Atomic Theory Timeline Model by Keaton Halyk

Democritus (Around 400 B.C.E.)

kh761 — 2025-02-24 19:21:24 UTC

Democritus' atomic theory was first introduced around 400 B.C.E. His idea for the atom was one of the first in history. Democritus wanted to prove that the atom existed, so he experimented by dividing a seashell into its smallest form. After this experiment, he stated that all matter is created using nature's indestructible building blocks (atoms). Many disregarded Democritus' atomic theory because it lacked supporting evidence. Later, Democritus would be labelled the original theorist of the atom.

John Dalton (1804)

kh761 — 2025-02-24 19:34:03 UTC

John Dalton brought back Democritus' atomic theory around 1804. Dalton's atomic theory consists of three main ideas: atoms joined together to form compounds. Atoms from the same element are alike and have the same mass; atoms from different elements are different and have different masses. Lastly, all substances are made of atoms, atoms are the smallest particles of matter and they cannot be divided, created or destroyed in any way. Unlike Democritus, Dalton's theory was immediately accepted by almost every scientist around this time, this is because he had great supporting evidence. John used experiments to prove his theory, Dalton would use many different compounds and gases to prove his theory definitively. To this day John Dalton's atomic theory is still mostly accepted today. John Dalton used wooden balls as models of atoms because they were hard and he believed atoms were also hard.

JJ Thomson (1904)

kh761 — 2025-02-24 19:36:47 UTC

In 1904 JJ Thompson was experimenting with some cathode ray tubes (sealed tubes with little air inside). These experiments showed that all atoms contain some tiny negatively charged subatomic particles or electrons. Thomson's atomic theory model was labelled the "plum pudding model". JJ's theory states that negatively charged electrons are embedded within a positively charged "soup" model. These experiments led to an even more major discovery, this discovery being the one that showed that the mass of each particle was much smaller than any atom. Thomson would repeat these experiments using different materials proving that no matter the difference the evidence stays the same.

Ernest Rutherford (1911)

kh761 — 2025-02-24 19:38:23 UTC

Ernest Rutherford developed his first atomic theory in 1911 (nuclear model). He proposed the atom as a tiny, dense, positively charged core (nucleus) with electrons that circulate at a distance similar to plants around the sun. Ernest Rutherford used gold foil to discover the nucleus of an atom. Also, Rutherford's most famous experiment the gold foil experiment used a beam of alpha particles directed into a thin layer of gold foil. Most of these particles went through the foil but some scattered backwards, this shows that an atom is mostly space but that it indeed has a tiny nucleus. This major discovery changed the course of everything in the scientific world from then on.

Henry Moseley (1913)

kh761 — 2025-02-24 19:40:00 UTC

In 1913 Henry Moseley proposed Moseley's law. This major discovery involving atomic numbers helped push the understanding of the atom even further. Later Moseley concluded that the atomic number is the number of positive charges in the atom's nucleus. Moseley's law is that the square root of the X-ray frequency emitted by an atom is proportional to its atomic number. Mosely's law has contributed to the discovery of many new elements on the periodic table. This law is used in many different ways but the two most useful ways are measuring the amount of energy exerted by an atom and finding the atomic number. Henry Moseley discovered Moseley's law by collecting the X-ray spectra of many different elements and then finding that the frequency of X-ray radiation released has a direct mathematical relationship to the atomic number of an atom.

Niels Bohr (1913)

kh761 — 2025-02-24 19:40:50 UTC

Niels Bohr proposed the theory for the hydrogen atom in 1913. This theory is based on the quantum theory that some physical quantities only take discrete values. Electrons can only move around the nucleus in a prescribed orbit. If electrons want to jump to a lower energy orbit the difference is sent out as radiation. Named the Bohr model after Niels Bohr, this model explains why atoms only emit light of fixed wavelengths. Niels Bohr didn't just discover everything on his own though, he used previous research from other brilliant minds to make this incredible discovery. Ernest Rutherford's description of the nucleus of an atom and Max Planck's quantum theory led Bohr to his groundbreaking theory. Bohr was able to account for X-rays while using heavy elements to show their emissions as electrons jumping around from outer orbits to inner orbits. He called this being "Hydrogen-like".

Ernest Rutherford (1918)

kh761 — 2025-02-24 19:43:02 UTC

After Rutherford's first breakthrough in 1911, he later updated his discovery in 1918 after further experiments. In 1911 Rutherford discovered the nucleus of an atom, but in 1918 he discovered protons what they are and how they work adding to his nuclear model. Ernest Rutherford proposed that the middle of an atom (the nucleus) was made up of positively charged particles called protons. Later Rutherford even predicted that neutrons existed before James Chadwick in 1932. Rutherford was also the first person in history to split the atom. He did this by bombarding lightweight atoms with alpha rays, he noted that he converted the original nitrogen atoms into oxygen atoms. This proved that he had split the atom, the result of this being called fission-products.

Schrodinger and Heisenberg (1926)

kh761 — 2025-02-24 19:44:38 UTC

In 1926, Erwin Schrodinger explored an idea about electrons and the way they move. Schrodinger had the idea that electrons moved more like waves than particles. Schrodinger's idea led Werner Heisenberg to develop the principle of uncertainty. The uncertainty principle states that if an electron moves like a wave and not a particle both the position and the movement of the electron cannot be known simultaneously. They called the model of their discovery the cloud model. The cloud model describes how electrons act and where they can be found in the orbit. Werner Heisenberg experimented with measuring the position of an electron with a gamma ray microscope. The electron would get a high-energy kick from the photon that would change the movement to become uncertain.

James Chadwich (1932)

kh761 — 2025-02-24 19:46:14 UTC

From Ernest Rutherford's prediction in 1918, James Chadwick proved that neutrons do exist inside the atom. He proved that not only protons and electrons are in the atom but neutrons also do in the Rutherford-Bohr model. Neutrons are a neutral subatomic particle that is about the same size as a proton and occupy the nucleus. James Chadwick was convinced that alpha particles couldn't produce powerful gamma rays with such little energy, so he performed beryllium bombardment experiments and found that radiation composed of particles of mass approximately equal to the mass of protons without an electrical charge or neutrons.