Can someone pls explain how I find the answers because my teacher didn’t explain it thoroughly.

KEY POINTS

Atomic no. = no. of protons

•No. of protons = no. of e- (For neutral )

•No. of neutron = Mass no. - atm no.(or no. of p)

Ex1- Atm no. =19

--->■ therefore no. of proton also 19

ii)no. of neutron=mass no. - atomic no.

=39 - 19

=20

iii) no. of e=no of proton (for neutral) ex He Ar H

□●■for charged species(ex Li+,cl-)

no. of e = no. of proton +/- charge

a))if charge is + then subtract that charge from proton then you will get no. of e

Ex K+

no. of e = no. of proton -1

=19-1=18

b)if charge is -ve then add that charge to no. of proton then we will get no.of e

Ex- cl-

no. of e- = no. of proton+ 1

=17+1=18

THANK YOU

your answer would be a. 12.5 kg