How are atoms and elements alike and different?

An element is an atom or group of the same type of atom in a concentrated, pure form. A molecule is a group of atoms that aren't all necessarily the same type of element. For example, in a sample of elemental iron, every atom in the sample will be an iron atom whereas in table salt, NaCl, half of the atoms are sodium atoms and the rest are chlorine atoms. Atoms are composed of a nucleus which has neutrally charged neutrons and  positively charged protons, surrounded by a cloud of negatively charged electrons. Number of electrons and neutrons may vary from atom to atom of the same element, but the number of protons in the atom determine what element it is.