Moles of N³⁻ = 4.2g / 14g/mol = 0.3 mol. A nitrogen atom has 5 valence electrons. A nitride ion (N³⁻) has 5 + 3 = 8 valence electrons. Total valence electrons = 0.3 mol * 8 electrons/ion * N_A ions/mol = 2.4 N_A.