Here are the atomic masses of the first 20 elements:
- Hydrogen (H) - Atomic Mass: 1.00784 u
- Helium (He) - Atomic Mass: 4.00260 u
- Lithium (Li) - Atomic Mass: 6.938 u
- Beryllium (Be) - Atomic Mass: 9.01218 u
- Boron (B) - Atomic Mass: 10.81 u
- Carbon (C) - Atomic Mass: 12.01 u
- Nitrogen (N) - Atomic Mass: 14.01 u
- Oxygen (O) - Atomic Mass: 16.00 u
- Fluorine (F) - Atomic Mass: 19.00 u
- Neon (Ne) - Atomic Mass: 20.18 u
- Sodium (Na) - Atomic Mass: 22.99 u
- Magnesium (Mg) - Atomic Mass: 24.31 u
- Aluminum (Al) - Atomic Mass: 26.98 u
- Silicon (Si) - Atomic Mass: 28.09 u
- Phosphorus (P) - Atomic Mass: 30.97 u
- Sulfur (S) - Atomic Mass: 32.07 u
- Chlorine (Cl) - Atomic Mass: 35.45 u
- Argon (Ar) - Atomic Mass: 39.95 u
- Potassium (K) - Atomic Mass: 39.10 u
- Calcium (Ca) - Atomic Mass: 40.08 u
Please note that atomic masses are often reported with more decimal places, but I have rounded them to two decimal places for simplicity.