Atoms are the smallest individual particle of a matter. Everything in this world are matter and comprises of atoms. But each and every atom is unique. They are not the same at all. Every atom consists of three basic units protons, neutrons, and electrons and these units determine the physical and chemical properties of an atom and matter. But the presence of a different number of these units( protons, neutrons, and electrons) makes an atom unique from the other atom. The number and arrangement of electrons in an atom define the chemical characteristics of elements. For example take the isotopes of carbon: C12, C13, and C14. The number of neutrons in these carbon atoms are different and hence have different properties. So we can clearly say that all atoms are not the same.