Atom

According to Wikipedia  ,"The atom is the smallest unit that defines the chemical elements and their isotopes. Every material object, or substance that can be touched and felt, is made up of atoms. Everything that is solid, liquid, or gas is made up of atoms."

The above statement is very true. Everything we see around us is made up of Atoms. Though the atom is not the smallest unit. According to Wiki atom is smallest unit that defines chemical elements and their isotopes, however in accordance with Physics it is not the smallest. I'll cover that later.