MODELS OF ATOMS Democritus(400 B.C):

said that matter is composed of indestructible particles. He called them “atoms”, a Greek word which means indestructible particle.

The Early History of Chemistry

The Early History of Chemistry

Before 16th Century– Alchemy: Attempts (scientific or otherwise) to change cheap metals into gold

17th Century– Robert Boyle: First “chemist” to perform quantitative experiments

18th Century– George Stahl: Phlogiston flows out of a burning material.– Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.”

Law of Conservation of MassLaw of Conservation of Mass

Discovered by Antoine Lavoisier

Mass is neither created nor destroyed

Law of Definite Proportion (Proust)

Law of Definite Proportion (Proust)

A given compound always contains exactly the same proportion of elements by mass.

Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.

Law of Multiple Proportions (Dalton)

Law of Multiple Proportions (Dalton)

When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.

The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).

John Dalton-1808

Dalton’s Atomic Theory (1808)

Dalton’s Atomic Theory (1808)

Each element is made up of tiny particles called atoms.

The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

Dalton’s Atomic Theory(continued)

Dalton’s Atomic Theory(continued)

Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.

Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Early Experiments to Characterize the AtomEarly Experiments to

Characterize the Atom

J. J. Thomson ( 1898-1903) - postulated the existence of electrons using cathode ray tubes.

Deflection of Cathode Rays by an Applied Electric Field

Thompson found electrons were produced from Electrodes of various metals.

Therefore, he concluded that all atoms must containelectrons.

Since atoms are neutral, he concluded that atoms mustcontain positive charge.

THOMPSON

Robert Millikan

His OIL DROP EXPERIMENT determined the charge and mass of the electron.

Mass = 0.000549 amu or 1/1850 amu or 9.109 x 10-28 g

Charge = -1 or 1.5921 x 10-19 Coulomb

Diagram of the Millikan Apparatus

Millikan calculated the mass of the electron = 9.11 x 10-31 kg.

The Nuclear Atom

Ernest Rutherford

Famous Gold Foil experiment in 1911.

CONCLUSIONS☻The atom contains a dense center called the

nucleus.

☻The nucleus contains a positive charge and most of the mass of the atom.  

☻The nucleus is approximately 100,000 times smaller than the atom.

CONCLUSIONS The nucleus is positively charged. The

amount of positive charge balances the negative charge of the electrons.

The electrons move around in the empty space of the atom surrounding the nucleus.

Incomplete Model

Did not explain where the electrons are located in the space surrounding the nucleus.