Atomic Number and Mass Number, Isotopes and Isobars

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11th Chemistry NCERT notes

Atomic Number and Mass Number

Atomic Number (Z):

The number of protons present in the nucleus is equal to the atomic number (Moseley 1913).

Atomic Number (Z)= Number of protons in the nucleus of an atom = Number of electrons in a neutral atom

Example:

  • The number of protons in the hydrogen nucleus is 1, in sodium atom, it is 11, therefore, their atomic number are 1 and 11.
  • In order to keep the electrical neutrality, the number of electrons in an atom is equal to the number of protons (atomic number, Z).
  • For example, the number of electrons in a hydrogen atom and sodium atom are 1 and 11 respectively.

Mass Number (A):

Sum of the number of protons and neutrons present in the nucleus.

Mass Number (A) = Number of protons (p) + Number of neutrons (n).

Isotopes

Atoms with an identical atomic number but a different atomic mass number are known as Isotopes.

Isotopes of Hydrogen:

Isotopes of Hydrogen

These three isotopes as shown in the figure below:

Isotopes of hydrogen

Isotopes of Chlorine:

These are two isotopes of chlorine with mass number 35 and 37.

Isotopes of chlorine

The two isotopes differ in their number of neutrons, having 18 and 20 neutrons, respectively.

Isobars

  • Isobars are the atoms with the same mass number but the different atomic number.

For example,

Isobars

  • Each of these has the same mass number but a different atomic number.

 

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