Here are the atomic number and atomic mass of the first 20 elements:
1. Hydrogen: Atomic number = 1, Atomic mass = 1.008
2. Helium: Atomic number = 2, Atomic mass = 4.003
3. Lithium: Atomic number = 3, Atomic mass = 6.941
4. Beryllium: Atomic number = 4, Atomic mass = 9.012
5. Boron: Atomic number = 5, Atomic mass = 10.81
6. Carbon: Atomic number = 6, Atomic mass = 12.01
7. Nitrogen: Atomic number = 7, Atomic mass = 14.01
8. Oxygen: Atomic number = 8, Atomic mass = 16.00
9. Fluorine: Atomic number = 9, Atomic mass = 19.00
10. Neon: Atomic number = 10, Atomic mass = 20.18
11. Sodium: Atomic number = 11, Atomic mass = 22.99
12. Magnesium: Atomic number = 12, Atomic mass = 24.31
13. Aluminum: Atomic number = 13, Atomic mass = 26.98
14. Silicon: Atomic number = 14, Atomic mass = 28.09
15. Phosphorus: Atomic number = 15, Atomic mass = 30.97
16. Sulfur: Atomic number = 16, Atomic mass = 32.07
17. Chlorine: Atomic number = 17, Atomic mass = 35.45
18. Argon: Atomic number = 18, Atomic mass = 39.95
19. Potassium: Atomic number = 19, Atomic mass = 39.10
20. Calcium: Atomic number = 20, Atomic mass = 40.08
Note that the atomic mass is the weighted average of the masses of all the isotopes of an element, taking into account their relative abundance, while the atomic number is the number of protons in the nucleus of an atom of that element.