₂₀⁴⁰Ca
Z=20--> atomul are 20 e⁻, in invelisul electronic, si 20 p⁺ in nucleu
A=40=Z+n⁰=> n⁰= 20 neutroni care se gasesc in nucleu
structura invelisului electronic: K2 L8 M8 N2
sau 1s2 2s2 2p6 3s2 3p6 4s2
