Here are the real atomic masses of the first 20 elements:
- Hydrogen (H) - 1.00784 amu
- Helium (He) - 4.00260 amu
- Lithium (Li) - 6.93800 amu
- Beryllium (Be) - 9.01218 amu
- Boron (B) - 10.81350 amu
- Carbon (C) - 12.01060 amu
- Nitrogen (N) - 14.00674 amu
- Oxygen (O) - 15.99940 amu
- Fluorine (F) - 18.99840 amu
- Neon (Ne) - 20.17970 amu
- Sodium (Na) - 22.98977 amu
- Magnesium (Mg) - 24.30500 amu
- Aluminum (Al) - 26.98154 amu
- Silicon (Si) - 28.08550 amu
- Phosphorus (P) - 30.97376 amu
- Sulfur (S) - 32.06000 amu
- Chlorine (Cl) - 35.45300 amu
- Argon (Ar) - 39.94800 amu
- Potassium (K) - 39.09830 amu
- Calcium (Ca) - 40.08000 amu
These atomic masses represent the weighted average masses of the naturally occurring isotopes of each element. The values may have slight variations depending on the isotopic composition of a given sample, as isotopes of an element can have different atomic masses due to differing numbers of neutrons.