The two isotopes of chlorine, have mass number 35 and 37 and number of neutrons 18 and 20 respectively. Which one will have higher valency? Do they have same physical or chemical properties?
Number of protons in ${}^{35}Cl$ =35-18=17
=> Number of electrons = Number of protons = 17
Similarly, number of protons in ${}^{37}Cl$ = 37 - 20 = 17 => Number of electrons = Number of protons = 17
Since, both have same number of electrons (i.e. 17).
Hence, they will have same valency (i.e. 1).
Isotopes having same number of electrons show same chemical properties but may show some different physical properties.