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Democritus
Democritus first proposed the idea of an ultimate particle, which he called the atom. He believed atoms were small, hard particles that were all made of the same material, and were dfferent shapes and sizes. They were infinate in number, always moving and were capable of joining together. -
Laws of Conservation of Mass
The laws of conversation of mass is the principal that states that matter cannot be created or destroyed in an isolated system. Antoine Lavoiser, a french scientist, was able to establish that heat played no role in adding or decreasing weight. He was able to assemble a number of experiments, in which the the weight remained the same. He had several large burning lenses made, and these reached high temperatures needed to cause chemical reactions. -
Spherical Model
John Dalton believed that atoms were solid, indestructable spheres. It did not include the existance of the nucleus, ions or isotopes, or subatomic particles. He proposed elements were made of atoms, and that combined atoms were called compunds. -
Atomic Theory
The atomic theory is the theory that all matter is made up of tiny invisable particles, otherwise known as atoms. Democritus, a greek philosopher, first suggest the idea of an atom in fourth century BC, but John Dalton created the atomic theory almost two millennia later. -
Laws of Definite Porportions
The law of definate porportion states that the chemical compound always contains exactly the same amount of elements by mass. The first expirements of this law were made by Joseph Proust, a French chemist, in 1804. The law of definite porportions was based on the theoretical basis by the atomic therory by Dalton. -
Proton
In 1886, E. Goldstein observed a cathode-ray tube and found rays traveling in the opposite direction of the cathode rays. He named these rays canal rays, and discovered that they were made of positive particles. These positvely charged subatomic particles were then named protons. Protons have a relative mass of about 1, and are loacted in the nucleus. -
Cathode Ray Tube Experiment
In 1897, J. J. Thomson perfromed the Cathode Ray Tube Experiment. This experiment was performed by passing elecrtic currents through gases at low pressure. He sealed these gases in glass tubes that were fitted at both ends with metal discs called electrodes. High volatge energy caused one electrode to turn one positive, and one negative. He saw that the rays were attracted to the positive plates. This expiremnt lead to the discovery of electrons. -
Electron
J. J. Thomson discovered electrons in 1897. He preformed the Cathode Ray Tube Expirement, and found that cathode rays are attracted to metal plates with a positive charge, and repel the negative plates. He knew opposites attract, so he proposed that the cathode ray is a stream of negatively charged particles moving at high speed. He named these electrons. Electrons have a relative mass of zero, and are located in the electron cloud. -
Quantum Theory
The quantum theory is a theory that is based on the principal that matter and energy have the properties of both particles and waves. Physicists had been trying to understand the surprising results they got from heating black bodies. Max Planck first formulated the theory in 1900, and he discovered that energy, which appeared to be inside wavelenghts, is discharged in small packages. -
Plum Pudding Model
J.J. Thomson proposed this in 1904 in order to add the electron to the atomic model. This model shows the atom made up of electrons that are floating in positvely charged pudding. The idea was that the positively charged pudding would cancel out the negative charge of the electrons. -
Robert Millikan
Robert Millikan was born on March 2, 1868 in Illinois. He was an expiremental physicist, who discovered the charge of an electron. In 1909, he preformed the oil drop expirement, which balanced the downward gravitational force with the upward drag and electric forces on tiny charged droplets of oil, suspended between two metal electrodes. Since the mass of the oil was known, he could calculate the force of gravity on one drop, then he could determine the charge of the oil, or the electron charge -
Nuclear Model
In 1911, Ernest Rutherford created the nuclear model. He proposed the idea of a central nuclues, that has a positive charge with electrons orbiting around it, and the rest of the atom is empty space. He disproved Thomson's plum pudding model. He determined that the radius of the nucleus was 100,000 times smaller than the radius of an atom. -
Gold Foil Expirement
In 1911 Ernest Rutherford tested the therory of atomic structure with the Gold Foil Expierement. Rutherford directed a narrow beam of alpha particles at a very thin sheet of gold foil. The alpha particles should have passed easily through the gold, with only a slight deflection. However, a majority of the alpha particles passed through the atoms without deflection, and a small fraction of them bounced off the gold foil at large angles. -
Nucleus
In 1911, Ernest Rutherford conducted the Gold Foil Experiment. He directed a narrow beam of alpha particles at a sheet of gold foil. He found that most alpha particles passed through the gold, while a small fraction bounced off at wide angles. Based on this, Rutherford suggested that the atom is mostly empty space, and that all postive charge and mass are concentrated into a small region, called the nuclues. It is the central core of the atom, made of protons and neutrons. -
Bohr Model
Introduced by Neil Bohr in 1913, he proposed that he electrons were outside the nucleus and in orbit around the nucleus. Bhor said the electrons were on a special orbit. -
Modified Nuclear Model
In 1913, Bohr changed Rutherford's model. Bohr combined Rutherford’s model with the quantum ideas put forward by Max Planck and said that the orbits were stationary states. Spectral lines were a result of transitions between these stationary states, where the atoms could exist and be stable. Electrons with higher energy travel further from the nuclues, and electrons with lower energy travel closer to the nucleus. -
Quantum Mechanical Model
In 1926 Erwin Schrodinger enhanced Bohr's model in the Quantum Mechanical Model. The Quantum Mechanical Model is based on the quantom theory, which says matter also has properties asscoiated with waves. This model uses complex shapes of orbitals, or electron clouds, volumes of space where there is likely to be an elecrton. It is based on probability, rather than certanity. -
Uncertainty Principal
The uncertaintly principal is the principal that the momentum and position cannot both be precisely determined at the same time.
Warner Heisenberg, a German physicist, determined that our obseravtions have an effect on the better behavior of quanta. -
Neutron
After the discovery of the atom, scientists were finding that the proton was not the only particle in the nucleus. The saw that the atomic number was less then the atomic mass, and since electrons has almost no mass, there had to be another particle. Chadwick was able to discover the neutron existed within the nuclues. It has a relative mass of one as well. His discovery enhanced Bohr's model. -
Aristotle
Aristotle disbelieved the theory that atoms were different sizes, regular geometric shapes, and being in constant motion. He developed the theory that matter consisted of four elements; earth, air, water and fire. He believed that no matter how many times you cut a form of matter in half, you will always have a smaller piece of that matter.
