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400 BCE
Democritus
Democritus theorized when dividing everything in half and half and half eventually we would have to stop and get something indivisible. He called this indivisible mass an atom. He said the structure of the atom were simple spheres. -
322 BCE
Aristotle
Aristotle disagreed with Democritus and said that all matter was continuous. That all matter fell under four categories/elements; earth, fire, wind, and water -
Birth of the U.S
The declaration of independence was signed by 13 delegates from the 13 colonies in the United States. This was the formal document annexing the United States from Great Britain. -
Laws mass and proportion
The law of conservation of mass was created. The law of conservation of mass states that mass can not be created nor destroyed only transferred. The law of definite proportions states that the ratio of each element in a compound is the same regardless of mass or size of sample. -
John Dalton
Dalton came up with 5 rules of how atoms are and how they interact. The laws are as follows:
1. All matter is made up of atoms
2. All atoms of the same element are the same size, have the same mass, and same other properties.
3. Atoms can not be created, destroyed or divided into sub-parts.
4. Different elements join in simple whole-number ratios to form chemical compounds
5. When a chemical reaction occurs the atoms are combined, rearranged, or separated. -
Stanislao Cannizzaro
Cannizzaro made a pamphlet explaining how Avogadro’s number was backed by empirical and numerical evidence that could be used to calculate elements in a mass of grams. He said “the conclusions drawn from [Avogadro’s theory] are invariably in accordance with all physical and chemical laws hitherto discovered.” -
Julius Meyer
Meyer published a textbook of a periodic table he had made. It arranged some of the 60 elements they knew so far. It was arranged by atomic mass. -
End of the U.S. Civil War
The war was started between the Union in the North and the Confederates in the south. The United States Civil war was fought for 4 years before it ended in 1865. The war was over more representation of congress seats for states in the south and the right to own slaves. -
Dmitri Mendeleev
Mendeleev published his first writing, the periodic table. It contained the known elements of the time about 70 elements. It was organized by atomic number or number of protons. -
J.J Thompson
Thompson found the electron. He found it by using a cathode ray tube. He found that if he used magnets that sometimes the tube bent away from the magnet. This is the law of charges in play; like charges repel. This led him to believe there were both positive and negative charges in the atom and so he created the plum pudding model. The plum pudding model is a sphere of positive charge with blobs of electrons, negative charges, throughout. -
Max Planck
Planck said the hot objects to not emit electromagnetic energy continuously as one might expect. Instead objects emit energy in small particular way and sizes called quanta. Quantum of the energy is the minimum amount of energy that can be lost or gained by a certain atom. -
Albert Einstein
Einstein expanded on Planck's theory and introduced another idea. The idea that electromagnetic radiation has a dual-wave particle nature. That each particle arrives/releases at a quantum of energy, a photon. A photon has an electromagnetic radiation having zero mass and carrying a quantum of energy. -
Robert A. Millikan
Began an experiment to see how much charge a single electron held. This was just the beginning but he started to find that 1 electron has 1 negative charge. -
Ernest Rutherford
He conducted the Gold Experiment that found the positively charged nucleus. He put alpha particles through a gold sheet and would see where they would go using a sensor. As he put them through some came back. Through this experiment he discovered that the middle of the atom has a positive charge, but around that is mostly empty space in the middle. The ones were the empty space hit the gold would go through but when it hit the positive charge it would bounce back. -
Niels Bohr
Bohr worked in Rutherford's lab after him. Believed that the electrons went around the positively charged nucleus. The further circle, electron shell, the electrons were on the more potential energy it contained is what Bohr claimed. -
Henry Moseley
Moseley created a theory based on Mendeleev's periodic table. He created the periodic law. The Periodic Law states that the properties of an element both physical and chemical are functions of their atomic number. -
Werner Heisenberg
Heisenberg created the theory of uncertainty. It stated that it is impossible to determine and see both the position and the velocity of an electron, or any other particle, at the same time. Which would later become part of Quantum theory. He also stated electrons had dual wave-particle nature. Meaning the electrons moved in waves and were particles. -
Erwin Schrödinger
Schrödinger operated under the hypothesis that electrons have a dual wave-particle nature and developed an equation that treated the electrons in the atom as waves. Schrödinger laid the foundation for present day Quantum theory. Quantum theory describes the mathematical properties of the waves of electrons and other subatomic and small particles. -
Werner Heisenberg and Erwin Schrödinger
Heisenberg and Schrödinger used the dual wave-particle nature to create a new atomic model. They showed the electrons moved around the nucleus in orbits and how the electrons had to keep moving or the electrons would be too drawn to the protons that had positive charge in the nucleus. -
James Chadwick
Chadwick discovered the neutron. He discovered it when bombarding beryllium atoms with alpha particles. While doing this experiment an unexplained radiation was produced. Chadwick saw the radiation as being made up of particles with a neutral electrical charge and having about the same mass of a proton. This particle he discovered and described has come to be the neutron. After the neutron was discovered it was added to atomic model. -
World War 2 starts
On September 1, 1939 Nazi Germany invaded Poland starting the 2nd World War.
