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400 BCE
Democritus
Democritus's idea of an atom was merely a round sphere with no protons, electrons, or neutrons. He hypothesized that all matter was made of atoms, and that atoms were solid, invisible, and indestructible. Atoms were separated from each other by an empty space called void. He coined the term atoms from the greek word "atomos", meaning uncuttable. -
350 BCE
Aristotle
Did not believe Democritus's view of the atom. He thought that everything on Earth was composed of the four elements of earth, air, fire and water. Fire was hot and dry. Air was hot and wet. Earth was dry and cold. Water was wet and cold. By changing the properties of each element, the element could become another. (For example, earth could become fire by becoming hot.) -
Antoine Lavoisier
Lavoisier was known as the father of modern chemistry, and discovered the law of conservation of mass, which states that the total mass of matter stayed the same after a chemical change. He also stated that mass could be neither created nor destroyed. Although he did not discover the elements of oxygen and hydrogen, Lavoisier named them as such. -
John Dalton
Dalton proposed the atomic theory. This first part of his theory proposes that all matter was made of atoms that were indivisible. The second part states that all atoms of a given element should be identical. The third part states that a compound is a combination of two or more different types of atoms. The fourth part states that a chemical reaction merely rearranges atoms. -
Henri Becquerel
Becquerel discovered radioactivity. He found that uranium salts produced radiation through an experiment that was meant to study the properties of x-rays. -
Marie and Pierre Curie
Marie and Pierre Curie were two married scientists, and Marie was a student of Becquerel. They discovered the elements radium and polonium, and coined the term radioactivity. -
J.J. Thomson
Thomson discovered electrons, and also created a new model of the atom called the plum-pudding model. The plum-pudding model proposed that the atom was a sphere of positive charge that had neutrons scattered about inside. -
Max Planck
Planck created quantum theory, which stated that only certain amounts of energy, called quanta, could be radiated. He also proposed that energy carried by electromagnetic radiation could be calculated through the formula E=hv where E is energy, h is Planck's constant, and v is the frequency of the radiation. -
Robert Millikan
Millikan performed the oil drop experiment. This experiment calculated the unit charge of each electron and led to the calculation of the mass of an electron. -
Ernest Rutherford
Rutherford did the gold-foil experiment and proposed that all of the mass of the atom was at the center, and the center was positively charged. He also suggested that most of the atom's volume was empty space. Rutherford was Thomson's student. He did not believe in the plum-pudding model and instead came up with his own model. -
Niels Bohr
Bohr presented the Bohr model, and proposed that electrons orbited the nucleus. They were restricted to fixed orbits, but electrons could jump from one orbit to another by absorbing or emitting photons. Bohr had collaborated with Rutherford to find that Rutherford's model was too unstable, and changed it. -
Henry Mosely
Mosely found that an element's atomic number was not arbitrary. He used X-ray spectroscopy to find a relationship between the wavelengths of the X-rays and the atomic numbers of the elements. Mosely predicted the existence of four new elements, all of which were later found. -
Werner Heisenberg
Heisenberg was a German physicist whom proposed the Uncertainty Principle, which stated that the location of an electron was unknown at any given time. Electrons did not travel in perfect orbits. This contradicted the Bohr model previously accepted. -
Erwin Schrodinger
Schrodinger built off of Bohr's model, but did not use orbitals. He instead depicted an electron cloud, as the given position of an electron was unknown. He stated that one could only determine the probability that an electron was at a given place but not a definite position. In his Schrodinger's equation, he showed that waves could be used to describe electrons. -
James Chadwick
Chadwick bombarded beryllium atoms with alpha particles, which caused an unknown radiation to be produced. The radiation was composed of particles with an approximate mass of a proton and neutral charge. Chadwick had just discovered the neutron.
