Early Chemical History

Early, early chemistry Chemistry has been around since people have been around Democritus- 400 BC

Next 2000 years 4 elements Metals to Au Robert Boyle- 1600’s Joseph Priestly- 1700’s

Real chemistry (late 1700’s) Antoine Lavoisier combustion O2 measurement Law of Conserv of Mass

Joseph Proust (early 1800’s) A given compound always contains exactly the same proportion of elements by weight Law of definite proportions

John Dalton Reasoned that not only the same weight per compound, but same number of atoms

Exp with C and O wt of O with 1 g C Cmpd 1 1.33 g Cmpd 2 2.66 g Cmpd 2 has 2 X’s the O

Law of Multiple Proportions When 2 elements form a series of compounds, the ratios of the masses of the 2nd element that combine with 1 g of the first element can always be reduced to small, whole #’s

Dalton’s Atomic Theory 1. Each element is made of tiny particles called atoms.

Dalton’s Atomic Theory 2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way.

Dalton’s Atomic Theory 3. Chemical compounds are formed when atoms combine with each other. A given cmpd always has the same # and types of atoms.

Dalton’s Atomic Theory 4. Chemical rxns involve the reorganization of the atoms- changes in the way they are bound. The atoms are not changed.

Water... Formulas are simple OH mass ratio 1:8 H = 1, O = 8 atomic mass

Joseph Gay-Lussac 1. Measured (under same T and P) volumes of gases that reacted

Amadeo Avogadro At same T and P, equal volumes of gases contain the same number of particles DIATOMICS

The Electron JJ Thomson studied electrical discharges in vacuum tubes called cathode ray tubes- high voltage produced negative cathode rays

The Electron Determined that ray was made of negative particles that he called electrons

Model of the atom If electrons are negative, atoms are neutral, so there must be a positive part of the atom Plum Pudding Model

Robert Millikan Found the mass of an electron using charged oil drops 9.11 x 10-28 g

Radioactivity Henri Becquerel found that uranium left marks on photographic plates spontaneous emission of radiation

Ernest Rutherford (early 1900’s) Shot positive alpha particles at gold foil If Thompson’s model was correct, the large alpha particles should crash through gold foil

The Nuclear Atom Some particles bounced back!! Defined the nuclear atom

Modern Atomic Structure Tiny nucleus with large cloud of e- Nucleus contains protons and neutrons

Different Properties All atoms composed of the same thing However… different number and arrangement

Neutral Nitrogen Has 7 protons Has 7 electrons Neutrons can vary ISOTOPE

Notation Mass # (A) top number protons + neutrons Atomic # (Z) bottom # just protons

Ions Elements that have gained or lost an electron changes charge

Ions CATIONS- elements lose e-’s (become positive) ANIONS- elements gain e-’s (become negative)