Atomic Timeline by Ceren Dagtekin

The Alchemists

500 BCE

The Alchemists examined the atomic theory about 2 centuries after the death of Aristotle. They used Aristotles idea on matter to make experiments with different metals to change their structure. Although they failed to make items into gold, they made a process that later led to atoms being discovered. They found that particles are in constant motion, and that material things are made of particles. They found that no matter can be eternal, because no matter can come from nothing.

Plato

428 BCE

Plato (~428 BC-348 BC) contributed to atomic theory by associating each classic element (water, fire, etc) with a regular solid (cube, tetrahedron, etc) (360 BC). Plato (+Aristotle) proposed that matter was continuous, infinite, present in every form, and always around us. They called this the Continuous theory of Matter.

Democritus

400 BCE

Democritus (~460BC-370BC) contributed to atomic theory by saying that the building blocks of matter was called atoms. He said that atoms were uniform solid, hard, incompressible, and indestructible, and they moved in infinite numbers through empty space until stopped. Although he was wrong, this was a belief. His theory—Democritus Theory (~400BC) said that all things are composed of these bits called atoms. At the time he thought that different shapes of atoms gave objects different properties.

Solar System Model

1543

One of the first heliocentric solar system models was made by Nicolaus Copernicus in his book that was published in 1543. At the time, people thought our solar system revolved around the Earth, rather than the Sun. Nowadays, it’s the opposite. Copernicus was one of the first people to draw a model that shows the correct positions/motions of planets and moons.

Robert Boyle

1662

Robert Boyle (1627-1691) is “The Father of Chemistry”. Robert made Boyle's Law: The volume of gas varies inversely with pressure. This means that since gas can be compressed, gas must be made of tiny particles (atoms) because of the space in between them. Robert Boyle was one of the founders of modern chemistry, in 1660 he became a Royal Society member.

Antoine Lavoisier

1777

Antoine Lavoisier (1743-1794), made the Oxygen Theory of Combustion (1777). He discovered that combustion was a reaction of metal or organic substance with common air. He defined elements as chemicals that couldn't be broken down into other substances. He also made a periodic table, which helped influence the modern day periodic table. He made the Law of Conservation of mass (1789) which states that in a chemical reaction, mass is neither created nor destroyed.

John Dalton

1803

John Dalton (1766-1844)’s atomic theory said that all matter was made of indestructible atoms (building blocks of matter, that can’t be destroyed) which determined the physical nature of their elements. He found that all the atoms of an element are identical. He learned that compounds are combinations of two or more different types of atoms. His theory was based on the Law of Conservation of Mass and the Law of Constant Composition. He was one of the first 2 recipients of the Royal Medal (1826).

Billiard Ball Model

1803

The Billiard Ball Model was made in 1803 by John Dalton. This model defined atoms as ball-like structures. At the time, electrons and nucleuses were unknown, which means the model would just be a ball. However today, the model would be more detailed.

Amedeo Avogadro

1811

Amedeo Avogadro (1776-1856) stated that equal volumes of gases at the same temperature/ pressure have the same number of molecules regardless of their chemical nature/ physical properties. Avogadro’s Number is 6.023 x 10 to the 23rd power (1811). In 1811, he found that elementary gases had 2 atoms. For example, oxygen, nitrogen, and hydrogen.

Dmitri Mendeleev

1869

Dmitri Mendeleev (1834-1907) discovered that, when all known chemicals were arranged by order of their atomic weight, there was a recurring pattern. He made this periodic table in 1869. In 1871 he predicted undiscovered elements (eka-aluminium) , or atomic masses (eka-boron, atomic mass: 44).

JJ Thomson

1897

JJ Thomson (1856-1940) discovered that atoms were made up of smaller components (1897). His biggest discovery is of the electron, which is a negatively charged subatomic particle in the atom (1897). He made experiments with cathode ray tubes to prove his theory.

Marie and Pierre Curie

1898

Marie (1867-1934) and Pierre Curie (1859-1906) both discovered the radioactive elements—polonium, and radium (1898). This proved that atoms are divisible because radium spontaneously disintegrated into other elements. Marie coined the term ‘radioactivity’ for emission of ionizing, penetrating rays by certain atoms.

Plum Pudding Model

1904

The Plum Pudding Model was proposed by JJ Thomson in 1904. This model represents the negatively charged particles (electrons) “swimming” in a positively charged sea.

Albert Einstein

1905

Albert Einstein (1879-1955) is known as one of the smartest people in history. Einstein contributed to the Atomic Theory by proving that atoms exist, and also by discovering that electrons could leave metal through the usage of light.

Robert Millikan

1910

Robert Millikan (1868-1953) contributed to atomic theory by showing the atomic structure of electricity. He did this by showing the charge carried by an electron (using the “falling-drop method”) to prove that it was a constant for all electrons. He found that all electrons have the same amount of a negative charge. His atomic theory demonstrated that electrons had a discrete quantifiable charge.

Ernest Rutherford

1911

Ernest Rutherford (1871-1937) contributed to the atomic theory by describing the atom as having a tiny, dense, positively charged core, called a Nucleus (1911). His description also talked about electrons circulating around the Nucleus at some distance. He also demonstrated that there are two types of radiation (alpha radiation, and beta radiation) (1899).

Henry G. J. Moseley

1912

Henry Moseley (1887-1915) demonstrated that the (major) properties of an element are determined by the atomic number rather than the atomic weight. In 1912, he discovered the first atomic battery. Moseley’s Law (1913): frequencies are proportional to the squares of whole numbers that are equal to the atomic number, plus a constant.

Niels Bohr

1913

Niels Bohr (1885-1962) proposed a model of hydrogen atoms, where the electron could only occupy orbits around the nucleus of an atom. This was the first atomic model to use Quantum theory, because electrons were limited in a certain orbit around the nucleus. His model on atoms explained why atoms only emit light of fixed wave lengths.

Werner Heisenberg

1925

Werner Heisenberg (1901-1976) contributed to atomic theory by creating the Uncertainty Principle and theory of quantum mechanics (1925) which set limits for how precise the position/momentum of a particle can be measured.

Erwin Schrödinger

1926

Erwin Schrödinger (1887-1961) proposed the quantum mechanical model of the atom, which treats electrons as waves. This calculated the energy level of the electrons in atoms. In 1926, Erwin combined equations for the behavior of waves with the Broglie equation to generate a mathematical model for the distribution of electrons of an atom.

Electron Cloud Model

1926

The Electron Cloud Model was made in 1926, by Erwin Schrödinger. The electron cloud is an area around an atom’s nucleus where electrons are usually found.

James Chadwick

1932

James Chadwick (1891-1974) conducted an experiment similar to Rutherford’s, but he used beryllium and a paraffin block rather than gold foil. He claimed that this unknown radiation was actually neutral particles that were called neutrons. He applied the law of conservation of momentum and conservation of energy, which led to him determining the mass of a neutron. This proved the existence of neutrons (1932). In 1932, he was also awarded the Hughes Medal of the Royal Society.