The Changing Models of the Atom

Democritus 460 – 370 BCE

Democritus 460 – 370 BCE• Democritus believed matter was made

up of “atoms” (Greek for indivisible)

• Democritus thought:– Each element had its own kind of atom. – Atoms of all elements had different shapes and

sizes. – All atoms moved around in space.

air

water

granite

copperwood

Aristotle 400 BCE

• claimed that there was no smallest part of matter

• different substances were made up of different proportions of fire, air, earth, and water.

• The four element theory

Four element example

• Wood: – contains “earth”

because it has weight– contains “fire” because

it can burn– contains “air” which is

released as smoke– contains water before

it dries out

• Copper:– More earth because it

is heavier– Less fire, but can still

be burned or melted– Very little air

– Very little water

Aristotle’s Fault

• Even though he was wrong, his theory persisted for some 2000 years!

John Dalton

John Dalton 1850• Dalton stated that all matter is made of

indivisible and indestructible atoms, which differ from element to element

• He also said atoms of different elements have different properties

• Different elements combine in specific ways to form new substances.

• The billiard ball theory

J. J. Thomson

Cathode ray tube

J.J. Thomson 1897

• Experimenting with vacuum tubes (cathode rays), scientists discovered that an electric current could pass through a tube as a bright light

What do you think that bright light might have been?

J.J. Thomson 1897

• Thomson found that the glowing stream would bend toward a positive charge, and bend away from a negative charge.– What could this mean?

• Thompson theorized that negatively charge particles were responsible and called them electrons.

Thomson’s Plum Pudding TheoryOr Raisin Bun Theory

Ernest Rutherford

Ernest Rutherford

• Rutherford fired tiny alpha particles at a very thin piece of gold foil.

Ernest Rutherford

• He found that most of the alpha particles passed right through the gold foil as he expected

• However, a small number of alpha particles were deflected or bounced right back.

• What can we conclude?

Rutherford’s Planetary model

• In 1911, Rutherford proposed a revolutionary view of the atom.

• Proposed that the atom consisted of a small, dense core of (+) particles in the nucleus of the atom.

• Electrons (-) floated around the nucleus like planets around the sun.

Rutherford’s planetary model

James Chadwick 1932

• The Chadwick problem:

• Positive charges repel positive charges• How then, can a nucleus not be pushed

apart as two magnets are?• Finally, if protons weigh 1amu, where is all

this extra mass coming from?

• What did he discover???

Point of Interest

This is why all of those giant elements at the bottom of the periodic table are

unstable or radioactive.

The Nuclei of the atoms have too many particles to stick together.

Too much charge!!

James Chadwick 1932 • Chadwick discovered the neutron.

• Neutrons reduce the repulsion between protons and stabilize the atom's nucleus.

• Neutrons are in the Nucleus and weigh 1amu

• Neutrons do not have any electrical charge, they are neutral

James Chadwick

The three isotopes of hydrogen

Neils Bohr

Bohr’s Model 1913• Bohr was studying electrons in atoms and

proposed the idea that electrons travel in specific shells around the atom's nucleus.

• Each of these shells can hold a certain amount of electrons which must be filled in a certain order

• We now know his theory is not entirely correct but it is still useful today.

Bohr’s model

The Quantum Mechanical Model

• This is the correct model used today.

• The electron cloud model uses the basic idea of Bohr’s model except that the electrons are not found in distinct orbits but their position can be thought of as in a 3D cloud that has a particular energy

• http://videos.howstuffworks.com/science/atoms-videos-playlist.htm#video-29292

• http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

Q’s

• Pg 177 # 1, 3, 4, 6, 9, 11