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Understanding Atomic Structure: Mass Number, Isotopes, and Average Atomic Mass

This section delves into atomic structure, focusing on mass number, isotopes, and relative atomic mass. The mass number, the total of protons and neutrons in an atom, is a whole number and not found on the periodic table. Isotopes are variants of elements that differ in mass number but maintain the same atomic number. We define relative atomic mass and average atomic mass, discussing how the abundance of isotopes affects calculations. This knowledge is essential for comprehending the fundamentals of chemistry and atomic behavior.

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Understanding Atomic Structure: Mass Number, Isotopes, and Average Atomic Mass

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  1. Ch. 2.2 - Atomic Structure II.Masses of Atoms(p.30-31) Mass Number Isotopes Relative Atomic Mass Average Atomic Mass

  2. © Addison-Wesley Publishing Company, Inc. A. Mass Number • mass # = protons + neutrons • always a whole number • NOT on the Periodic Table!

  3. Mass # Atomic # B. Isotopes • Atoms of the same element with different mass numbers. • Nuclear symbol: • Hyphen notation: carbon-12

  4. © Addison-Wesley Publishing Company, Inc. B. Isotopes

  5. B. Isotopes • Chlorine-37 • atomic #: • mass #: • # of protons: • # of electrons: • # of neutrons: 17 37 17 17 20

  6. © Addison-Wesley Publishing Company, Inc. C. Relative Atomic Mass • 12C atom = 1.992 × 10-23 g • atomic mass unit (amu) • 1 amu = 1/12 the mass of a 12C atom • 1 p = 1.007276 amu1 n = 1.008665 amu1 e- = 0.0005486 amu

  7. Avg. Atomic Mass D. Average Atomic Mass • weighted average of all isotopes • on the Periodic Table • round to 2 decimal places

  8. Avg. Atomic Mass D. Average Atomic Mass • EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. 16.00 amu

  9. 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. 35.40 amu

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