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400 BCE
Democritus
Democritus was a Greek philosopher who lived between 470-380 B.C. He made a theory that all material bodies are made up of indivisibly small “atoms.” He conducted a simple experiment to prove this. He took a seashell and broke it in half and realized even though they were divided, the two halves would still have the same properties as the whole. -
John Dalton
In 1803, John Dalton, made a theory of chemical combination. 1) Elements consist of indivisible small particles (atoms). 2) All atoms of the same element are identical; different elements have different types of atoms. 3) Atoms can neither be created nor destroyed. He described atoms as solid spheres and said that different types of sphere made up different elements. -
JJ Thomson
On April 30, 1897, he announced his discovery that atoms were made up of smaller components. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He then created a model for the structure of the atom, the plum pudding model. His work led to the invention of the mass spectrograph. -
Ernest Rutherford (1)
Ernest Rutherford discovered the nucleus of the atom in 1911. He conducted an experiment, the gold foil experiment. A beam of alpha particles was aimed at a piece of gold foil, most particles passed through, but some were scattered backward which showed that the middle on an atom is where the most mass is located. -
Ernest Rutherford (2)
He discovered that there are two types of radiation, alpha and beta particles. He found that the atom consists mostly of empty space, with its mass concentrated in a central positively charged nucleus. -
Neils Bohr (1)
In 1913 Niels Bohr proposed a new model of the atom. This model consisted of four basic principles. 1) Electrons can only exist in fixed orbits, or shells, and not anywhere in between.
2) Each shell has a fixed energy.
3) When an electron moves between shells electromagnetic radiation is emitted or absorbed.
4) Because the energy of shells is fixed, the radiation will have a fixed frequency. -
Neils Bohr (2)
He also said that the shells of an atom can only hold fixed numbers of electrons, and that an element's reactivity is due to its electrons. Atoms will react to gain full shells of electrons. When an atom has full shells of electrons it's stable and does not react. -
Gilbert Newton Lewis (1)
In 1916 Gilbert Newton Lewis published a paper that stated, “A chemical bond is a pair of electrons shared by two atoms.” Lewis’ Theory of Chemical Bonding. Covalent Bonds. He discovered that two atoms attract each other (create a covalent bond) by sharing a pair of electrons. -
Gilbert Lewis (2)
He is also known for the Lewis Dot Structure. It is a diagram that represents the valence electrons of an atom. This structure helps to see whether the valence electrons of atoms and molecules exist as lone pairs or within bonds. -
Erwin Schrodinger
In 1926 Erwin Schrodinger, an Austrian physicist, made an advancement on the Bohr model. He described “how a cat in a box could be in an uncertain predicament.” This theory meant that we don’t know the cat’s state until the box was opened. He used mathematical equations to describe he likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. -
James Chadwick
In 1932, Chadwick made a fundamental discovery in the domain of nuclear science; he proved the existence of neutrons – particles devoid of any electrical charge. James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. This particle became known as the neutron