Atomic Theory

Events

Democritus

400 BC

-He discovered and named the atom.
-Atoms are indivisible and indestructible.
-Atoms are always moving.
-Atoms are different shapes.

Law of Conservation of Mass

1778

-This law was discovered by Antoine Lavoisier.
-It says that mass cannot be created or destroyed by chemical reactions or physical transformations.
-The mass of the products in a chemical reaction must equal the mass of the reactants.

Antoine Lavoisier

1788

-He discovered the Law of Conservation of Matter after heating mercury oxide. It's weight decreased, but the oxygen gas released had the exact same weight as the amount lost by the mercury oxide.

Dalton's Atomic Theory

1803

-Dalton's Atomic Theory states:
1. All matter is made up of atoms that 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.

John Dalton

1803

-Dalton discovered the Atomic Theory.
-He said all matter is composed of atoms of different weights and is combined in ratios by weight.
-He also found that atoms are spherical and are always in motion.

Dmitri Mendeleev

1869

-Mendeleev developed the first periodic table.
-His periodic table was arranged by increasing atomic weight and left blank spaces for elements that hadn't been discovered yet.

J.J. Thomson

1897

-J.J. Thomson first discovered the electron using a cathode ray tube.
-He created a model of the atom known as the "Plum Pudding Model."
-He found the first evidence that stable elements can exist as isotopes.

Cathode Ray Tube

1898

-A Cathode Ray Tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images.
-It was used by J.J. Thomson in his discovery of the electron.

Plum Pudding Model

1904

-This model was created by J.J. Thomson.
-Thomson believed that an atom is composed of electrons surrounded by a soup of positive charge to balance the electrons negative charge.
-His model showed the negatively charged "plums" being surrounded by the positively charged "pudding."

Robert Millikan

1909

-Millikan measured the charge of a single electron using the "Oil Drop Experiment."
-This charge is known as the Elementary Charge and is one of the fundamental physical constants.

Rutherford Model

1911

-This model was created by Ernest Rutherford.
-After his Gold Foil Experiment, Rutherford concluded that the only way alpha particles could have been deflected backwards was if most of an atom's mass was in the nucleus.
-The Rutherford Model of an atom puts all the protons in the nucleus with the electrons orbiting around the nucleus.

Gold Foil Experiment

1911

-This experiment was performed by Ernest Rutherford.
-Alpha particles were shot at a piece of gold foil.
-This was used to allow the observance of the scattering of alpha particles to prove the existence of the nucleus of an atom.

Ernest Rutherford

1911

-Rutherford is known as the"Father of Nuclear Physics"
-He developed the theory for the structure of the atom using the "Gold Foil Experiment."
-He proved the existence of the nucleus.

Neils Bohr

1913

-Neils Bohr discovered that chemical properties are determined by how many electrons are in the outer orbits of an atom.
-He created the Bohr Planetary Model of an atom which shows electrons traveling in orbits around the nucleus of the atom.

Bohr Planetary Model

1913

-This model was created by Neils Bohr.
-In this model, the atom is a small positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus like the planets around the sun.

Henry Moseley

1913

-Moseley discovered that the energy of x-rays emitted by elements increased with each successive element in the Periodic Table.
-He proposed that this was due to a positive charge in the nucleus.
-He rearranged the Periodic Table using atomic number instead of mass.

Electron Cloud Model

1925

-This model was created by Erwin Schrodinger.
-It shows the atom surrounded by a cloud-like region where the electron is likely to be. In places where the cloud is dense, the probability of finding an electron is greater. The dense regions are known as electron orbitals.

Erwin Schrodinger

1926

-Schrodinger described how electrons move in a wave form.
-He developed the Schrodinger Equation which describes how a quantum state of a system changes over time.

Quantum Mechanical Model

1926

-This model was created by Erwin Schrodinger.
-It is based on probability rather than certainty.
-It uses mathematical equations to describe the likelihood of finding an electron in a certain position.

James Chadwick

1932

-James Chadwick discovered the neutron component of an atom.
-This discovery led to the discovery of nuclear fission and later to the atomic bomb.