Answer:
It means these compounds are only made up of two elements.
Explanation:
Binary compounds:
The compounds which are made up of the atoms of only two elements are called binary compounds.
For example:
The following compounds are binary:
HCl
H₂O
NH₃
HCl is binary because it is composed of only hydrogen and chlorine. Ammonia is also binary compound because it is made up of only two elements nitrogen and hydrogen.
water is also binary because it is also made up of only two elements hydrogen and oxygen.
While all polyatomic compounds are not binary. for example,
H₂SO₄
It is made up of hydrogen, sulfur and oxygen. There are three elements are present in this compound. So, it is not binary compound.
