3.1 Early Atom Theories and the Origins of Quantum Theory

Electron

A negatively charged subatomic particle

Discovery of atom

• Democritus has first speculated — not based on science

• 1700s, Antoine Lavoisier & others used experiments to gather first accurate quantitative measurements of chemical reactions

• John Dalton proposed first modern atomic theory: elements consist of atoms which cannot be destroyed, created, or divided

Phlogiston Theory

Fire like element (phlogiston) in flammable material released during combustion to explain combustion and rusting

Lavoisier

• Disproved the phlogiston theory using experiments with oxygen

• first to apply stoichiometry

• father of modern chem

John Dalton

Atomic theory

• matter is composed of indestructible, indivisible atoms

• which are identical and unique for an element

• have the same size and mass

J.J. Thomson

• Who? J.J. Thomson (English physicist)

• What? Discovered the electron, a negatively charged subatomic particle, wanting to understand the structure of an atom

• How? Used a cathode ray tube (high voltage across a partially evacuated tube with metal electrodes)

• Observation: A ray originated from the negative electrode (cathode)

• Conclusion: The ray was made of negatively charged particles → electrons

Blueberry Muffin Model/ Plumb Pudding Model

Thomson’s model of the atom

• Proposed by J.J. Thomson — atoms are made of a positive "cloud" with negatively charged electrons embedded throughout, like blueberries in a muffin. Explained why atoms are neutral overall.

Robert Millikan

In 1909, Millikan used the oil drop experiment to measure the charge of an electron by balancing gravity with electric force on charged oil drops.
Using this and Thomson’s charge-to-mass ratio, he calculated the mass of an electron:
9.11 × 10⁻³¹ kg.

Radioactivity

the spontaneous decay of an atom's nucleus

Rutherford

Gold foil experiment

• to prove Thomson’s blueberry muffin model, did the gold foil experiment expecting alpha particles to break through the thin foil (like bullets through paper)

• instead, most particles passed straight through, and some alpha particles were scattered/reflected back

Rutherford’s model issues

• if atoms are orbiting around nucleus, they are constantly acceleration (changing direction)

• should be giving off electromagnetic radiation, therefore losing energy

• therefore, should crash into nucleus (but does not)

Nucleus

the dense center of an atom with a positive charge

Proton

 a positively charged subatomic particle

Neutron

an electrically neutral subatomic particle

Photon

• A massless particle of light that carries energy.

• Photons can excite or eject electrons from atoms, affecting their energy levels.

photoelectric effect definition

electrons are emitted by matter that absorbs energy from shortwave electromagnetic radiation

• The photoelectric effect is when light hits a metal surface and knocks electrons out of the metal.

ex: UV light, visible light

photoelectric effect discovery

1887 by Hertz

• light shining on metal surface causes emissions of electrons from the metal

• frequency, not intensity, was more important in determining the energy of the emitted electrons

threshold frequency

the minimal frequency required to emit an electron

_____ photoelectric theory

Einstein’s

• one electron can be ejected upon collision with one photo, w photon losing all of its energy