History of the Atomic Model

Democritus

400 BCE

•First person to propose the "ultimate particle"
•Believed that matter could only be divided a limited number of times, eventually, there would be a "smallest piece"
•Called it "atomos", meaning “indivisible”
•Thought that atoms were small particles that were made out of the same material but were all different sizes and shapes
•Believed that they were in constant motion

John Dalton

1803

• Recognized the existence of atoms and elements
• Assigned a relative atomic weight of one to hydrogen, from that developed a relative atomic weight scale from assumed atomic ratios
• Law of multiple proportions
• Proposal of the atomic theory in 1803
•Proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together

Antoine Henri Becquerel

1896

•Discovered radioactivity through his investigations of uranium and other substances
•Radioactivity demonstrated that the atom was neither indivisible nor immutable
•He was studying the fluorescent properties of uranium salts and placed a piece of the uranium salt on top of a photographic plate wrapped in black paper. He discovered, upon development, that the plate was exposed in the shape of the uranium sample.

J.J Thomson

1897

•Discovered the electron, using properties of cathode rays
•Developed the "plum pudding" model
•discovered atoms of the same element but different atomic weight
•Determined the charge to mass ratio of electrons
•Experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. He proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."

Ernest Rutherford

1900

•Thought Thomson was wrong
•Thought of an atom with a positive nucleus with electrons surrounding it
•Atom was mostly empty
- Came to this conclusion through the "Gold Foil Experiment"
-Had particles pass through gold foil with detective screen surrounding them
-The particles that didn't pass made him come to the conclusion that most of the atom is empty space
-Flaws: thought electrons contained charges that are accelerating
•Discovered alpha, beta, and gamma rays in radiation

Niels Bohr

1913

•Decided to make a new model based off of Rutherford's model, but changed the orbit of the electron
•Created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom

Erwin Schrödinger

1926

•Described how electrons move in wave form, and developed the Schrodinger equation which describes how the quantum state of a system changes with time
•Discovered that electrons don't move in orbits (or in a set path at all)
•Theorized that electrons move in waves & have no exact location
• Atomic model is known as the quantum mechanical model of the atom
•The model describes the probability that an electron can be found in a given region of space at a given time
•Based on mathematics

James Chadwick

1932

•Discovered the neutron in 1932 by bombarding beryllium atoms with alpha particles
•Discovery of the neutron led directly to the discovery of fission because he found out that Sice neutrons have no electrical charge they could spilt the nucleus without anything to get in their way
•Discovers the neutron component of the atomic nucleus, explaining the nuclear fission of uranium 235. This also made it possible to produce elements heavier than uranium in the lab.