Atomic History

Democritus

Had the idea that there were small, indivisible, and identical particles called atoms that always moved, were infinite in number, and could be joined in groups.

Boyle

Believed that everything was made of very small yet divisible particles made of one universal thing and that atoms could only be differentiated by their shape and motion. Discovered a relationship between pressure and gases, such as the volume of a gas decreasing with more pressure and vice versa.

Lavoisier

Discovered the Law of Conservation of Mass through his extensive and thorough experiments. Also proved that atoms were not created or destroyed in a reaction. Also opposed phlogiston, a disproved scientific theory.

Dalton

Had five postulates:

1. All matter is made of atoms, which are indivisible and indestructible

2. All atoms of an element are identical in mass and properties

3. Compounds are formed by a combination of two or more different kinds of atoms

4. A chemical reaction is a rearrangement of atoms

5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged.

Some of these postulates would be proven either false or partly false in the future. For example, atoms can be destroyed and divided through nuclear reactions, but not through chemical reactions.

Thomson

Created the plum pudding atomic model in which negatively charged particles are contained within a positively charged mass. The electrons were thought by Thomson to be embedded within the positively charged mass. Thomson found out about these properties by doing the cathode ray experiment in which he shot electrons at positively and negatively charged plates. He found out that electrons were negatively charged, and that atoms were divisible into smaller components.

Millikan

Discovered the mass of the electron through the oil drop experiement, spraying charged oil droplets and seeing how they fell in the presence of postiively and negatively charged metal plates. The strength of the charge over the droplet determined the mass of the electron.

Rutherford

Conducted the gold foil experiment which showed that atoms are mostly empty space and that there is a nucleus with most of the atom’s mass, 10,000 times smaller than the atom itself. Experiement was conducted by shooting alpha particles at a gold foil sheet and seeing the trajectory of the particles. Most passed through, but some seemingly bounced off of the sheet at a 90 degree angle. Confusion still surrounded the electrons’ movements and whether they moved around the nucleus, spending their energy, or were stationary, which would still lead to the same result: the electrons falling into the nucleus.

Bohr

Believed that electrons could travel in an atom without losing energy in specific paths called orbitals or shells, and that each orbital had a fixed energy level. When an atom gained or lost energy, an electron could move between orbits.

Schrodinger, Heisenberg, Einstein, de Broglie, Born et al.

Schrodinger prosposed wave mechanics, which explained how electrons behaved like waves, continously orbiting around the nucleus in certain areas.

Heinsenberg created the Heisenberg Uncertainty Principle, which states that it is impossible to know with perfect accuracy the position and momentum of particles at any time. The principle states that the more precisely the position is known the more uncertain the momentum is and vice versa.

As a result, we only know the area of the atom in which there is a high probability of an electron’s presence called the electron cloud.

Chadwick

Discovered that the nucleus of an atom is made up of neutrons, no-charge particles, which comprised of most of the atom’s weight. Modeled the nucleus as being made of protons and neutrons, with electrons flying around the nucleus in orbitals.