Chlorine is an element of atomic number 17. It is a mixture of two isotopes having mass number of 35 and 37.

(i) What is meant by “atomic number of an element” ?

(ii) Write down the electronic configuration of the Chlorine atom.

(iii) State the number of protons, electrons and neutrons in the following isotopes : ${}_{17}{}^{35}\mathrm{Cl}$ & ${}_{17}{}^{37}\mathrm{Cl}$

(iv) Explain why the two atoms in ${}_{17}{}^{35}\mathrm{Cl}$ & ${}_{17}{}^{37}\mathrm{Cl}$ have the same chemical reactions.

(i) “Atomic number of an element” represents number of protons in the nucleus of atom.

For Example Chlorine is having Atomic number 17, which means it has 17 protons in its nucleus.

(ii) Electronic configuration of Chlorine will be-

${\mathrm{Cl}}_{12}=\underset{\mathrm{K}}{2},\underset{\mathrm{L}}{8},\underset{\mathrm{M}}{7}$

(iii) Isotopes of elements have same atomic number, but different atomic mass. Their atomic mass vary due to the change in number of neutrons while the number of electrons and protons remains same in case of Isotopes. Therefore,

in ${}_{17}{}^{35}\mathrm{Cl}$

Number of Electron = 17

Number of Proton = 17

Number of Neutron = 18

in ${}_{17}{}^{37}\mathrm{Cl}$

Number of Electron = 17

Number of Proton = 17

Number of Neutron = 20

(iv) Isotopes have same atomic number which means they have same number of protons and electrons so both ${}_{17}{}^{35}\mathrm{Cl}$ & ${}_{17}{}^{37}\mathrm{Cl}$ have same electronic configuration, same number of valence electrons, and same valency due to which both of these Isotopes ${}_{17}{}^{35}\mathrm{Cl}$ & ${}_{17}{}^{37}\mathrm{Cl}$ shows similar chemical reactions.