History of the Atom

Democritus READ NOTE

100

NOTE: the software dosn't allow events in BC. This actually occured at about 400BC
Democritus' theorized that all matter is made of smaller divisable parts and there would be a final, indivisible piece that he called "atomos". This was the first atomic theory

Aristotle READ NOTE

170

NOTE: The software wouldn't allow events in the BC era. This actually occured at about 330 BC.Tis is spaced correctly from the rist event.
Aristotle believed that all matter is composed of a combination of four elements: fire, earth, wind, and water. Unlike Democritus, he believed that matter can be infinitely divided. This model held for the next 2000 years

Antoine Lavosiser's Works

Mar 1, 1774

Antoine proposed the Law of Conservation of Mass and showed that water is composed of oxygen and hydrogen, disproving Aristotle's theory. The Law of Conservation of Mass is the base of modern chemistry.
NOTE: the date listed here is roughly when he started researching burning

John Dalton's Atomic Theory

Sep 30, 1803

Dalton based his theory off of the works of Democritus.
However, unlike Democritus, Dalton used the scientific method and conducted expirements with chemical reactions. The main ideas in his theory were:
1. All matter is composed of atoms
2. Atoms are indestructable and cannot be created
3. Atoms of the same element have the same mass and properties
4. Compounds are atoms of different elements chemically combined
5, When atoms rearrange, chemical reactions occur

Henri Becquerel

Sep 30, 1896

Henri discovered natural radiation by covering a photographic plate with opaque paper and placing uranium salts near by, casuing the plate to fog.

Joseph John Thomson Part 1

Apr 30, 1897

While studying cathode rays, a glass tube with an electric current, Jospeh theoized that the negatively charged part of the current was composed of fundamental particles that reside inside of the atom. This was contridictory to the then popular belief that atoms are indivisible. He then measured the charge of the particles. These particles are known today as electrons. Later, he studied the positively charged rays, known now as protons.

Joseph Thomson Part 2

Oct 1, 1897

Combining the two, he proposed a new atmoic model. The majority of this model is a large positive section. Electrons are scattered through this portion. This is known as the rasin pudding model, where the positive mass is the pudding and the rasins the electrons.

Marie and Pierre Curie

Sep 30, 1900

This husband and wife team investigated radioactivity, based oof of Hernri Bequerel's work. In fact, in 1903, the couple and Henri shared a Nobel Prize. The word "radioactivity" comes from their research. In addition, they discovered two radioactive elements: polonium and radium.

Max Planck

Sep 30, 1900

At the turn of the century, Planck was studying the radiation of heated matter. He found that heat is absorbed or radiated in bundles, called quanta. Then, he developed an equation for the energy of the quantum-E=f*h, where E is energy, f is the frequency of radiation, and h is Planck's constant.

Robert Millikan

Sep 30, 1909

Using an "oil drop" machine, Robert Millikan measured the charge of the electron. Water or oil was sprayed into a chamber. Through a hole, some particles fell into a x-ray ionzied middle chamber between a positive and a negative plate. If a electrons bonded to a particle, the particle would float, and if not, sink.

Ernest Rutherford

Sep 30, 1911

Ernest Rutherford set up an experiment where he shot alpha particles through a slit onto gold foil. This foil was surrounded by zinc sulfide. Only approximately 1 out of 8,000 particles collided with the gold atoms.Thus, Rutherford proposed a new atmoic model where most of the atom is empty space, unlike the clumped previous model, and the real mass of the atom is in the center. Electrons surround this center (nucleus).

Henry Mosley

Sep 30, 1914

Henry Mosley resdesigned the periodic table, listing elements by atomic number, not mass. He even left blank spaces for future discoveries. By discovering that the energy of x-rays released by elements grows linearly.

Niels Bohr's Atomic Model Part 1

Sep 30, 1915

Based off of the work of Rutherford, Niels Bohr designed yet another atomic model. By applying quantum theory, Max Planck's theory, to Rutherford's model, Bohr found that electrons, without any lost energy, orbit the nucleus, and do so in energy shells. A electron is placed in these shells based on the amount of energy it contains. However, a shell has a maximum capacity, and, to be stable, must fill shells completely. If an electron gains enough energy, it will shoots to a higher energy shell.

Neils Bohr Part 2

Oct 1, 1915

Upon returning to its normal shell, the electron releases a photon, a particle of light. We now know that there are many holes in this model, but it does hold true for hydrogen atoms.

Erwin Schrodinger

Sep 30, 1926

Schrodinger proposed the electron cloud model, a cominbation of wave functions and the de Broglie equation. In this model, electrons are waves, instead of particles, and one determines the areas where they are most likely to be found. This model allows 3-dimensional electron movement, unlike the 1-dimensional Bohr model, and thus needs 3 coordinates: angular (l), principle (n), and magnetic (m).

Werner Heisienberg Part 1

Sep 30, 1927

Heisienberg took the slit expirement and analyzed it mathmatically. The result was the Uncertainty Principle, ∆p∆x≥1/2 h, where h is planck's constant, delta p is the standard devitaion of momentum, and delta x is the standard deviation of position. Basically, the Uncertaintly Principle states that the more accurately you measure one variable of certain pairs (the most common is location and momentum as shown above), the less accurately you measure the other,

Werner Heisenberg Part 2

Oct 1, 1927

This isn't due to observer effect, but to the wave properties of particles. Note that the effect is so small, that it dosn't effect normal human life,

James Chadwick

Sep 30, 1932

Based off an expirement where alpha particles collided with beryllium, then the beryllium emitted a mysterious ray that could dislodge protons. The particles, to dislodge photons, must be very heavy and were uneffected by magnetic fields (meaning neutral). Chadwick proposed that they were new particles that reside in the nucleus-neutrons. Neutrons now are a proven nucleon.