A molecule is a particle that makes up matter. It’s made of two or more atoms that are bonded together chemically, or by chemical bonds. Molecules are reversible and can be broken down into their respective atoms, chemically. Like atoms, even molecules are not easily visible and require an electron microscope. For a clearer size estimate, a human body has more molecules than the total number of stars in the sky.

Composition

The atoms making up a molecule may belong to the same element, or could be from different elements. For instance, H2O (water) is a molecule that is made of two hydrogen atoms and one oxygen atom. Similarly, O2 (molecular oxygen) is a molecule comprising two oxygen atoms. The chemical bonding doesn’t happen between all atoms. For example, chlorine and oxygen atoms attach, but atoms of argon and neon don’t come together. A molecule that has atoms from different elements is called a compound. In the above example, the H2O molecule is a compound.    

Motion and Arrangement

Molecules are constantly in motion. The speed and frequency of their movement depends on the state they are in. In solids, molecules are packed tightly together having a specific arrangement, and they can only slightly move or jiggle. The space or gap between molecules is called inter-molecular space, which is the least in solids. Thanks to this negligible space, molecules in solids attract to each other much stronger, giving the solid its rigid form. 

Image credit: Flickr
Image credit: Flickr

In liquids, there is no particular arrangement and the movement is much more pronounced. The molecules still stay closely knit together despite the much larger intermolecular space. In gases, molecules have no arrangement and they move at high speeds haphazardly. There’s almost zero attraction force between the molecules in a gas.