next paragraph). In a compound the molecule is the smallest part that has all the characteristics of  the  compound.  Consider  water,  for  example. Depending on the temperature, it may exist as a liquid  (water),  a  solid  (ice),  or  a  gas  (steam). Regardless of the temperature, it will still have the same composition. If we start with a quantity of water, divide this and pour out one half, and continue this process enough times, we will end up   with   a   quantity   of   water   that   cannot   be further divided without ceasing to be water. This quantity  is  called  a  molecule  of  water.  If  this molecule of water is divided, instead of two parts of water, we will have one part of oxygen and two parts  of  hydrogen  (H₂O). ATOMS Molecules  are  made  up  of  smaller  particles called ATOMS. An atom is the smallest particle of an element that retains the characteristics of that element. The atom of one element, however, differs  from  the  atoms  of  all  other  elements,  Since over 100 elements are known, there must be over 100 different atoms, or a different atom for each element. Just as thousands of words are made by a   combination   of   the   proper   letters   of   the alphabet,   so   thousands   of   different   materials are  made  by  the  chemical  combination  of  the proper  atoms.  Any  particle  that  is  a  chemical combination  of  two  or  more  atoms  is  called  a molecule. The oxygen molecule has two atoms of oxygen,  and  the  hydrogen  molecule  has  two molecules of hydrogen. Sugar, on the other hand, is  a  compound  composed  of  atoms  of  carbon, hydrogen,  and  oxygen.  These  atoms  are  combined into sugar molecules. Since the sugar molecules can  be  broken  down  by  chemical  means  into smaller and simpler units, we cannot have sugar atoms. In  figure  2-1  you  will  see  that  the  atoms  of each element are made up of electrons, protons, and, in  most  cases,  neutrons,  which  are collectively  called  subatomic  particles.  Further- more, the electrons, protons, and neutrons of one element  are  identical  to  those  of  any  other element. The reason there are different elements is that the number and arrangement of electrons and protons within the atom are different for the different  elements. The  electron  is  considered  to  be  a  small negative  charge  of  electricity.  The  proton  has  a positive charge of electricity equal and opposite to  the  charge  of  the  electron.  Scientists  have measured the mass and size of the electron and proton.  They  know  how  much  charge  each  has. The  electron  and  proton  each  have  the  same quantity  of  charge,  although  the  mass  of  the proton is about 1837 times that of the electron. In some atoms, a neutral particle exists called a neutron.  The  neutron  is  a  mass  about  equal  to that of a proton, but it has no electrical charge. According  to  a  popular  theory,  the  electrons, protons, and neutrons of the atoms are thought to be arranged in a manner similar to a miniature solar  system.  The  protons  and  neutrons  form  a heavy  nucleus  with  a  positive  charge,  around which  the  very  light  electrons  revolve. Figure 2-1.—Structure of simple atoms. 2-2