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The Atom I- Theories About the Atom: A- The term Atom comes from the Greek word meaning “invisible” or “that which cannot be further cut”. This comes for the Greek thinker Democritus more than 2000 year ago. 1-The Greeks believed that all matter was made up four basic elements Fire Air Earth Water B-Dalton’s Model: in the early 1800’ John Dalton said the atom was a solid particle that could not be divided, and each element had its own kind of atom. C-Thomson’s Model: in 1897 J.J. Thomson discovered that the atom was made up of smaller particle. Thomson discovered the electron. 1- The atom was known to be neutral. The atom must also have a positive charged particle. a- Thomson never discovered the positive particle but he knew it existed. b- His model known as “Plum Pudding” D-Rutherford’s Model: In 1908, Ernest Rutherford fired a stream of tiny positively charged particle at a very thin sheet of gold foil. He discovered the Nucleus of the atom and found that it contained what Thomson was looking for the positively charged particle the Proton. 1-Ruther Model is known as “The Solar system”. E-Bohr Model: In 1913 Niel Bohr said that the electrons move in a definite orbit around the nucleus known as. 1-Energy Level: is the path an electron takes around the nucleus of the atom, it related to the amount of energy it has. 2-Electrons Energies: By heating up or passing electricity through the atom you can increase its energy. a-If the atom gains enough energy the electron can jump to a higher energy level called an Excited State. b- To return to it’s original state the atom releases the extra energy in the form of a PHOTON. c-Spectrum is the finger print of an element each element has its own spectrum. F-Chadwick’s Model: In 1932 Chadwick discovered a particle in the nucleus of the atom. The Neutron has NO CHARGE but has more mass than the proton. This completed the model of the atom. Electron Neutron Proton 1-Today: the atom is described as a central mass surrounded by an electron cloud. II- How Atoms Differ A- All atoms of an element have the same number of protons. The number of protons determines what the element is. 1- Atomic Number is the number of protons of an atom. 2- Mass Number: is the total number of Protons and Neutrons in an atom. a- To find the number of neutrons in an atom all you do is subtract the atomic number from the mass number. Mass Number B-Isotopes: Atoms of the same element with the same atomic number but a different number of neutrons. III-Classifying Elements: A-The Periodic Table: Was created by Mendeleev, he put the elements with the same characteristics into columns. 1-The elements line up in increasing atomic numbers. B- Arrangement of Table: 1-Each row is called a period and each column is called a group. 2-If you know something about an element in a group you know something about them all. 3-Each box has an element’s symbol, name, atomic number, mass number, and the number of electrons in each energy level. Atomic Number Symbol Name 13 2 Al 8 3 Aluminum 26.98 Number of electrons in each energy level Average atomic mass