A (masa atomica) Al=27 u.a.m.
nr de protoni=Z=13 (in fiecare atom)
1 mol de Al are 6,023x10^23 atomi rezulta ca 0,25 moli Al au 0,25x6,023x10^23 atomi
1 atom de Al......................13 protoni
0,25x6,023x10^23 atomi de Al.....x protoni
x=2x10^24 protoni
1 atom de Al are A-Z neutroni
1 atom de Al............14 neutroni
0,25x6,023x10^23....y neutroni
y=2x10^24 neutroni
nr de nucleoni=x+y=4x10^24
(ca o mica paranteza, in caz ca nu stii, 10^23 inseamna 10 la puterea 23 si 10^24 de asemenea)