-
400
Democritus
400 BC: Democritus introduces the theory of atoms. He believed that everything was made up of small things that cannot be split. At that time there was little to no technology so it was impossible for Democritus to confirm his theory. However, Democritus based this theory on assumptions and experience. -
John Dalton
John Dalton was the first person to prove Democritus' theory and further develop it. He had 4 laws in his theory. The first law was that everything is made up of tiny things called atoms. The second law was that every element have similar atoms but still have different properties. His third theory was that when a chemical reaction occurs, atoms aren't destroyed but instead they are just shuffled or reconstructed. His last law was that atoms only combine in whole number ratios. -
JJ Thomson
JJ Thomson contributed to the atomic theory when he discovered that atoms have electrons and that there are positive and negative charges in an atom. He also introduced isotopes. He found out about electrons when he experimented with electricity and magnets. He discovered that it was repelled from negative and charged to positive. He concluded that there were negative particles in an atom called electrons that is causing this attraction. -
The Curies
The Curies (Marie Curie and Pierre Curie) were best known for their work with radioactivity. Also, they discovered two elements; Polonium and Radium. They discovered the elements when they were looking at the ore Uraninite and they realized that there was radiation stronger than Uranium in the ore. That led them to the idea that there were other elements in the ore. She concludeed that the element Radium was radioactive decay of Uraninite. -
Ernest Rutherford
created the Rutherford model which showed an atom like a solar system with electrons circling a nucleus. -
Ernest Rutherford
Ernest Rutherford was a scientist who further proved the atomic theory. He conducted an experiment that showed that there were two different charges in an atom. What he did was he shot particles into atoms to see if they deflect. If they deflect, then it means that JJ Thompsons theory of positive and negative charges is true. If they did not deflect, then it would have meant that his theory was false. In the end, the particles did deflect and JJ Thompson was proven right. In addition, Rutherford -
Robery Millikan
Robert Millikan was a physicist who discovered the electron mass and the charge of an electron. He did this by doing an experiment with oil drops and charged plates. He dropped oil in between the charged plates and found out the rate in which the droplets fall and therefore getting the charge and mass of the electrons. The oil was first turned into mist. -
Linus Pauling
Linus Pauling is a chemist and bio chemist best know for his work on chemical bond. He published a book in the 1920's which explained the nature of chemical bonds. He talked about electronegativity, ionic bonding, and covalent bonding which are still used today. He stated that the amount of electronegativity between atoms predicted how strong the atoms will bond. -
Irving Langmuir
Irving Langmuir was a chemist and phycist who is best know for his work with valence electrons and isotopes. He explained valence electrons and valence shells. He stated that the electrons in the outer shell of the atom are called valence electrons. The amount of valence electrons told how attractive an atom is to another atom. Also, the introduction of valence electrons helped tell the different groups that elements are in (alkali earth metals, noble gases, etc.). -
Werner Heisenburg
Werner Heisenburg created the uncertainty principle. The uncertainty principle states that no particle's position can be accurately measured at a given moment because they are always moving fast. Heisenburg first proposed this principle in a physics experiment. -
James Chadwick and the Neutron
James Chadwick was a physicist in the 20th century known for discovering the neutron. He discovered it when he realized that the atomic mass of a proton and an electron do not equal the atomic mass of hydrogen (which has one proton and electron). He concluded that there must have been something else in the atom that is negatively charged called a neutron. Also, he got more evidence for this theory because he realized that an atom needed a "neutral" particle to be stable. -
Niels Bohr and His Model
Niels Bohr was a physicist who created the Bohr model of an atom. Bohr was the student of Rutherford who also created a model of an atom. The Bohr model was different from the Rutherford model because it explained the energy levels of an atom while Rutherford hypothesized that electrons move randomnly and not in levels. The Bohr model is still being used to this day to sketch how an atom looks like. Bohr found out about energy levels because of his experiments with light. -
Arnold Sommerfield
Arnold Sommerfield was a physicist who stated that electrons can move elliptically and not just spherically. In addition, he stated that electron shells do not orbit in the same axis/plane as each other which means they can move in a random direction but they still follow Bohr's theory of electron shells. Sommerfield's theory fixed what Bohr did not include in his theory/model.
