Atom Timeline



460 BC - 370 bc

• The first to come up with the theory that atoms are in all forms of existence.
• Believed that the atom is the smallest particle possible so there is no way to separate them.
• Believed there are different sizes of atoms, but none of them can be separated.
• Came up with the word atomos, which mean not divisible.
• His theory was later taken over by the more popular Aristotle’s atomic theory.
• He was partially right about the atom as being an indestructible particle and also right about the atoms moving around to combine with other atoms to form objects.
• Democritus came upon the theory when thinking about how finely one can cut something.
• His ideas were kept alive by the Roman poet Lucretia and survived through the dark ages to be rediscovered in 1417.
• Atoms have different characteristics: metal is stiff and hard and water is wet and slippery.
• Atoms combine with hooks or balls and sockets.
• Atoms exist in a void.


384 bc - 322 bc

• All matter is made up of fire, earth, water, and air.
• They all have four qualities: hotness, coldness, dryness, and moistness.
• There are two forces in the between atoms, conflict and harmony.
• He was right about the conflict and harmony because atoms do have positive and negative ends, but he was wrong about all of the matter in the universe being made up of four elements, the four qualities of matter, and the possibility of a form of matter being able to get infinitely smaller.
• He rejected the most widely believed theory at the time, which was that matter is made of very tiny particles that cannot get infinitely smaller.
• Aristotle came up with his atomic theory because he working on just about everything there was to work on 2000 years ago.
• Did not believe in atoms being in void and infinite motion.



• Known as the Father of Modern Chemistry.
• He discovered that chemical elements are neither created nor destroyed- the Law of Conservation of Mass, this happened while he was working on the balance and combustion: the combination of a substance with oxygen.
• Named Oxygen and found out that it is an element because he could not separate it into more substances.
• He was correct in many areas such as the conservation of mass and discoveries concerning elements including the publication of a table of elements that contained substances identified as elements today.

• Matter as in elements instead of atoms.
• Determined that water is made out of oxygen and hydrogen.
• Found out that fire is not an actual substance, but related to oxygen.



• He concluded that equal volumes of gas expand equally with the same increase in temperature.
• At the same temperature and pressure, when two gases are combined, the ratio of the volume of the reactants to the product is in whole numbers.
• Two volumes of hydrogen combine with one volume of oxygen to create one volume of water.
• He made several trips to high altitudes to study how altitude affects the temperature and composition of the atmosphere.



• Believed that all matter is made out of atoms, which are indestructible and indivisible, all atoms of the same elements are exactly the same, compounds are a combination of different types of atoms, and a chemical change is a rearrangement of atoms.
• Published a table of atomic weights based on the analysis of different substances by chemist at the time.
• Some of his theories still remain valid today except that atoms can be destroyed by nuclear reactions and there are isotopes in an element that differs in mass from the other atoms.
• Atoms combine in whole number ratios; it means that one atom cannot pair up with half an atom.
• Atoms are in constant motion.
• His ideas were very similar to Democritus.



• He believed that in equal volumes of gases, there is the same amount of molecules at the same temperature and pressure.
• Created Avogadro’s number: 6.022 * 1023 molecules of any gas in one mole and occupies a volume of 22.41L.
• He saw Gay-Lussac’s work with gases and the ratios of reactants and products as basis for understanding molecules.
• His hypothesis was accepted until 50 years later.
• Avogadro’s number still stands true today.



• Discovered radioactivity in uranium.
• He put uranium on photographic plates and put them in a drawer, when he took the plate out and developed it, he found clear images of the uranium crystals.
• The uranium produced radiation without external source, and it can be deflected by a magnetic field so charged particles were being radiated.
• Won 1903 Nobel Prize for physics.

The Curies


• Discovered radioactive metals, radium and polonium which she extracted chemically from pitchblende ore.
• Radiation is not a reaction of different elements coming together, but something occurring within uranium.
• Discovered Thorium, another radioactive element.
• The level of radiation depends only on the amount of uranium there is.
• Radioactivity cause atoms to break down and release energy and subatomic particles.
• Died of Leukemia from overexposure to radiation.



• Discovered the electron through experimentation with the cathode rays.
• Discovered isotopes when shooting ionized neon through magnetic and electric fields and measured the deflection on photographic plates.



• Stated that energy is in discontinued, small units called quanta.
• Energy is not continuous or flowing.
• Quanta explain why a hot iron poker glows distinctly red and white.
• His works influenced Einstein, who used the Planck constant.



• Photoelectric theory- light is carried in quanta instead of waves. Based on Planck’s theory.
• Brownian motion- the random movement of particles suspended in liquid or gas.
• Mass-energy equivalence- mass is a measure of energy. Mass is a property of all energy and energy is a property of all mass. E=MC2.
• Proposed special relativity.



• Helped developed the Geiger counter.
• Correlated nuclear charge with atomic number.



• Atom has central positive nucleus with orbiting negative electrons, the rest is just empty space.
• Most of the mass of the atom is contained in the nucleus.
• Used the gold-foil experiment to determine most of the atom is empty space.
• Differentiated between the different types of radiation emitted by uranium and thorium.
• Used the rate of decay of radioactive materials to determine the age of the earth, which was a lot older than scientists believed at the time.
• Proposed the neutron.



• Justification of the atomic number.
• Showed that the frequencies of certain characteristics of X rays emitted from an element is close to the square of an element’s atomic number.
• Before him, atomic numbers were thought to be arbitrary.



• Created Bohr model, which was based on Rutherford’s model. Electrostatic forces created attraction instead of gravity.
• Applied quantum concepts to atomic and molecular models.
• Electrons orbit around the nucleus.



• Determined the charge of an electron through finding the charge of an oil droplet.
• He let a drop of oil go through a small charged hole and then put it into an electric field and by observing the speed of the particles; he found the charge of a single electron- 1.6 * 10-19 Coulombs.
• Against Einstein’s theory on light.



• Devised equipment to count alpha particles; inventor of Geiger counter that detects and counts atomic particles.
• Worked with Rutherford and Marsden.



• Discovered the neutron.
• Neutron made it possible to create elements heavier than uranium.
• Led to discovery of nuclear fission when uranium is bombarded with neutrons and eventual development of nuclear weapons was made possible.