Which formula contains only two elements?

1) Bi (NO3) 3

2) KOH

3) NaClO3

4) Al2S3

Elements are made of one type of atoms. Elements have characteristic symbols for their identification.

1) Bi (NO₃) ₃ - this is made of 3 elements - Bismuth, Nitrogen and oxygen

2) KOH - made of 3 elements - potassium (K), oxygen and hydrogen

3) NaClO₃ - made of 3 elements - sodium (Na), chlorine and Oxygen

4) Al₂S₃ - made of 2 elements - Aluminium and sulphur

we are asked for the formula with 2 elements, correct answer is 4)