The Structure of Matter

1. The Structure of Matter “Nothing exists except atoms and empty space; everything else is opinion” – Democritus of Abdera

2. Historical Overview • By the middle of the 19th century, much indirect evidence for the existence of atoms, the building blocks of matter. • Around the end of the first millennium, experimental observation became the standard for scientific exploration – advanced largely in the Arab world … alcohol, alchemy are from Arabic • Alchemy – search for the magic formula for perpetual life – the elixir … also, how to turn lead into gold. • Following Aristotle – used fire and water …. Heating, dissolving in water, stirring, blending, grinding, … • 1780’s Antoine Lavoisier – elements and compounds • Elements cannot be broken down into simpler substances

3. Chemical Evidence for Atoms • Law of Definite Proportions • 10 g of substance A + 2 g of B yields 12 g of compound. • 10 g of substance A + 10 g of B still leaves 12 g of compound, with 8 g of A left over. • Therefore, A and B form a compound in the ratio 5:1 • This is very difficult to explain in the Aristotlean model of the world • Of course, we know understand this in terms of atoms combining together to form molecules. • But, still it was difficult to figure out the number of atoms in each molecule – all scientists had was information on the masses … 8g of oxygen combined with 1g of hydrogen made 9g of water. • What are the relative masses of the atoms?

4. Masses and Sizes of Atoms • New evidence from combining gaseous elements … Gases combined in definite volume ratios, when their temperatures and pressures were the same. • Amedeo Avagadro – under identical conditions, ANY GAS contains the same number of molecules … 6.022 x 1023 molecules per mole. • 1 mole of gas will 22.4 litres, at 20oC and normal atmospheric pressure.

5. Quiz • Question: We combine 44.8 litres (2 mol) of hydrogen with 22.4 litre (1 mol) of oxygen, to form water vapour (H2O). How much water vapour is produced? • Answer: 44.8 litres (2 mol) !!! Remember, it is the number of ATOMS or MOLECULES that matters, not the volume. • 2 mol H2 gas contains 2.4 x 1024 H atoms • 1 mol O2 gas contains 1.2 x 1024 O atoms • The combine to form 1.2 x 1024 H2O molecules • 1.2 x 1024 H20 molecules = 2 mol = 44.8 litres

6. Flawed Reasoning • Student 1 – “If two ideal gases are both at the same temperature and pressure, then equal volumes will contain equal numbers of atoms. The means that 1 mole of ammonia (NH3) would take up twice as much volume as 1 mole of nitrogen (N2) because each ammonia molecule has four atoms, and each nitrogen molecule has only two.” • Student 2 – “No, equal volumes will contain equal numbers of molecules, not atoms. One mole of ammonia would contain the same number of molecules as one mole of nitrogen, so they would take up the same volume.”

7. Molecular Masses • Atoms are made of electrons, protons, and neutrons • The proton’s and neutron’s mass is 1.66 x 10-27 kg • The electron’s mass is 0.911 x 10-30 kg • So, almost all of the mass of the atom comes from the nucleus. • The number of protons = the number of electrons • Each element in the periodic table has a unique number of PROTONS. For stable atoms, there is usually a certain specific number of neutrons in the nucleus as well. • Hydrogen Atom – 1 proton. Oxygen Atom – 8 protons, 8 neutrons. Water Molecule – 10 protons, 8 neutrons.

8. Pressure • Pressure is defined as the force per unit area: • Atmospheric Pressure is caused by the weight of the earth’s atmosphere pressing down: • 1 atm = 101,325 Pa

9. Atomic Speeds and Temperature • Imagine a container full of some gas • The molecules of the gas are constantly in motion (Brownian motion) • The molecules travel around the container and strike the walls. With each collision, they bounce off the wall. Their momentum is reversed, and hence there is a force on the wall. This is the source of PRESSURE in the container. • The more collisions there are with the walls, the greater the pressure. • The FASTER the molecules are moving, on average, the more collisions there will be, and hence the greater the pressure. • It turns out that the AVERAGE SPEED of the atoms of the gas is directly related to the TEMPERATURE of the gas.

10. Temperature Scales • Celsius – Anders Celsius (Swedish) – defined by the freezing and boiling points of water • Fahrenheit – Gabriel Fahrenheit (German) • 0 F = freezing point of ice/salt/water mixture – lowest temp. in a lab at the time • 96 F = body temperature (again, easy to produce in the lab) • Later, this was meshed with the Celsius scale, and the freezing of boiling points of water were set to be 32F and 212F, making body temperature 98.6F. • Kelvin - thinking about atomic speeds, at what temperature does the motion STOP? It turns out that it is at -273.15 on the Celsius scale. This is called ABSOLUTE ZERO, and is the zero of the Kelvin scale. The Kelvin scale is the one that we need to use in most calculations involving temperature. • For example, the PRESSURE of an ideal gas, inside a container, is proportional to the TEMPERATURE, in Kelvin.

11. The Ideal Gas Law • The three macroscopic properties of a gas – pressure, temperature, and volume • These are related by the ideal gas law n is the number of moles of gas R is the ‘gas constant’: R = 8.314 (Pa m3)/(mol K)