Atoms are the smallest units of . They are formed by particles with different charges and locations. Protons have charges, electrons have charges and neutrons are not charged. Protons and neutrons make up the of an atom and define the atomic . The atomic is determined by protons. Atoms interact through bonds to form groups called , including the large categories (protein, carbohydrate, lipid, nucleic acid). Three bonds that join atoms are: - bonds formed between atoms that donate or receive electrons and become charged. -strong bonds that exist between atoms of a molecule that share electrons. These bonds can be polar or nonpolar, depending on whether or not electrons are shared equally. Oxygen and nitrogen are atoms, which have greater pull on electrons creating a molecule with unequal charges. For example, within a water molecule the bonds are all covalent because there is unequal sharing of electrons between the oxygen and the two hydrogens. Methane, on the other hand, has all covalent bonds because electrons are shared equally among the carbon and hydrogens. -weak bonds are formed between two atoms with slight charges on them like the partially negative oxygen atom of one water molecule and the partially positive hydrogen atom of another water molecule. Even though atoms and molecules are not living, they are the building blocks of cells, which are the smallest units of .