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400
Democritus (400 BC)
Democritus was a philosopher who lived in ancient Greece in 400 BC. He discovered that there are atoms that are present in the atmosphere and created the first atomic model of which there is written evidence. He was the first to use the term atom and said the atoms are bound together with a system like a hook and eye
His atomic model states that:
- Atoms are in all forms of existence
- Atoms are inseparable
- Atoms can come in many different sizes. -
Period: 400 to
Important Notes
The excat date of each event is unknown so they are all listed as the 1st of January, just take the year into account. -
Antoine Lavoisier (1789)
Atonine Lavoisier was a chemist born who lived in France in 1789. He proved the law of conservation of mass and also proved that combining oxygen and hydrogen creates water. He named oxygen and hydrogen. His law of conservation of masses states that the sum of the weights of two chemical substances is equal to the weight of the compound formed by combining these two chemicals -
John Dalton (1803)
John Dalton developed his atomic theory in 1803, he discovered that evaporated water exists in the atmosphere as a gas.
His atomic theory states that:
- Atoms can be distinguished by their weights
- All matter is made of atoms
- Atoms can not be made or destroyed
- Atoms of the same elements are identical
- Different elements have different atoms
- When atoms are rearranged chemical reactions occur -
Robert Brown (1827)
Robert Brown was a Botanist who lived in Scotland. In 1827 He noticed when looking through a microscope at pollen grains in a clear liquid that the grains moved around as if they were alive. He discovered that this movement now called Brownian Motion was not caused by the pollen grains being ‘alive’ but that the grains were being pushed around by molecules within the water. His work provides a base for proving the existence of atoms. -
Dmitri Ivanovich Mendeleev (1869)
Dmitri Mendeleev was a chemistry professor in Russia who created the periodic table of the elements in 1869. He documented the elements in a pattern of symmetry. He discovered that the elements are made of smaller particles and their masses determined how many of these particles they contained. The periodic table of elements allowed Mendeleev to predict new elements. -
Marie Curie (1898)
Marie Curie was a chemist in France who was born in Poland. She studied the radiation of compounds that contained the known radioactive elements & found out that you can accurately measure the strength of the radiation from uranium, the intensity of radiation is proportional to the amount of uranium or thorium in the compound, the ability to emit radiation does not depend on the arrangement of the atoms but it is instead linked to something inside the atom. -
Marie Curie (1898)
She also thought that compounds with a stronger radiation than uranium must have an unknown element in the compound that had a stronger radiation than uranium or thorium. In 1898 Currie and her husband Pierre discovered two new radioactive elements: radium and polonium. In 1902 they discovered these two elements in their pure state -
Joseph John Thomson 1904
Joseph John Thomson (J.J.) was a physicist who lived in Britain in 1897. He discovered the electron after a series of experiments designed to study the electric discharge. In 1904 he developed an atomic model that says the atom is a sphere of positive matter in which electrons are positioned by electrostatic forces. -
Albert Einstein (1905)
Albert Einstein studied Brownian Motion and was able to calculate the number of water molecules per square inch.He also made statistical & mathematical formulas for the motion. His theory was based on a belief that as particles move around they are being pushed by smaller atoms in every direction. He learned that the movement was completely random & unpredictable but that it obeys laws of probability & made a mathematical formula for the probability laws of the atoms now known as the Random Walk -
Robert Millikan (1909)
Robert Millikan was an American physicist who won the 1923 Nobel Prize in Physics ‘for his work on the elementary charge of electricity’. He discovered the charge of an electron in 1909 by using his oil drop experiment. This allowed him to calculate the mass of an electron. He worked on J.J. Thomson’s hypothesis that the mass of an electron was at least 1000 times smaller then the smallest atom. The charge on the oil drops was always 1.59 x 10-19 Coulombs. -
Ernest Rutherford (1911)
Ernest Rutherford was a physicist who lived in Britain in 1911. He developed an atomic model describing the atom as a tiny, dense, positively charged core (nucleus) in which most of the mass is located, with electrons circling the nucleus at some distance, like planets revolving around the sun. His model also states that the main weight of the atom iis in the nucleus and the rest of the atom is mainly empty space. -
Frederick Soddy (1912)
Frederick Soddy was a chemist in England who in 1912 proposed that elements can exist in different forms that have the same number of protons but a different number of neutrons. His theory of isotopes explains that different elements can be chemically identical but have different atomic weights and characteristics. In 1920 he showed the importance of isotopes in dating the age of organic materials (carbon dating). His theory was disputed until James Chadwick discovered the neutron in 1932. -
Niels Bohr (1913)
Niels Bohr was a student of Ernest Rutherford who developed a new atomic model in 1913. He proposed that electrons orbited around the nucleus like the solar system. His model states that electrons occupy only certain orbits; these orbits are stable, each orbit has an energy associated with it, energy is absorbed when an electron jumps from a lower one to a higher one and the energy and frequency of light emitted/absorbed can be calculated by using the difference between the two orbital energies -
Erwin Schrödinger (1926)
Erwin Schrödinger was a physicist in Austria who used mathematical equations to describe the probability of an electron being in a certain place in 1926. This atomic model is known as the quantum mechanical model. His model doesn’t define the path of an orbit but predicts the location of the electron. His model can be described as a nucleus surrounded by an electron cloud and where the cloud is the densest the higher the probability of finding an electron. -
Werner Heisenberg (1927)
Werner Heisenberg was a physicist who is well know for his discovery of quantum mechanics and the uncertain principal. One of the biggest discoveries from the uncertain theory is that electrons do not travel in neat orbits as Bohr had thought. He calculated the behaviour of electrons and sub-atomic particles. His discovery clarified the modern view of the atom because scientists can now compare the number of atoms by the movements of their electrons and how many electrons an atom holds. -
James Chadwick (1932)
James Chadwick discovered an unknown radiation in 1932 by bombarding beryllium atoms with alpha particles. He explained this radiation as particles with a neutral electrical charge (neutrons) that had roughly the same mass as a proton.
