Question

Which of the following substances are elements?

Air, water, hydrogen, ammonia, oxygen, marble, salt

Answer 1

Elements

• They are pure substances that are made up of some kind of atoms.
• We can not divide elements further by any other physical or chemical processes.
• It includes oxygen (O) and hydrogen (H).

Compounds

• They are made up of different kinds of atoms that are chemically bonded.
• It includes ammonia$\left({\mathrm{NH}}_3\right)$, marble$\left({\mathrm{CaCO}}_3\right)$, and salt$\left(\mathrm{NaCl}\right)$.

Mixtures

• They are materials made up of different substances which are not chemically bonded.
• It includes air.

Among the substances hydrogen, and oxygen are the elements, they are present in group 1 and group 16 of the periodic table respectively.