first 20 mass numbers

Here are the mass numbers of the first 20 elements, excluding cases where the rounded number is the same as the unrounded number:

1. Hydrogen (H): 1

2. Helium (He) 4

3. Lithium (Li):

• Unrounded: 6.94

• Rounded: 7

4. Beryllium (Be): 9

5. Boron (B):

• Unrounded: 10.81

• Rounded: 11

6. Carbon (C): 12

7. Nitrogen (N): 14

8. Oxygen (O):

• Unrounded: 15.999

• Rounded: 16

9. Fluorine (F):

• Unrounded: 18.998

• Rounded: 19

10. Neon (Ne): 20

11. Sodium (Na):

• Unrounded: 22.990

• Rounded: 23

12. Magnesium (Mg) 24

13. Aluminum (Al):

• Unrounded: 26.982

• Rounded: 27

14. Silicon (Si): 28

15. Phosphorus (P):

• Unrounded: 30.974

• Rounded: 31

16. Sulfur (S): 32

17. Chlorine (Cl): 35

18. Argon (Ar):

• Unrounded: 39.948

• Rounded: 40

19. Potassium (K): 39

20. Calcium (Ca): 40