A Very Brief History of Chemistry (loosely from Chapter 0 of Jespersen & Hyslop 7TH ed) Dr. C. Yau Spring 2015

Overall View of 4 Main Concepts of Chem Atomic theory – John Dalton, 1813 Described atoms and how they interact with one another 2. Careful laboratory observation Can lead to understanding the atomic world 3. Energy changes and probability Lead to predictions about chemical interactions Geometric shapes of molecules are important Affect properties, reactivity, and function (e.g. DNA, RNA, and proteins)

Macroscopic vs. Particulate View Macroscopic View: What we can actually see & measure. Particulate View: What we imagine matter to be composed of based on the Atomic Theory.

The Big Bang: As the Big Bang cooled 14 billion years ago Explosion of energy and subatomic particles Extreme temperature, pressure, and density Universe has been expanding ever since As the Big Bang cooled Initially only quarks exist After 1 second, quarks form protons and neutrons After 3 minutes, nucleosynthesis begins of light nuclei (e.g., helium, lithium) After further cooling, electrons join nuclei to form atoms.

Supernovas and the Elements Universe was 91% hydrogen, 8% helium, 1% other light atoms Uneven distribution of matter resulted in star formation Formation of elements occurred in the stars Small atoms combined, due to high pressure at the center, to create slightly heavier elements New elements concentrated in the stars’ centers Heavier elements then combined into new, even heavier elements Cycle kept repeating

Supernovas and the Elements Iron is the heaviest element created in stars Causes the nuclear reactions to stop and the star to cool and collapse in on itself allows for even heavier elements to form Eventually, the star disintegrates Called a supernova Spews its content into space Remnants rejoin to form new stars Cycle begins again Some of the debris combines to form moons, planets, and asteroids

Distribution of the Elements Earth formed 4.5 billion years ago Result of gravitational forces Earth heated up Iron and nickel melted Migrated to the core Outer core is superheated lava Mantel is superheated rock Crust is the surface 10 miles thick Contains the familiar elements (gold, silicon, carbon, etc.)

3 Scientific Laws Law of Conservation of Matter (Mass) Law of Definite Proportions (Ratios) Law of Multiple Proportions (Ratios) A scientific law is a merely a summary of observations with no exceptions. It is not an explanation for the observed. John Dalton (1800’s) developed a theory to explain these laws: ATOMIC THEORY

Dalton’s Atomic Theory Matter consists of tiny particles called atoms Atoms are indestructible In chemical reactions, atoms rearrange but do not break apart In any sample of a pure element, all atoms are identical in mass and other properties Atoms of different elements differ in mass and other properties In a given compound, constituent atoms are always present in same fixed numerical ratio

STM* Image of atoms of palladium (Pd) deposited on a graphite surface *STM = Scanning Tunneling Microscope (invented the early 1980’s) Figure 0.3

Thomson’s Cathode Tube (mid-1800’s) Rays observed to flow from negative anode to positive cathode: Particles in ray must be negative. Figure 0.4

Modified Cathode Ray Tube Magnetic field bent beam to Point 2 Charged electrodes (metal plates) bent beam to Point 3 Charge was adjusted to cancel the 2 effects, leading to charge to mass ratio of particles. Regardless of what gas is in tube, same ratio is observed. Particle must be a fundamental part of all matter: the electron Figure 0.5

Millikin’s Oil Drop Experiment (1909) By adjusting charge of electric plates, he determined the charge of the electron to be -1.60x10-19 Coulombs Figure 0.6

Mass to Charge Ratio* = -1.76x108 C/g Charge** = -1.60x10-19 C Mass of the Electron Mass to Charge Ratio* = -1.76x108 C/g Charge** = -1.60x10-19 C C = coulomb, a unit of charge mass of electron *from the Modified Cathode Ray Tube Experiment **from the Millikin’s Oil Drop Experiment

Thomson’s Plum Pudding Theory Since electrons are negative and protons are positive, it would make sense that the negative and positive particles would be evenly dispersed through the atom.

Rutherford’s Gold Foil Experiment (early 1900’s) Alpha particles are deflected by a thin gold foil. Most beams went straight through. A few bounced right back. Atom must have massive particles (the protons) concentrated in a small space (the nucleus). The atom is made of mostly empty space! Figure 0.7

The heavy particles (p and n) must be concentrated in a VERY small space in the atom (the nucleus)....how small? Figure 0.8

Discovery of the Neutron (Chadwick, 1932) First postulated by Rutherford and coworkers Estimated number of positive charges on nucleus based on experimental data Expected mass to be same as + charge BUT, expected mass always far short of actual mass (about ½ actual mass) Therefore, must be another type of particle Has mass about same as proton Electrically neutral Chadwick (1932) was awarded the Nobel Prize for the discovery of the neutron.

u = atomic mass unit (amu), a very small unit of mass No need to memorize the exact mass listed above Instead, learn table below: u = atomic mass unit (amu), a very small unit of mass defined as exactly 1/12 mass of the C-12 atom. Table 0.3