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Structure of the Atom CHEM Basic Parts of the Atom nucleus –the positively charged center of the atom protons – positive charged particle with a relative mass of 1 neutrons – neutrally charged particle in the nucleus with a relative mass of 1 electron cloud – negatively charged, mostly empty space electrons – negatively charged particles with a relative mass of 1 / 1840 Atomic Structure Forces in Atoms Electromagnetic Force: Strong Force: Opposes the electromagnetic force between protons in the nucleus. The strongest of the four forces, but only acts over short distances. Weak Force: Like charges repel, opposite charges attract. The force responsible for radioactive decay. Gravity: The force of attraction between all objects in the universe. The weakest of the four forces. Atomic Number The atomic number is equal to the number of protons in the nucleus of an atom This number is expressed as a whole number on the periodic table. Since + and - charges are equal in atoms, it is also the number of electrons Mass Number The mass number is equal to the sum of the protons and neutrons in the nucleus of an atom. This number is expressed as a Number of neutrons in a atom = (mass number, as a whole number) - (atomic number) How many neutrons are in an atom of zinc? . . . . 65 - 30 = 35 neutrons in Zn Atomic Number and Mass Number Isotopes Atoms which have the same number of protons with different amounts of neutrons. Carbon-12 has 6 neutrons, while Carbon14 has 8 neutrons. The mass number for an element on the periodic table represents the average for all the isotopes in a sample of the element. Hydrogen has 3 common isotopes 1 2 3 Carbon also has 3 common isotopes 1)6p, 6n 2)6p, 7n 3)6p, 8n mass # = 12 mass # = 13 mass # = 14 nuclide name carbon - 12 carbon - 13 carbon - 14 Isotopes Atomic Mass The average mass of the mixture of an elements isotopes Atomic mass is measured in Atomic Mass Units where 1amu = (1/12) mass of carbon measured in grams. In order to find this average you need to know: 1. 2. 3. number of stable isotopes of the atom mass of isotopes percent abundance in nature of isotopes Atomic Mass Example: A sample of cesium is 75%133Cs, 20% 132Cs, and 5% 134Cs. What is its average atomic mass? Answer: .75 x 133 = 99.75 .20 x 132 = 26.4 .05 x 134 = 6.7 Total = 132.85 amu