Atomic Models Through Time

Democritus• @400 B.C.• Theorized that if you were to cut

something in half, then cut it in half again and again…

• Eventually you would be left with something too small to be cut.

• Therefore, Democritus gave us “atomos,” or “uncuttable,” to be the word for the smallest individual particle of a substance.

John Dalton

• Dalton’s Atomic Theory ~ 1808– All matter is composed of atoms.– Atoms of each element are the same size,

mass, etc., but are different from atoms of other elements.

– Atoms can’t be subdivided (cut) or destroyed.• Envisioned atoms as “tiny, solid balls”

Cathode Ray Tube• If you run electricity through a gas filled tube,

– you see streams of particles• How do we know they are particles?

– A cross left a “shadow”, so something must be traveling in a straight line.

• How do we know it isn’t light?– J.J. Thomson put a magnet next

to the tube and it bent the beam.• Light is not magnetic!• Beam had a negative charge

• Thomson’s Atomic Theory ~ 1897– Discovered negatively charged

electrons.– Envisioned atoms as a “positively

charged sphere with electrons embedded in it.”

• “Muffin” Model– Positively charged bread– Negatively charged “blueberries”

floating around (electrons)

J.J. Thomson

Hantaro Nagaoka• Nagaoka’s Atomic Theory~ 1904

– Proposed a nucleus– Envisioned electrons orbiting a

positively charged sphere in the center.

• “Planetary” Model– Electrons = planets– Positive Center Sphere = Sun

Alpha Particle Cannon• Ernest Rutherford shot fast moving positively

charged alpha particles at thin gold foil…– If Thomson’s model was

correct, the alpha particles would pass right through the foil.

– Instead, some deflected back at large angles!

Alpha Particle Cannon• Conclusions:

– Thomson’s model was inaccurate

– A new model must be created

1. The atom is mostly empty space2. There is a dense, positively charged

nucleus in the center

Ernest Rutherford

• Rutherford’s Atomic Theory ~ 1911– The atom is composed of mostly empty

space.– Small, dense, positively charged nucleus

containing almost all of the atom’s mass.• “Peach” Model

– Peach Pit = Nucleus– Meat = Empty space with electrons

randomly orbiting nucleus.

Refining Atomic Theory in the Early 1900’s

• Masses of protons are measured.– Protons are 2000 times more

massive than electrons.• Charge of protons is

determined to be positive.– Protons provide equal &

opposite charge to electrons.

Niels Bohr

• Bohr’s Atomic Theory ~ 1913– Electrons do not “randomly” orbit the

nucleus.– Electrons move in specific “layers” or

“energy levels” / “shells” (3-dimensional).– Atoms absorb or give off energy when

electrons move from one “level” to another.

• Bohr’s Model– “onion” model

• Electrons move on layers of an onion.

Valence Electrons• Valence electrons are the electrons located

furthest from the nucleus– They are a determining factor in chemical

bonding.– If you visualize the atom like Bohr (an onion), the

valence electrons are the electrons located on the outer layer.

New Problem• As scientists find the masses of atoms,

they always have about twice the mass predicted by the number of protons– Remember, the mass of electrons is so small

it is insignificant• So, there must be something else in the

atom…

James Chadwick

• Contribution to Modern Model ~ 1932• Discovered the neutron

– Same mass as proton– No electrical charge

• This is why it was not “discovered” earlier

• Existence of neutron explained why atoms were heavier than the total mass of the protons and electrons.

Modern Model• Electron Cloud Model ~

1920’s to present– Electrons form a

negatively charged cloud around the nucleus.

– It is impossible to determine exactly where an electron is at any given time. (probability instead)

– Higher probability of electrons near nucleus.