Atom Periodic Table Timeline



400 BC

Democritus propose the idea of the atom. The atom was an indivisible particle that all matter is made of, but the methods to verify it did not exist in that era, so it was not widely adopted for many centuries.

Antoine Lavoisier


French chemist who, through a conscious revolution, became the father of modern chemistry. Lavoisier demonstrated with careful measurements that transmutation of water to earth was not possible, but that the sediment observed from boiling water came from the container. He burnt phosphorus and sulfur in air, and proved that the products weighed more than he original. Nevertheless, the weight gained was lost from the air. Thus he established the Law of Conservation of Mass.

Law of Conservation of Mass


Antoine Lavoisier discovered that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. If we account for all reactants and products in a chemical reaction, the total mass will be the same at any point in time in any closed system. Lavoisier's finding laid the foundation for modern chemistry and revolutionized science.

John Dalton

1803 - 1808

John Dalton is credited as the father of the modern atomic theory. The theory is composed of five parts. Which includes elements being composed of atoms and that element's atoms are identical in mass.

Dalton's Atomic Theory


The theory proposed a number of basic ideas:
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the constituent elements

Quantum Mechanical Model

1838 - 1926

The quantum mechanical model is based on mathematics. Although it is more difficult to understand than the Bohr model, it can be used to explain observations made on complex atoms.

Dmitri Mendeleev


Mendeleev is best known for his work on the periodic table; arranging the 63 known elements into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties.

J J Thomson


Thomson determined that all matter is made up of tiny particles that are much smaller than atoms. He originally called these particles 'corpuscles,' although they are now called electrons. This discovery upended the prevailing theory that the atom was the smallest fundamental unit.

Cathode Ray Tube

1894 - 1896

Thomson used a charge-to-mass ratio because he knew the weight of the cathode ray tube, the heat, the electrical current, and how much heat had been added from the electrons firing. He concluded that the negative cathode ray particles were a thousand times tinier than an atom. Along these lines he proved the existence of subatomic particles.

Plum Pudding Model


The plum pudding model, also known as the blueberry muffin model, of the atom by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model, the atom is composed of electrons, and surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "raisins" surrounded by positively charged "pudding".

Robert Millikan


Using the falling-drop method which balanced downward gravitational pull with buoyant and electrical upward forces, between two electrodes, Millikan found the charge of an electron to be slightly negative. It's approximately described as 1/1840 the mass of a Hydrogen atom. This discovery would later allow scientists to better understand and find the weight of other subatomic particles

Gold Foil Experiment


The Gold foil experiment consisted of a series of tests in which positive charges were sent through a layer of gold foil. The expected result was the the charges would just pass through because of the theory of the plum pudding model. However, the results were unexpected, as various charges were aimed all directions, bouncing off of something in teh atom. This led to the discovery of the atomic nucleus, the planetary model of the atom, and the downfall of the plum pudding model.

Rutherford's Model


Description of the structure of atoms proposed by the New Zealand-born physicist Ernest Rutherford. The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun. The Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom.

Bohr Planetary Model


The Bohr Planetary Model, introduced by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with attraction provided by electrostatic forces rather than gravity.

Henry Moseley


When atoms were arranged according to increasing atomic number, the few problems with Mendeleev's periodic table had disappeared. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements.

Ernest Rutherford


He is widely credited with first "splitting the atom". In a nuclear reaction between nitrogen and alpha particles. Where he also discovered (and named) the proton.

Niels Bohr

1920 - 1930

Bohr discovered a way to understand the structure of an atom. He conducted three different experiments. He also won the Noble Prize.

Electron Cloud Model


It was developed by Erwin Schrödinger and Werner Heisenberg. It makes it easier to visualize the electrons. The model provides the means of visualizing the position of electrons in an atom.

Erwin Schrödinger


He took further Bohr's Planetary Model. He wanted to improve it. Erwin Schrödinger formulated a wave-equation that accurately gave the energy levels of atoms.

James Chadwick


Chadwick discovered the neutron. He performed experiments to prove his theory. He worked in Ernest Rutherford's lab.