Question

An atom contains 3 (protons ,electrons )and 4 neutrons.what is its mass no ,atomic no, and valency ??????

Answer 1

atomic no. = no. of protons/electrons
thus,
atomic no.= 3

mass no. =no. of protons+ no. of neutrons
thus,
mass no. =3+4=7

since it has 1 valence electron(s) it will lose it to gain stability.
Thus, valency=1

Answer 2

Atomic number = Number of protons = 3
Mass number = Number of protons + number of neutrons =3 + 4 = 7 
Electronic configuration of the atom is  2,1
Hence, its valency is 1.