Question

an atom has 19 proton and 20 neutron state its mass number and valency

Answer 1

Its mass no is 39 . its valency is Plus 1

Answer 2

NO. OF PROTON =NO. OF ELECTRON   Therefore it has 19 electrons  

Hence, it needs to loose one electron to complete 2,8,8.

 Valency ==>1  

MASS OF AN ATOM=NO.OF(NEUTRON +PROTON   THEREFORE 39-19=20