The first 20 elements in the periodic table, in order of increasing atomic number, are:

1. Hydrogen (H)

2. Helium (He)

3. Lithium (Li)

4. Beryllium (Be)

5. Boron (B)

6. Carbon (C)

7. Nitrogen (N)

8. Oxygen (O)

9. Fluorine (F)

10. Neon (Ne)

11. Sodium (Na)

12. Magnesium (Mg)

13. Aluminum (Al)

14. Silicon (Si)

15. Phosphorus (P)

16. Sulfur (S)

17. Chlorine (Cl)

18. Argon (Ar)

19. Potassium (K)

20. Calcium (Ca)