Bonjour,
1) 121 nucléons dont 51 protons donc 121 - 51 = 70 neutrons.
2) e ≈ 1,60.10⁻¹⁹ C
3) charge du noyau = charge des 51 protons
= 51 x e = 51 x 1,60.10⁻¹⁹ = 8,16.10⁻¹⁸ C
Charge des e⁻ = - charge des protons = -8,16.10⁻¹⁹ C
4) m(Sb) = 121 x mn = 121 x 1,7.10⁻²⁶ kg ≈ 2,1.10⁻²⁴ kg
5) N = m/m(Sb) = 57/2,1.10⁻²⁴ ≈ 2,8.10²⁵ atomes