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# Atomic Mass Presentation

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### Atomic Mass Presentation

1. 1. Ch. 3 - Atomic Structure II. Masses of Atoms ♦ Mass Number ♦ Isotopes ♦ Relative Atomic Mass ♦ Average Atomic Mass
2. 2. A. Mass Number ♦mass # = protons + neutrons ♦always a whole number ♦NOT on the Periodic Table! © Addison-Wesley Publishing Company, Inc.
3. 3. B. Isotopes ♦Atoms of the same element with different mass numbers. C12 6 Mass # Atomic # ♦Nuclear symbol: ♦Hyphen notation: carbon-12
4. 4. B. Isotopes © Addison-Wesley Publishing Company, Inc.
5. 5. B. Isotopes ♦Chlorine-37 • atomic #: • mass #: • # of protons: • # of electrons: • # of neutrons: 17 37 17 17 20 Cl37 17
6. 6. IONS ♦Sometimes an atom can lose or gain electrons. If this happens it becomes charged. ♦If it loses (an) electron(s), it becomes positive. ♦If it gains (an) electron(s), it becomes negative.
7. 7. IONS ♦ If lithium loses one electron, its symbol is: ♦ If oxygen gains two electrons, its symbol is: + iL7 3 −2 O16 8
8. 8. C. Relative Atomic Mass ♦12 C atom = 1.992 × 10-23 g ♦1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu ♦atomic mass unit (amu) ♦1 amu = 1 /12 the mass of a 12 C atom
9. 9. D. Average Atomic Mass ♦weighted average of all isotopes ♦on the Periodic Table ♦round to 2 decimal places 100 (%)(mass(mass)(%) )+ = Avg. Atomic Mass
10. 10. Avg. Atomic Mass D. Average Atomic Mass ♦EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16 O, 0.04% 17 O, and 0.20% 18 O. = ++ = 100 (18)(0.20)(17)(0.04))(16)(99.76 16.00 amu
11. 11. Avg. Atomic Mass D. Average Atomic Mass ♦EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37. = + = 10 (37)(2)(35)(8) 35.40 amu
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