Matter is anything that has mass and occupies space. There are three different states of matter- solid, liquid, and gas.
Two atoms of oxygen element are represented as "20". The two oxygen atoms are unreactive state and are unstable readily reacts and form molecules.
O₂ reprersents one molecule oxygen. When the two oxygen atoms reacts with each other forms one molecule of Oxygen. After the formation of Oxygen molecule, it (oxygen) has got stability.
