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John Dalton: First coherent atomic theory
John Dalton is the one who came up with the first coherent atomic theory, they can be summarized as follows:
1. Matter is composed of small particles called atoms
2. All atoms of an element are the same, but atoms differ from one element to the other
3. During chemical reactions, atoms are rearranged
4. Atoms of different elements combine in simple whole-number ratios -
Dmitri Mendeleev: First periodic table
After scientists were able to determine atomic weights, Mendeleev was able to arrange the elements in order by their atomic weights; from the atomic weights he noticed that there is a repetition of the elemental properties. He created a table (known as the periodic table today) that elements with similar properties were arranged veritcally with each other. From that, he was able to say that there are unidentified elements that exist. -
Marie Curie and Pierre Curie: Radioactivity discovered
After Henri Becqueral disregarded his work, Marie and Pierre Curie continued his work. They discovered other radioactive elements such as polanium, radium, and thorium. Pierre passed away, but Marie continued the work. Marie said that uranium, and these other elements break apart over time and produce radiation. She called it "radioactivity". Though it was unknown what type of radiation was produced from these elements was. -
J.J. Thomson: Electron discovered
The same time radioactivity was being discovered he was working on experiments with cathode tubes.Throughout his experiments he discovered that rays were made up of particles:negative charge of partcles.Thomson summarized his experiments;negatively charged particles are subatomic particles and are included in every atom,atoms are neutral which meant that there had to be positive charges in the atom,since it had positive charges he suggested that there are "corpscles" which are known as electrons -
Ernest Rutherford: Radioactivity identified
What type radiation produced from these elements was known by Ernest Rutherford. After discovering that the radiation prodcued by Uranium were two different components, he tried to seperate them using different materials; he was successful in doing so by using two opposite charged plates. He called these two plates positive particles and negative particles. Note: A person named Paul Villard was able to identify a third type of radioactivity; gamma rays which has no mass or charge. -
Max Planck: Quants
Max Planck showed that when a person vibrates atoms strong enough you can measure the energy, but only in discrete units which he called quanta. Planck doubted his theory and therefore he did not try to develop it but instead Albert Einstein did so. -
Albert Einstein: Photons
Albert Einstein believed that the quanta behaved liked discrete light partcles which physicists called "photon". "Atoms not only give away protons but they can also gain them. Einstein discussed that as a result of light absorption electrons can be released from their atoms", which is known is as the "photoelectric effect". -
Ernest Rutherford and Hans Geiger: Identification of atomic nucleus
Previously stated: J.J. Thomson was able to identify that the atom is made up of positive and negative charge with the atom being a solid mass. Which meant that any particle shot at the atom would bounce back, except if there is a certain particle arrangement.Through experiment Rutherford and Geiger concluded that there is a large of concentration of positive charge in the atom; located in the center of the atom known as the "nucleus". -
Niels Bohr: Theories of electrons
Niels Bohr's theory was that electrons did not curve into the nucleus, he listed two rules of what he believed actually happened: 1. "Electrons can orbit at certain allowed distances from the nucleus. 2. " Atoms give away energy when electrons jump from higher energy orbit to lower ones. But absorb energy when electrons from low to high energy orbit. -
H. Moseley: Atomic number determined, and the periodic table is reorganized
"When an atom is charged with eneregetic electrons, a rearrangement of the core electrons (closest to the nucleus) causes x-ray to emit". The emitted frequency of the x-ray showes the characteristic of the atom/element since each atom has it own emitted frequency. Based on that he concluded that since the weight of elements increase at a constant step, "quantity can only be the charge on the central positive nucleus" which where the atomic number comes from. -
E. Rutherford: Proton existance confirmed
While experimenting Rutherford found a partical with similiar properties to that of a hydrogen nucleus. He found that when nitrogen is under great force of collision, a particle from the nitrogen atom nucleus was released. Based on that, Rutherford called the hydrogen nucleus a proton. Ruthermore suggested the existence of the neutron but it was never proven till Chadwick came. -
Bohr and Arnold Sommerfeld: Bohr-Sommerfeld model
During the 1920's, the Bohr model was considered too simple to explain heaver elements, which in truth it did. Bohr and Arnold Sommerfeld developed Bohr's theory and concluded that: "electrons travel in orbits but these orbits have different shapes and could tilt depending on the magnetic field". They also stated that these orbits can be circural, elliptical, and can "swing back and forth through the nucleus in a straight line". -
J. Chadwick: Neutron identified
Beforehand, only the proton and electron was identified. It was known that protons equalled the atomic number and for the atom to be neutral the number of had to equal the number of electrons, which was proven wrong till Chadwick proposed the neutron. Through experiment he found a particle with a similar mass to protons exist. The difference between both though was that the particle wasn't by electrical fields which is why it is called the neutron.
