Science Timeline



400 BCE

Democritus was a Greek scientist who made his discoveries with another Greek scientist named Leucippus. Democritus and Leucippus both thought that everything was not "one" but that everything is made of atoms. They hypothesized that if they broke off a piece of gold and kept breaking the same piece they would eventually get to the size of an atom. Democritus and his partner both discovered the atom.


1803 - 1805

Dalton created the idea of the atomic theory, that matter consists of tiny particles called atoms. He also found out that atoms cannot be created nor destroyed, which later was named the Law of Conservation of Matter.



Crookes invented the radiometer and the spinthariscope. He also developed a sealed tube that allowed him to produce cathode-rays.



Becquerel, a French scientist, discovered that Uranium rays affect photographic films. His discovery led to another discovery of radioactivity.



Thomson had discovered the small charged particles that orbit the atom's nucleus, known today as electrons. In 1904 Thomson made a diagram of a sphere of an atom that is the whole volume of the atom. Later, though, his work was abandoned on electrons and scientists were more focused on the nucleus.



In 1911, Rutherford made a hypothesis that the charge of the atom came form the nucleus. This model now shows the electrons scattered around the nucleus.

Niels Bohr


Bohr had a theory of the hydrogen atom. He proposed that electrons move in a certain, circular orbit. Today scientists still use Bohr's atom model.

Model Currently Used to Represent the Atom

1913 - 2013

Two models are used today, the Bohr model and the quantum mechanical model. The Bohr model is the easier and simpler of the two. The quantum mechanical model is based on mathematics and it can be used to explain observations made on complex atoms.



An English physicist named James Chadwick discovered the neutron. A neutron in a particle in the nucleus that contains no charge. His model of the atom consisted of the protons and neutrons in the nucleus and the electrons in a discrete orbit around them.


William Crookes


"Development of the Atom." ThinkQuest. Oracle Foundation, n.d. Web. 28 Jan. 2013.

N.p., n.d. Web. -(picture)

Hernry Becquerel + Other Scientists


Current Atom Model . N.d. n.p. Web. 28 Jan 2013.
N.p., n.d. Web. -(picture)

J. J Thomson


"New Page 1." New Page 1. N.p., n.d. Web. 28 Jan. 2013.

Neils Bohr


"Science Clarified." Atomic Theory. N.p., n.d. Web. 28 Jan. 2013.

Chadwick Picture


N.p., n.d. Web.