Question

the mass no. of an element is 18 it contain 7 electron what is the no. of proton and neutron in it

Answer 1

no of protons=no of electrons
so no of protons=7
mass no=nof neutrons+no of protons
no of neutrons=18-7=11

Answer 2

since no. of proton =no. of electron
proton = 7
neutron=mass no. -proton
18-7=11
hence it contain 18 neutron