What is the simplest form of matter with a unique set of properties?

The substance which has mass and volume is said to be matter. Matter is classified as Pure substance (Element and Compound) and Mixture (Homogeneous and heterogeneous).

The simplest form of matter that has unique properties is known as element.

Elements can't break down into simpler units or substances by physical or chemical process. It is a  pure substance which is made up of single type of atom such as hydrogen is made up of hydrogen atoms only.

Every element has unique name and symbol such as name: iron and its symbol is .

Hence, element is defined as the simplest form of matter that has unique properties.