number of protons and neutrons.

Sol: Given, Atomic number= 13
We know that, Z= Atomic Number= Number of protons
Therefore, number of proton=13
We know that Mass no. of an atom = No. of protons + No. of neutrons
Mass no= atomic mass= 27 u
27 u = 13 + n Or, n = 27-13 = 14
Thus, Number of proton=13 and number of neutron= 14

Q: Calculate the number of neutrons for the symbol 35 Cl17

Sol: We know that, Z= Atomic Number= Number of protons
Atomic number=Z= 17
Given, Mass no= A= 35
We know that Mass no. of an atom = No. of protons + No. of neutrons
35= 17 + no of neutrons
Or, No of neutrons= 35-17= 18
Therefore, Number of neutrons=18

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