Atoms and the Periodic Table Part I: Historical Background

Current Atomic Theory 1.What makes up the atom? 2.Where are things found in the atom? 3.What kind of evidence lead to our current understanding of this theory?

History of Chemistry Practical Arts and Crafts (---to 600 B.C.E.) Greek Period (600 to 300 B.C. E.) Alchemy (300 B.C.E. to 1650 C.E.) Phlogiston (1650 to 1790) Modern Chemistry ( ) Note: This listing focuses on the Middle East and Western Europe. Dates are very approximate.

Greeks and the Atom Leucippus and Democritus (ca 400 BCE) thought that matter was made up of tiny particles that could not be separated into smaller units. These particles were called atomos (uncuttable) Each atom had distinct shape and size.

Dalton’s Atomic Theory All matter consists of tiny indivisible and indestructible particles called atoms 2.Atoms of a given element are the same and have identical properties and masses. Atoms of different elements have different properties and masses

3. Atoms of different elements combine in small whole number ratios to form compounds 4. Chemical reactions involve rearranging atoms into new combinations.

Rearrangement of Atoms in a Chemical Reaction

Dalton’s Theory still holds, with two exceptions: Not all atoms of element are identical (isotopes) Possible to destroy or create elements (nuclear methods)

Dalton’s Atomic Theory explained several scientific laws Law of Conservation of Matter (Mass): Matter is neither gained nor lost during a chemical reaction Antoine Lavoisier Mass before = Mass after

Law of Definite Proportions: The elements in a compound are always present in a definite proportion by mass Joseph Proust H 2 O 2 g H and 16 g O

Law of Multiple Proportions Elements can combine in more than one set of proportions, with each set of proportions corresponding to a different compound. John Dalton CO3 g C 4 g O CO 2 3 g C8 g O

Dalton’s Theory (1803) still holds, with two exceptions: Not all atoms of element are identical (isotopes) Possible to destroy or create elements (nuclear methods)

Discovery of Subatomic Particles Electrons “Cathode Rays” 1875 William Crookes Protons1886 Eugen Goldstein Neutrons1932 James Chadwick

Vacuum Tube Pass high voltage through tube Electrode (negative electrode) Anode (positive electrode) Screen coated with Zinc Sulfide Gives green fluorescent color when current hits screen

Fig. 2.5

J. J. Thomson 1897 Used cathode ray tube to discover negatively charged particles Placed electrical plates around above and below tube

If you remove electrons from an atom, you wind up with a positively charged “ion” Can do this by shooting a stream of electrons at atoms Resulting positive ions move toward negative electrode.

Goldstein used vacuum tube which had holes in the cathode Some positive particles were formed, moved toward the negative cathode and passed through the holes to hit the fluorescent screen Particles called “canal rays” (protons)

Ernest Rutherford How are the protons and electrons distributed in atom? Thought that particles were evenly spread throughout atom (Plum Pudding Model) Designed experiment to test this model

Rutherford’s Experiment 1911 Shot positive alpha particles at thin gold foil Expected that the alpha particles would pass straight through foil

Most particles passed straight through foil Some were deflected at an angle A few were deflected straight back to source

New Model to Explain Data Small dense nucleus in center with positive charge (most of mass) Nucleus must contain the protons (+) Electrons (-) are outside the nucleus

Atom Nucleus located in center of atom is small, dense and positively charged. Contains protons and neutrons Region outside the nucleus contains electrons

Sub-Atomic Particles ParticleMass Charge Location (amu) Proton Nucleus Electron 1/ Outside nucleus Neutron 1 0 Nucleus