Average atomic weight: 4.002602(2) u
| Isotope | Fraction | Atomic weight/u |
| 3 | 0.00000134(3) | 3.0160293201(25) |
| 4 | 0.99999866(3) | 4.00260325413(6) |
Atomic weights and isotopic compositions were obtained from NIST Database 144
He I (He-like): 1106 levels, 60299 lines (E1: 59662 IC: 636 M1: 1), transition probabilities for 58229 lines
He II (H -like): 1310 levels, 26678 lines (E1: 26678), transition probabilities for 26678 lines
(Atomic Line List v3.00b5)
