Step-by-step explanation:
Mass N3- ion = 14 g/mol. So,
4.2 g X (1 mol/14 g) = 0.3 mol N3-
Number of N3- ions = 0.3 mol X 6.02 X 10^23 ions/mol = 1.8 X 10^23 ions
Each N3- ion has 8 valence electrons, so total number of valence electrons = 1.8 X 10^23 X 8 = 1.4 X 10^24 valence electrons.
hope it helps