-
332
Aristotle (B.C.)
He proposed that all matter was composed of four elements: earth, air, water, and fire (dryness, hotness, coldness, and wetness). -
450
Democritus of Abdera
Considered the father of modern science by many, his early speculations on atoms said all matter is composed of atomos (a – not, tomos – to cut); atoms of the same element are the same; atoms of different elements were different; characteristics of a substance were determined by the shape of its atoms; sweet things were made of smooth atoms; bitter things were made of sharp atoms; atoms remained unchanged but combined with other atoms to form objects. -
Period: 500 to
Alchemists (starts in B.C.)
They theorized that atoms were composed of mercury and sulfur, and they also thought that base metals could be changed into gold. -
Isaac Newton
He theorized that the Earth was made up of small, solid masses that were constantly moving. He also published the book “Optics” which showed movement of atoms and how they can cause other things to move. -
Benjamin Franklin
He conducted the famous kite experiment which led him to believe that there are positive and negative charges, and that lightning was static electricity. He attached a key to a kite with a Leyden jar toward the bottom, and the electricity attracted by the key traveled down the wet silk string into the jar. -
Antoine Lavoisier
He discovered the Law of Conservation of Mass which stated that matter could not be created or destroyed in a reaction. -
Joseph Proust
He discovered the Law of Constant Composition which stated that a given compound always contains the same elements in the same proportions. -
John Dalton
He stated all matter is composed of extremely small particles called atoms, atoms of a given element are identical in size,
mass, and other properties; atoms of different elements differ in size, mass, and other properties, atoms cannot be subdivided, created, or destroyed, atoms of different elements combine in simple
whole-number ratios to form chemical compounds, and in chemical reactions, atoms are combined, separated, or rearranged. -
Michael Faraday
He suggested that the structure of atoms was
somehow related to electricity. -
Dmitri Mendeleev
Created the Periodic Law, and then created his own version of the periodic table. He then used this new version to correct already found element’s properties and predict future elements properties. -
William Crookes
He discovered that cathode ray tubes travel in straight lines, exhibit a negative charge, and can be redirected by electric fields and magnets. -
E. Goldstein
He used a cathode ray tube in order to examine “canal rays” which possessed properties opposite to electrons. Goldstein concluded that in addition to the electrons, or cathode rays, that travel from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction, from the anode toward the cathode. -
George Johnstone Stoney
He proposed the idea that electricity was composed of electrons, which are negatively charged particles. Also, he estimated the mass of an electron. -
Henri Becquerel
Helped to discover radioactivity along with Marie and Pierre Curie. -
Joseph John Thomson
He discovered negatively charged particles in a series of experiments created to test electron discharge, and he named these particles "electrons." -
Marie & Pierre Curie
They discovered and named the spontaneous decay process as “radioactivity” after studying uranium and thorium. They also discovered radium and polonium. -
Hantaro Nagaoka
He opposed Thomson’s model and proposed one that is similar to Saturn and its rings-- a positively charged center in the middle of moving electrons. -
Joseph John Tomson (again)
He suggested a model of the atom that was a sphere of positive matter positioned by electrostatic forces. -
Albert Einstein
He published the papers that defined the Brownian theory; it made certain that matter was composed of things smaller than the eye could see (atoms) because of their movement. -
Hans Geiger
He developed a machine that could “click” when alpha particles came into contact. He affirmed Rutherford’s idea that the nucleus takes up a small volume at every atom and that the alpha particle was the nucleus of the helium atom. -
Robert Millikan
Starting in 1908, he worked on an oil drop experiment to measure the charge of a single electron. JJ Thomson had figured out the charge to mass ratio, but the actual values were unknown. Using his experiment he figured out the charge and mass of the electrons. -
Ernest Rutherford
Although he didn’t know whether it was positive or negative, he theorized atoms have certain charges compacted into a small nucleus, and came up with the Rutherford Model of the atom. -
Niels Bohr
Niels Bohr's model of the atom is a very small, positively charged nucleus which contains protons and neutrons surrounded by negatively charged electrons. His model improved upon the plum-pudding model and the planetary model. -
Henry Moseley
He found the relationship between atoms of elements and their positively charged nuclei. This helped to figure out that the number of protons determines the atom’s identity. -
Max Planck
He invented quantum theory and utilized quanta (discrete units of energy) to explain hot, glowing matter. -
Francis William Aston
He used a mass spectrograph to discover isotopes. He first found the two isotopes of neon, then applied the same principle he used to figure out many other isotopes. From the results of this work he was able to formulate the so-called Whole Number Rule which states that, the mass of the oxygen isotope being defined, all the other isotopes have masses that are very nearly whole numbers. -
Louis deBroglie
He discovered electrons’ dual nature (they’re similar to waves and particles) which supported Einstein. He studied wave mechanics: Dirac's electron theory, the new theory of light, the general theory of spin particles, and wave mechanics applied to nuclear physics. -
Werner Heisenberg
He formulated the Principle of Indeterminacy-- you can’t know the position and velocity of a particle. He is responsible for providing a theory for the structure of co-ordination complexes. -
Robert J. Van de Graaff
He created the first particle accelerator (Van de Graaff generator). It uses a moving belt to collect high charges on a hollow globe made of metal. -
Erwin Schrodinger
He viewed electrons as clouds and introduced the idea of wave mechanics, and how electrons do not necessarily move in orbits. They behave as particles and waves. -
Paul Dirac
He made large contributions to quantum mechanics and quantum electrodynamics. He was first to develop quantum field theory. He also was presented the Nobel Prize for physics in 1933 for discovering more productive forms of the atomic theory. -
James Chadwick
He is famous for discovering the fourth subatomic particle, the neutron, and then theorizing that it is present in order to stabilize the atom. He also established that the atomic number is determined by the numbers of protons in an atom. -
Enrico Fermi
He conducted the first controlled chain reaction releasing energy from the atoms nucleus. He is one of the men referred to as the “father of the atomic bomb.”
