-
Period: 500 BCE to 500
Classical Civilisation
Throughout this period great thinkers of Ancient Greece, the Roman Empire, Egyptian empire and other civilisations began to ponder the workings of the world. Often through beautifully elegant and elaborate thought experiments, these thinkers would create theories which would shape modern science and remain the accepted beliefs for centuries to come -
Period: 500 BCE to 200 BCE
The atomists of Ancient Greece
Some philosophers of Ancient Greece believed that the world was made of minute particles which they dubbed atoms. They came to this conclusion through the thought experiment that if a piece of material was repeatedly cut in half, it would eventually reach a size too small to cut in half. They believed correctly that these atoms were invisible and consisted of empty space. However, their theories were rejected by Aristotle's teachings, and therefore largely ignored. -
400 BCE
Democritus and Leucippus
Regarded as the most scientific of ancient philosophers, Democritus is widely regarded as the father of atomism, alongside his tutor Leucippus. They believed that atoms consisted of empty space, which accounted for their ability to move and change compounds, and had physical properties relating to the structure of atoms. Very few writings from Democritus survive, though he reportedly quoted, 'But in reality atoms and void.' His theories were mostly given little attention, especially by Plato. -
Period: 500 to
The Middle Ages
Throughout the early Middle Ages, Known as the dark ages, science progressed very little. However from the 14th and 15th century onwards science and discovery were reborn in the Renaissance. However, these scientists and thinkers were more concerned with the physical aspects of the world and little progress occurred regarding atomic structure. -
Giordano Bruno
Although relatively unknown, Bruno was an important atomist in medieval Italy. He believed in atomic theory similar to that of Democritus, however this and his other dangerous ideas about the heliocentric solar system and the church led to his burning at the stake in 1600. -
Robert Boyle
Boyle believed atoms must exist based upon his work with gases and pressure, saying that for Boyle's Law to work atoms must exist. However his work was criticised by the church and he focused on his other projects. -
Period: to
Discovery and the Industrial Revolution
As the industrial revolution and modernisation of Europe began to occur, interest in atomic structure began to grow. The microscope had been born in the 1600s, and the first microorganisms were seen in 1676. The ability to see invisible things possibly began this revived interest in atoms, and with an improving quality of life for all more science began to occur. -
Issac Newton
Isaac Newton radically proposed a new atomic theory. Hindered by the church, Newton suggested that throughout the universe small moving masses were present. However these atoms more reflected modern molecules in their appearance and properties and his work of this theory was brief and fleeting. -
John Dalton
John Dalton was one of the first scientists to propose an atomic theory that seemed similar to that of modern science. Dalton’s book ‘A New System of Chemical Philosophy’ observed that when compounds decompose into elements, the same elements were always found, and had a predictable mass (through his use of ratios). Therefore he concluded that all compounds were made of small, mass units; otherwise known as atoms. He also used this to explain the differing solubility of gases in water. -
Dalton's Discoveries and their Reception
Dalton also found out that atoms have a differing mass and size depending on the element, he would go on to classify the masses of hydrogen, oxygen, nitrogen, carbon, sulphur and phosphorous. However, his theories were initially rejected as too complicated and were unable to determine the number of atoms in a compound until Avogadro's discoveries were made. -
Brownian Motion
Although a botanist by nature, Robert Brown would soon discover ‘Brownian Motion’, which would become hugely important in modern physics and chemistry. The random jerking motion he observed in pollen grains suspended in water suggested the presence of tiny particles which moved quickly and randomly, but had very little mass. This was one of the first observations of molecules and would be used to prove the existence of atoms and molecules. -
The Modern Periodic Table
Russian chemist Dmitri Mendeleev created what is now recognised as the periodic table in 1870 based off the table of fellow chemist Meyer. Mendeleev used the atomic number of the elements to arrange them in rows, with each column (or group) filled with similar elements. Mendeleev also left space for elements which were yet to be discovered. His sorting of elements based on atomic number shows the basic knowledge of the nucleus at the time, as a result of an evolution of Dalton's theory. -
Cathode Ray Tube/Crookes Tube
The Crookes Tube was invented by William Crookes in the 1870s, and consisted of a stream of electrons (a cathode ray) through a near vacuum. The electrons cause the tube to glow. The primitive Crookes tubes first suggested the existence of small particles in 1869, which was confirmed by J.J Thompson as the electron. These electrons were found to be negatively charged. Furthermore, the Crookes Tube was used to identify plasma as a state of matter and was used to discover x-rays. -
J.J Thompson
One of the most important scientists in the evolution of modern atomic theory, Thompson was instrumental in the discovery of the electron and also proposed one of the first atomic model which included sub-atomic particles. J.J Thompson put forward the 'plum pudding model', which speculated that a positive 'sponge' contained small electron negative 'plums'. Whilst this was proven wrong, it seemed a step in the right direction for atomic theory. Thompson did more work on electron charge as well. -
Period: to
Modern Science
From 1900 onwards, scientific knowledge of the atom grew massively. This was due to the evolution of complex mathematics, the emergence of quantum physics and the availability of modern equipment. Since the midst of the 20th century, the model of atomic structure has been largely unchanged. -
Geiger-Marsden Experiment
The Geiger-Marsden Experiment was the first experiment to lead to a modern atomic structure. An alpha particle beam passed through gold foil, however the particles were reflected and deflected, which was not expected in Thompson's model. This gave rise to a new model, in which the mass and positive charge was concentrated in a small nucleus. The rest of the atom was empty space, in which the electrons were positioned. This new model of the atom had a nucleus as the sun, and orbiting electrons. -
Bohrian Atom Model
Niels Bohr was a physicist who contributed to the development of quantum physics. However, before this he created an atomic model in which electrons sat in orbits. This in itself was not that different from the Rutherford model, but Bohr stated that these electrons were relegated to certain shells and energy levels. -
Isotopes and Atomic Mass
The discoveries of isotopes and atomic mass helped develop the Rutherfordian model of the atom and its nucleus. The isotope was suggested first in 1913, by Thompson and Soddy, as a way of explaining some of the difference in properties and radioactivity of the same elements. The discovery of atomic mass recently before Soddy by physicist Moseley, who described that each ascending atomic number meant an increased proton count of 1, and that atomic mass equaled the sum of protons and neutrons. -
Quantum Models
Since the Neutron was discovered in 1932, the basic model of the atom we are familiar with today was established. However scientists like Schrodinger developed a theory in which electrons existed in a cloud surrounding the nucleus. Later in the century, elementary particles were discovered like Quarks, Neutrinos, Bosons and antiparticles. The discovery of nuclear fission by Otto Hahn also allowed for the forces of the atoms; the strong force and the weak force to be explored.
