Neutron,Valency,isotope and Isobars Lessons for Class 9th NCERT

What is Neutron

  • In 1932, J. Chadwick discovered another subatomic particle which had no charge and a mass nearly equal to that of a proton.
  • It was eventually named as neutron.
  • Neutrons are present in the nucleus of all atoms, except hydrogen.
  • In general, a neutron is represented as ‘n’.
  • The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus

The Mass of An Atom = Number of Neutrons + Number Of Protons

James Chedwick

How are Electrons Distributed in Different Orbits (Shells)? in Different Orbits (Shells)

  • Bohr and Bury gave the theory for distribution of electrons in different orbits
  • The maximum number of electrons present in a shell is given by the formula 2n2
  • where ‘n’ is the orbit number or energy level index, 1,2,3,….
  • Hence the maximum number of electrons in different shells are as follows:
  • first orbit or K-shell will be = 2 × 12= 2,
  • Second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42 = 32, and so on
  • The maximum number of electrons that can be accommodated in the outermost orbit is 8.
  • Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.


  • The electrons present in the outermost shell of an atom are known as the valence electrons
  • It was observed that the atoms of elements, having a completely filled outermost shell show little chemical activity. In other words, their combining capacity or valency is zero.
  • An outermost-shell, which had eight electrons was said to possess an octet.
  • Atoms would thus react, so as to achieve an octet in the outermost shell.
  • This was done by sharing, gaining or losing electrons.
  • The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element,
  • Example Hydrogen/lithium/sodium atoms contain one electron each in their outermost shell, therefore each one of them can lose one electron. So, they are said to have valency of one
  • If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way.
  • For example, the fluorine atom has 7 electrons in the outermost shell, and its valency is not  7.
  • For fluorine it is easier  gain one electron instead of losing seven electrons.
  • Hence, its valency is determined by subtracting seven electrons from the octet and this gives you a valency of one for fluorine

Atomic Number 

  • It is the number of protons of an atom, which determines its atomic number.
  • It is denoted by ‘Z’. All atoms of an element have the same atomic number
  • For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus.
  • Similarly, for carbon, Z = 6.

Therefore, the atomic number is defined as the total number of protons present in the nucleus of an atom


  • Protons and neutrons are also called nucleons
  • Carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u.
  • Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons).

The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom


The Elements having same atomic Number but different mass Number are called Isotopes.

  • Taking the case of Hydrogen . It has three atomic species, namely protium , deuterium and tritium .

  • The atomic number of Each hydrogen are 1, but the mass number is 1, 2 and 3, respectively.
  • Therefore, we can say that there are three isotopes of hydrogen atom, namely protium, deuterium and tritium
  • Similarly For Carbon we have 3 isotopes as carbon-12,carbon-13,carbon-14
Isotope of Carbon
  • The chemical properties of isotopes are similar but their physical properties are different.
  • The mass of an atom of any natural element is taken as the average mass of all the naturally occuring atoms of that element
  • If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it.
  • But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated.

Applications of ISOTOPE 

Since the chemical properties of all the isotopes of an element are the same, normally we are not concerned about taking a mixture. But some isotopes have special properties which find them useful in various fields. Some of them are :
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in the treatment of goitre


  • Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18.
  • The number of electrons in these atoms is different, but the mass number of both these elements is 40.
  • That is, the total number of nucleons is the same in the atoms of this pair of elements.

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars

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