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Democritus*
Democritus's atomic theory was that everything was made up of invisible particles called atoms, they are indestructible, solid, homogeneous (all of the atoms are the same,) and they are different in size, shape, mass, position, and arrangement. Solids are made up of small, pointy atoms, liquids are made up of large, round atoms, and oils are made up of very fine, small atoms that easily slip past each other. *Year is 432 BC -
Robert Boyle
Robert Boyle's main contribution to the atomic theory was the publishing of his book "The Skeptical Chemist," in which he disproved Aristotle's theory (the idea that everything was made up of earth, fire, water, and air.) He also helped developed the definition of an element, any substance that can be broken into two or more substances is not an element. -
Antoine Lavoisier
Antoine Lavoisier is known for his Law of Conservation of Mass, which declares that mass is neither created nor destroyed in any chemical reaction. Or more simply, the mass of substances produced by a chemical reaction is always equal to the mass of the reacting substances. -
Joseph Proust
Joseph Proust is recognized for his Law of Definite Proportion, which states that a chemical compound always contains the same proportion of elements by mass. -
Spherical Atomic Model
John Dalton's atomic theory led him to develop the spherical atomic model of the atomic. This describes that one sphere cannot be broken into smaller parts. -
John Dalton
Dalton's theory was based on the fact that all matter is made of atoms, and they cannot be created nor destroyed, all atoms of a given element are identical in their properties and their mass, compounds are formed by the combination of two or more different kinds of atoms, and a chemical reaction is a rearrangement of atoms. Today we know that atoms be destroyed through nuclear reactions but not chemical reactions and that there are different kinds of atoms within an element called “isotopes." -
Dmitri Mendeleev
Dmitri Mendeleev’s periodic table provided a necessary and massive step in the development of atomic theory. In 1869, Mendeleev’s first periodic table was made by putting elements in ascending order based on atomic weight and similar properties. His first published periodic table has spaces left between some elements because Mendeleev knew that not every element has been discovered, and there were also uncertainties with the atomic mass of some elements. -
Eugen Goldstein
Eugen Goldstein is often considered the discoverer of protons, he worked on an experiment dealing with cathode rays. The particles that were negatively charged moved toward to anode, which was the positively charged end. During the experiment he noticed that some rays traveled in the opposite directions, he called these rays "canal rays." His discovery proved the existence of protons. This experiment is called the Cathode Ray Tube Experiment. -
J.J. Thomson
J.J. Thomson is known as the discoverer of the electron. In 1897, he made the plum pudding model, which is tiny electrons embedded in a soft sphere of positively charged model. This model was abandoned in 1911 in favor of Rutherford’s model in which electrons orbit the positively charged nucleus. -
Ernest Rutherford
Ernest Rutherford is known for his nuclear model, also known as Rutherford’s model. This model described the atom as miniature, thick, and a positively charged core (the nucleus) that contained most of the mass. On the outside of the nucleus, electrons orbit the nucleus a lot like the earth orbits the sun. -
Gold Foil Experiment
Ernest Rutherford is famous for his “Gold Foil Experiment.” In this experiment, Rutherford took a piece of gold foil and shot alpha particles (two neutrons and two protons bounded together into one particle,) into the gold foil. He expected them to go right through the foil, but some of the bounced back off the foil. The thing that turned out to be deflecting the particles was the nucleus and that alike particles repel and opposites attract. -
Neils Bohr
Niels Bohr is recognized for his Bohr Model. He proposed that the electrons moved in orbits at different levels in the atom and the energy that the electrons have separates them from one level to another. In the model, neutrons and protons occupy the nucleus and electrons orbit the nucleus a lot like the earth orbits the sun. Bohr’s model is essentially a modified version of Rutherford’s model. -
Erwin Schrödinger
Schrödinger is known for his quantum mechanical model. This model showed that atoms have energy levels for electrons, paths of electrons are not circular orbits, and it can only tell us the probability of finding an electron a certain distance from the nucleus. The electrons are found inside a blurry electron cloud. This atomic theory predicts the odds of the location of an electron. -
Werner Heisenberg
Werner Heisenberg is well known for his “uncertainty principle.” This principle states that nothing has a definite position, trajectory, or momentum. Heisenberg’s contribution to the atomic theory was that he calculated the behavior of electrons, stating that they do not travel in a definitive orbit. -
Quantum Theory
In 1925, Heisenberg published work on the quantum theory which revolutionized human understanding of atomic and subatomic processes. The quantum theory is a theory of mass and energy based on the concept of quanta, which is the certain amount of something. -
Terms associated with atoms
Protons are positively charged subatomic particles in an atom that are equal in size to electrons, except both have opposite signs.
Neutrons are about the same mass of a proton but without an electric charge.
Electrons are negatively charged subatomic particles in an atom that are equal in size to protons, except both have opposite signs.
The nucleus of an atom is made up of protons and neutrons and is surrounded by the electron cloud. -
Aristotle*
Aristotle claimed that atoms were made of four elements: fire, water, air, and earth. Aristotle believed that no matter how many times you cut a form of matter in half, you would always have a smaller piece of matter. Each atom has different properties depending on what they were made of: hotness, dryness, coldness, or wetness. *Year is 384 BC
