PA RT O N E

ATOMIC THEORY

ATOMIC THEORY

• Over the course of thousands of years our idea of what matter is made of and what the atom looks like has changed dramatically• There are 6 major contributors, although there

are many many many others who have helped out along the way• The 6 major contributors to the atom were

Democritus, John Dalton, J.J. Thomson, Ernest Rutherford, Neils Bohr, and Erwin Schrodinger

DEMOCRITUS

• Democritus (460 – 370 B.C.) was one of the first people to describe the atom• Unlike all of the other contributors to the atom,

his thoughts were NOT based on scientific evidence and research, but just his own thoughts• Democritus thought that atoms were indivisible

and indestructible

JOHN DALTON

• John Dalton (1766 – 1844) was the first person to describe the atom using scientific evidence and research• He studied the ratios in which elements combine

in chemical reactions to come up with his theory

JOHN DALTON

John Dalton’s Atomic Theory can be explained in 4 parts:1. All elements are composed of tiny indivisible particles

called atoms2. Atoms of the same element are identical. The atoms of

one element are different from the atoms of all other elements

3. Atoms of different elements can physically mix together (mixtures) or chemically combine in whole ratios to form compounds

4. Chemical reactions occur when atoms are separated, joined, or rearranged in different combinations. Atoms of one element will never change into atoms of another element as a result of a chemical reaction.

J. J. THOMSON

• In 1897 J.J. Thomson discovered the electron!• With this discovery we no longer think that the

atom is indivisible, because we know that it is now made up of even smaller parts

J. J. THOMSON

His experiment is the cathode ray experiment:

• He passed an electric current through gases at low pressure in a sealed glass tube with electrodes at both ends, this produced a cathode ray (a glowing beam)

• He then did tests on the cathode ray with electrically charged plates, and he found that the ray went away from the negatively charged plate and towards the positively charged plate

• He knew that like charges repel and opposite charges attract so he reasoned that the ray was negatively charged

CATHODE RAY EXPERIMENT

J. J. THOMSON

• J.J. Thomson knew that the atom had an overall neutral charge, so he thought the negative electrons were evenly distributed through the positive atom• His model is known as the plum pudding model

ERNEST RUTHERFORD

• In 1911 Rutherford performed the gold foil experiment• He shot alpha particles (Helium atoms that are

super positively charged) at a thin sheet of gold• Because they thought the gold sheet had its

positive charged scattered throughout they expected that the particles should either go through or be slightly deflected • Many did that, but some also bounced off the

gold foil really strongly, which was really surprising and unexplained

GOLD FOIL EXPERIMENT

ERNEST RUTHERFORD

Based on his experiment Rutherford concluded that:• The atom is mostly empty space• All of the positive charge and all of the mass of the

atom are found in the middle• This positively charged, dense middle region was

called the NUCLEUS• Electrons are found evenly distributed around the

nucleus

His model is known as the “nuclear atom”*When Chadwick found the neutron it was added to Rutherford’s model and put in the nucleus

OTHER IMPORTANT CONTRIBUTIONS

• Robert A. Millikan – found the charge of an electron using the oil drop experiment• Eugen Goldstein – using a similar experiment to

J.J. Thomson he found the positively charged proton, and found that it’s mass was much much larger than an electron (1840 times heavier)• James Chadwick – found the neutron and found

that it has no charge but a mass the same as the proton

OVERVIEW OF THE ATOM

• The atom is the smallest piece possible of any given element while still keeping all of the properties of that element• The atom is made up of 3 subatomic particles: The

proton, neutron and electron• “Subatomic” Sub = under or below• Atomic = of or relating to an atom or atoms

• The atom is TINY!!! One penny (if it were made of pure copper) would contain 2 x 1022 copper atoms• Technology has improved so much that we can now

see individual atoms using an electron microscope

OVERVIEW OF THE ATOM

What we know (as of Rutherford/Chadwick):• Electrons are negative and have almost no mass

and are distributed around the nucleus• Protons are positive & neutrons have no charge.

Both are much heavier and are found in the nucleus