Atomic Theories

Early Greek Theories

Aristotle (350 B.C.)• 4 Elements

Democritus (400 B.C)• Matter: atoms and a void (empty space)• Atoms are indivisible

Dalton’s Billiard Ball Model (1805)• All matter is made of atoms.• Atoms of an element are identical.• Each element has different atoms.• Atoms of different elements combine in constant

ratios to form compounds.• Atoms are rearranged in reactions, but are not

created nor destroyed.

Thomson’s Raisin Bun Model (1897)

• Cathode Ray experiment: Sent electricity through a gas, producing negatively charged particles “cathode rays”

• Discovered the electron: a negatively charged particle with a small mass

• Atoms consisted of a positively charged sphere embedded with negative electrons

Rutherford’s Gold Foil Experiment (1910)

• Alpha particles (positively charged helium ions) were fired toward a very thin gold foil.

• Most alpha particles passed through.

• Some positive -particles deflected or bounced back!

• Conclusion: Atoms are mostly empty space. Most of the mass of an atom is concentrated in a positive core, called the atomic nucleus.

Shortfalls of Rutherford’s Model

• Orbiting electrons should emit light, losing energy in the process

• This energy loss should cause the electrons to collapse into the nucleus

• However, matter is very stable, this does not happen

Bohr’s Planetary Model (1913)

• Electrons orbit the nucleus in energy “shells”• Electrons in these shells have a specific and

constant amount of energy, without losing energy in the shell

• The greater the distance between the nucleus and the energy level, the greater the energy level

• An electron changes energy levels by emitting or absorbing a specific quantity (quantum) of energy in the form of light (photons)

• Electrons at their lowest possible energy state are in their ground state

• If light or heat is directed at an atom, electrons can absorb the energy, and move to a higher energy shell

• When the electron returns to a lower energy state the energy is released in the form of a photon, which we see as visible light

• The energy of the photon determines its wavelength or colour

• Each element has its own frequencies of color

Summary of Atomic Models1) Dalton’s “Billiard ball” model (1800-1900)

Atoms are solid and indivisible.

2) Thomson’s “Raisin bun” model (1900)Negative electrons in a positive framework.

3) Rutherford’s “Nuclear” model (~1910)Atoms are mostly empty space.Negative electrons orbit a positive nucleus.

4) Bohr’s “Planetary” model (~1920)Negative electrons orbit a positive nucleus.Quantized energy shells

5) Quantum Mechanical model (~1930)Electron probabilities (orbitals)

Indivisible Electron Nucleus Orbit Electron Cloud

Greek X

Dalton X

Thomson X

Rutherford X X

Bohr X X X

Wave X X X