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400 BCE
Democritus
Democritus proposes that all matter is made up off small, uncuttable units called atoms. He had interesting ideas about what atoms looked liked. For example, he thought iron atoms had hooks that would lock them into a solid. -
Period: to
Discovery of most elements
From 1669-1930, most of the elements were discovered, including phosphorus, oxygen, and hydrogen. -
Dalton
While analyzing data from the composition of tin oxides, John Dalton noticed that the amount of oxygen in one compound was exactly twice the amount of oxygen in the other. Using this, he developed a model. In his model, all matter was made of atoms, atoms were like steel balls and atoms of one element possessed the same properties and mass. Atoms of different elements had different properties, compounds were combinations of multiple atoms, and chemical reactions were rearrangements of atoms. -
Mendeleev
In 1869, Dmitri Mendeleev created the first version of the periodic table using atomic weight to arrange the elements. If he found an element did not fit into a group, he would rearrange them. He predicted the properties of many undiscovered elements as he left gaps for elements in groups for elements that had not been discovered. The positions of some elements still could not be explained, however. -
Becquerel
In 1896, Henri Becquerel accidentally discovered radioactivity while experimenting with uranium. This discovery drastically affected scientists' ideas about atoms. Along with Marie and Pierre Curie (who furthered studies about his newfound discovery), he earned a Noble Prize for his efforts. -
Thomson
While experimenting with a cathode ray tube, J.J. Thomson noticed that something interesting happened when he used a magnet with the cathode rays. After further experimentation, he discovered electrons. He later created his own model, which was named the "Plum-Pudding" model because he thought that the atom was made up of positively charged "pudding" with electrons embedded throughout like plums in plum pudding. -
Rutherford
Ernest Rutherford was curious about where the electrons were in the atom, so he experimented further to see if Thomson's model was correct or not. He fired alpha particles through gold foil, and some particles rebounded, which disproved Thomson's model since the experiment showed that there was a highly concentrated area of positive charge. He created his own model where positive charges were centralized and negative electrons orbited the nucleus. -
Bohr
In 1913, Niels Bohr introduced a new structure for the electrons orbiting the nucleus. He created a model where electrons would be in shells around the nuclei, with a specific arrangement for each shell in his mode. This arrangement also explained the arrangement of the periodic table. -
Soddy
Frederick Soddy discovered that there could be variants of known elements, while further studying the discovery of radioactivity. He found that some atoms would have the same chemical properties as an element but might have different properties when it came to radioactivity. In 1913, he introduced isotopes, which he eventually won a Noble Prize for. -
Lewis
Gilbert Lewis is known for his discovery of the covalent bond, the concept of electron pairs, and Lewis structures. In his 1916 study, he introduced the idea that a chemical bond was a pair of electrons that were shared by two atoms. He also introduced Lewis dot structures in the same study that are used to describe covalent bonds in models. -
Chadwick
James Chadwick is best known for the discovery of the neutron in 1932. The neutron is a subatomic particle with no positive or negative charge. Neutrons are located in the nucleus with protons and serve as a "glue" to hold the nucleus together.