The smallest unit or the basic building blocks of matter which make up all objects. Made up of equal number of protons and electrons.

Two or more atoms of the same or different elements joined together.

Smallest unit of a living thing.

A group of cells working together to perform a particular function in the body of the living organism.

Groups of different types of tissues organized together to carry out a particular set of functions.

Groups of organs that function together to carry out broad sets of functions.

A group of organ systems working together that has a distinct existence as a complex, self reproducing unit.