-
500 BCE
Democritus
One of the first to acknowledge the existence of atoms; believed that atoms were invisible and indestructible. Though his ideas lacked experimental support. -
Antoine Lavosier
He helped to transform chemistry from the science of observation to a science of measurement. He also helped to develop a system for naming compounds. -
John Dalton
created the scientific theory of atoms using experimental methods. Atoms make up everything, all atoms of the same element are the same, elements can mix chemically and physically to form compounds, and chemical reactions occur when atoms are joined, separated, or rearranged -
Dmitri Mendeleev
Published the table of elements and used it as a way to show the relationships between more than 60 different elements, later grouped them together into the periodic table based on repeating properties. Mendeleev organized the elements in the table in order of increasing atomic masses. -
Eugen Goldstein
He observed cathode rays and found that there were canal rays which went in the opposite direction of cathode rays. He concluded that these were positively charged particles called protons. -
Max Planck
He was the originator of Quantum theory of energy which then helped people understand the atomic theory better. -
J.J. Thomsen
Discovered electrons and learned they are negatively charged subatomic particles using an experiment involving an electric current passing through gases at a low pressure. -
Ernest Rutherford
Discovered atoms have a nucleus by shooting alpha particles at a thin sheet of gold. Many particles continued through but others bounced off because the alpha particles hit the nucleus of particles in the gold sheet. His model is known as the nuclear atom. In the model, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of an atom. -
Robert Millikan
experimented to find the quantity of charge carried by an electron. His values are close to the values that are accepted today. He also calculated the mass of an electron using the charge to mass ratio. -
Henry Moseley
He determined an atomic number for each known element. -
Niels Bohr
He proposed that an electron is found only in specific circular paths, or orbits, around the nucleus; each electron orbit has a fixed energy. -
Werner Heisenberg
the Heisenberg uncertainty principle states that it’s impossible to know exactly both the velocity and the position of a particle at the same time. It is important when dealing with small particles, like electrons -
Erwin Shrodinger
created a mathematical equation to describe the behavior of electrons in a hydrogen atom. The quantum mechanical model is the modern description of the electrons in an atom, which comes from the results of Schrodinger’s equation. -
James Chadwick
confirmed the existence of a neutron, particles with no charge but have a mass nearly equal to a proton.
