How do you know hydrogen and oxygen and nitrogen are elements?

Hydrogen, oxygen, and nitrogen are classified as elements because they cannot be broken down into simpler substances through chemical means. Each of these substances consists of only one type of atom, which defines them as fundamental building blocks of matter. Additionally, they are listed on the Periodic Table, where elements are organized based on their atomic number and properties. Their unique atomic structures contribute to their distinct chemical behaviors and roles in various compounds.