Molecules and Atoms

• The physical world is quantified.
  Take any material, water for example : It seems to be continuous, but if you take 18g of water which is about 18 cm³ you cut it in two 10 times; you have 1 thousandth of the initial water (2¹⁰ ≈ 10³), you go on cutting in two 70 times : 2⁷⁰ = (2¹⁰)⁷ = (10³)⁷ = 10²¹. you get a particule called molecule of water.
• Matter is made of molecules.
  It is the minimum quantity of water you can get.
  If you cut this molecule, you will no longer have water, but one atom of oxygen and 2 atoms of hydrogen.
• Molecules are made of atoms.
  It was easy to cut water in two until you get a single molecule.
  To break this molecule, it is much more difficult.
  That is why matter does not change so easily.
• In 18 g of water, there are about 6. 10²³ molecules.
  • exercice : what is the volume occupied by of a water molecule ?
    answer : 18 cm³ / 6. 10²³ ~ 3. 10⁻²³ cm³
  • exercice : if this volume is a cube, what is its side ?
    answer : ³√(3. 10^-23) cm³ ∼ 3. 10⁻⁸ cm = 3. 10⁻¹⁰ m = 3 Angstroms (Å)
    so the radius ∼ side / 2 = 1.5 Angstrom
    (to have simple numbers, we will use the Angstrom unit for the distances between atoms : 1 Angstrom = 10⁻¹⁰ m)
• An atom is made of a nucleus and electrons.
  The nucleus has a positive charge which is a integer number of an elementary charge : e = 1.6 10⁻¹⁹ Coulomb
  This nucleus is surrounded by the same number of electrons of the opposite elementary charge.
  So that the total charge of the atom is 0.
• Sometimes, it is said that there is a lot of vacuum inside an atom.
  It is a wrong image : electrons, due to their low energy, have the size of the "cloud".
  And we can say that the electron at the ground state energy level fills all the "vacuum" around the nucleus.
  (Electrons have only discrete possible levels of energy)
• The nucleus is made of nucleons : protons and neutrons.
  protons have a positive elementary charge
  neutrons have no charge.
• Avogadro's number : N_A = 6.02 10²³ is the number of atoms (or molecules) contained in a mole.
  It is an arbitrary number well chosen so that the mass in gramms of a mole of pure isotope is very near the number of nucleons (proton or neutron)
  The mass of a mole extends from 1g to a few hundreds depending on what kind of atoms it contains.
  • Hydrogen : 1 mole of H weights 1 g
  • Carbon : 1 mole of C weights 12 g
  • Oxygen : 1 mole of O weights 16 g
  When it is not near a round number, it means that it is a mix of isotopes like ³⁵Cl + ³⁷Cl : M(Cl) = 35.5 g
  • 75% of ³⁵Cl : which is Chlorine isotope with 18 neutrons (and 17 protons)
  • 25% of ³⁷Cl : which is Chlorine isotope with 20 neutrons (and 17 protons)