Periodic Table: Structures of Atoms

The periodic table organizes elements according to their properties. Chapter 1. Discoveries of Atomic Structures 6 min Discovery of the Electron J.J. Thomson in 1897 was investigating the then mysterious cathode rays using an apparatus which is replicated in the video. By measuring the deflection of the cathode ray by a positive and a negatively charged plate above and below the ray, Thompson brilliantly calculated that the ray was composed of tiny negatively charged particles, parts of atoms. He had discovered the electron and for that won the Nobel Prize. Discovery of the Nucleus In 1911, Ernest Rutherford’s experiment is replicated in which he passes high energy alpha particles through gold foil, and particle detectors record that most pass straight through, but unexpectedly, a few are deflected backwards. Rutherford reasons that the few deflected particles must have hit a very small dense center in the gold atom, a center he chose to call a nucleus, which was surrounded by vast empty space which was why most of the alpha particles passed straight through. Since Thomson and Rutherford, scientists have discovered many more atomic particles, three of which you should know more about, protons, neutrons, and electrons. Chapter 2. Atomic Numbers and Mass Numbers 5 min. The atomic number is simply the number of protons in the nucleus. There is always one negatively charged electron for each positively charged proton in the nucleus. Hydrogen has one proton so its atomic number is 1. Helium with two protons has the atomic number 2, and Lithium with three protons, has atomic number 3. Each element in the periodic table reading left to right in each row (called a Period), has one more proton, and so has an atomic number which increases by one. The mass number is the number of protons plus neutrons in the nucleus. Hydrogen has one proton, no neutrons, so its mass number is 1. Helium has two protons plus two neutrons, so its mass number is 4. Lithium has 3 protons and 4 neutrons, so its mass number is 7. Chapter 3. Electron Shell Configurations 7 min. Electrons moving around the outside of an atom occupy a series of shells at different distances from the nucleus. This orbital picture is visualized using a group of roller blade skaters, each representing one electron moving around a set of rings drawn on the floor representing electron shells. The players illustrate how the first three shells can hold a fixed number of electrons (2.8.8) and how they fill up starting with the shell closest to the nucleus. The skaters visualize how the electrons fill up shells in Hydrogen (1 electron), Helium (2), Lithium (3), Fluorine (9), Neon (10), and Sodium (11). Elements in the same column, called a group, of the periodic table have the same number of electrons in their outer shell, which gives them similar chemical and physical properties. Group 18 on the far right, the noble gases, has no free electrons on their outer shell, and their common chemical property is virtually no reaction with other elements. In the far left group, group 1, starting with Lithium top left, and reading down the group, each element in this and every other group has one additional shell, which holds a of varying maximum number of electrons. But the outer shell of each element in this group 1, holds just one electron. Reading left to right along a row, called a period, each element will have one additional electron in its outer shell.

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Wednesday, September 28, 2016 - 9:30am
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