`color{green}(•)` In 1932, J. Chadwick discovered another subatomic particle.

`color{green}(•)` It had no charge and a mass nearly equal to that of a proton.

`color{green}(•)` It was eventually named as neutron `color{red}(’n’)`.

`color{green}(•)` Neutrons are present in the nucleus of all atoms, except hydrogen.

`color{green}(•)` The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.

Bohr and Burydiscussed about the distribution of electrons in different orbits.

The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) The maximum number of electrons present in a shell is given by the formula `color{red}(2n^2)`, where `color{red}(‘n’)` is the orbit number or energy level index, `color{red}("1,2,3,….")`

Hence the maximum number of electrons in different shells are as follows: first orbit or K-shell will be `color{red}(= 2 × 1^2 = 2)`, second orbit or L-shell will be `color{red}(= 2 × 2^2 = 8)`, third orbit or M-shell will be `color{red}(= 2 × 3^2 = 18)`, fourth orbit or N-shell will be `color{red}(= 2 × 4^2= 32)`, and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is `color{red}(8)`.

(iii) 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

`color{green}(•)` The electrons present in the outermost shell of an atom are known as the valence electrons.

`color{green}(•)` The atoms of elements, having a completely filled outermost shell show little chemical activity i.e. their combining capacity or valency is zero.

`color{green}(•)` Of these inert elements, the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outermost shell.

`color{green}(•)` An outermost-shell, which had eight electrons was said to possess an octet.

`color{green}(•)` Octet can be completed by sharing, gaining or losing electrons.

`color{green}(•)` The number of electrons gained, lost or shared so as to complete the octet gives us directly the combining capacity of the element.

`color{green}("𝐀𝐓𝐎𝐌𝐈𝐂 𝐍𝐔𝐌𝐁𝐄𝐑")`

`color{green}(•)` Atomic number is determined by the number of protons of an atom.

`color{green}(•)` It is denoted by `color{red}(‘Z’)`.

`color{green}(•)` All atoms of an element have the same atomic number, `color{red}(Z.)`

`color{green}(•)` For hydrogen, `color{red}(Z = 1)`, because in hydrogen atom, only one proton is present in the nucleus and for carbon, `color{red}(Z = 6.)`

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

`color{green}("𝐌𝐀𝐒𝐒 𝐍𝐔𝐌𝐁𝐄𝐑")`

`color{green}(•)` Mass of an atom is practically due to protons and neutrons present in the nucleus of an atom.

`color{green}(•)` Hence protons and neutrons are also called nucleons.

`color{green}(•)` Therefore, the mass of an atom resides in its nucleus.

`color{green}(•)` For example, mass of carbon is `color{red}(12 u)` because it has 6 protons and 6 neutrons, `color{red}(6 u + 6 u = 12 u.)`

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

Isotopes are the atoms of elements that have the same atomic number but different mass numbers. For example, take the case of hydrogen atom, it has three atomic species, namely protium (`color{red}(text()_(1)^(1)H)`), deuterium ( `color{red}(text()_(1)^(2) H)` or `color{red}(D)`) and tritium ( `color{red}(text()_(1) ^(3)H)` or `color{red}(T)`). The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.

Other such examples are (i) carbon, `color{red}(text()_(6)^(12) C)` and `color{red}(text()_(6)^(14) C)`, (ii) chlorine, `color{red}(text()_(17)^(35) Cl)` and `color{red}(text()_(17)^(37) Cl)`, etc.

`color{green}(•)` Many elements consist of a mixture of isotopes.

`color{green}(•)` Each isotope of an element is a pure substance.

`color{green}(•)`The chemical properties of isotopes are similar but their physical properties are different.

Chlorine occurs in nature in two isotopic forms, with masses `color{red}(35 u)` and `color{red}(37 u)` in the ratio of `color{red}(3:1)` .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 calculate the percentage of each isotopic form and then calculate the average mass. The average atomic mass of chlorine atom, on the basis of above data, will be
`color{red}([ ( 35xx 75/100 + 37 xx 25/100) = ( 105/4 + 37/4) = 142/4 = 35.5 u])`

This does not mean that any one atom of chlorine has a fractional mass of `color{red}(35.5 u)`. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is `color{red}(35.5 u.)`

`color{green}("𝐀𝐩𝐩𝐥𝐢𝐜𝐚𝐭𝐢𝐨𝐧𝐬")`

(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.