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.
The gravitational potential energy of the ball decreases as it rolls down the hill while the gravitational kinetic energy of the ball increases due to its acceleration in velocity.
