Evolution of Atomic Theory: From Democritus to Chadwick
Democritus
Ideas:
All matter is made up of water, air, fire, and earth.
Matter is composed of atoms, which move through empty space. They are solid, homogeneous, indestructible, and indivisible. There are different kinds of atoms, having different sizes and shapes. Matter can be divided, but at some point, it can’t be divided any further, and its name is atom.
Dalton
Background:
Making experiments with gases, law of conservation of mass defined, and multiple proportions.
Ideas:
- Matter is composed of extremely small particles called atoms.
- Atoms are indivisible and indestructible.
- Atoms of the same element are identical in size, mass, and chemical properties.
- In a chemical reaction, atoms are separated, combined, or rearranged.
Atomic Model:
Billiard Ball Model
Thompson
Background:
Electricity, model of cathode ray tube.
Experiment:
Cathode Ray Tube
Ideas:
- Atoms were composed of particles called electrons, which were electrically charged.
- Atoms are divisible.
- Mass-to-charge ratio.
Atomic Model:
Plum Pudding Model
Rutherford
Background:
Discovery of radioactivity, alpha particles have 2 protons and 2 neutrons.
Experiment:
Gold Foil Experiment
Ideas:
- The positively charged protons are present in the center of an atom called the nucleus.
- The size of a nucleus is very small compared to the atom.
- The mass of an atom is concentrated in the nucleus.
- Most of an atom is empty space.
Atomic Model:
The Nuclear Model
Goldstein
Experiment:
Variation of Cathode Tube
Ideas:
He proved the existence of the proton.
Milikan
Experiment:
Oil Drop Experiment
Ideas:
Determined exactly the mass and charge of the electron (1.602 x 10^-19 Coulombs).
Bohr
Background:
Colors, Rutherford’s work, radiation spectrum.
Ideas:
- Electrons traveled in stationary orbits.
- The outer orbits could hold more electrons, and these determined the properties of the atom.
- Energy levels closer to the nucleus require less energy than others; electrons can jump between energy levels.
Atomic Model:
Planetary Model
de Broglie
Ideas:
- He believed that electrons can act like particles and waves.
- Waves produced by electrons contained in the orbit around the nucleus.
- We cannot know the time and place of an electron; we can only know one.
Schrödinger
Background:
Quantum mechanics and dual behavior of matter.
Ideas:
- Wave equation.
- He developed a mathematical equation that states the probability of finding electrons somewhere in the atom.
Atomic Model:
Quantum Mechanical Model
Heisenberg
Ideas:
- Used the frequencies based on the spectrum lines to describe atoms.
- The Principle of Indeterminacy states that you cannot know both the position and velocity of a particle at any moment in time.
Chadwick
Ideas:
Discovered neutral atomic particles with a similar mass to a proton by using alpha particles.