Here's why:

* Nucleic acids (DNA and RNA) are composed of nucleotides. Each nucleotide has a phosphate group, a sugar, and a nitrogenous base. The phosphate groups form the backbone of the nucleic acid strand.

* Proteins are made up of amino acids. Amino acids contain carbon, hydrogen, oxygen, and nitrogen, but not phosphorus.

Therefore, phosphorus is a key component of nucleic acids but not proteins.