Question

how to write chemical formula of first 20 elements.

Answer 1

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca
You can spell them according to their spelling and remember their sequence by their pronunciation.

Answer 2

Hydrogen H  

Helium He  

Lithium Li  

Beryllium Be

Boron B

Carbon C

Nitrogen N

Oxygen O

Fluorine F

Neon Ne

Sodium Na

Magnesium Mg  

Aluminum Al  

Silicon Si  

Phosphorus P

Sulphur S

Chlorine Cl  

Argon Ar  

Potassium K  

Calcium Ca