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UNIT 2 – ATOMIC STRUCTURE

Learn about the basic subatomic particles, atomic number, atomic mass, and isotopes. Explore how isotopes affect the weighted average atomic mass on the periodic table.

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UNIT 2 – ATOMIC STRUCTURE

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  1. UNIT 2 – ATOMIC STRUCTURE • Basic Subatomic Particles • A. Reproduce this table in your notes:

  2. B. A typical square on the periodic table – Get out your table! • The Atomic Number (Z) • *The whole #, the smaller of the two numbers • *Corresponds to the # of p+ in an atom • *Determines the identity of the atom • *In an electrically neutral atom, #p+ = #e-

  3. 2. The Atomic Mass (A) *The larger of the two numbers, not usually a whole number *The mass of an atom in atomic mass units (amu), the combined mass of p+ and n0. *What is reported on the periodic table is the weighted average of the element’s ISOTOPES. *For an individual isotope of that element: #p+ + #of n0 = Atomic mass

  4. C. ISOTOPES – Atoms of the same element (same # of p+, same atomic number) that have a different number of n0 and therefore a different atomic mass. 1H H-1 1 1 1 0 1 1 2H H-2 2 1 1 1 2 1 3H 1 1 3 H-3

  5. Many elements have multiple isotopes. To determine the mass seen on the periodic table the weighted average is calculated: Magnesium: Isotope Mass (amu) % Abundance Mg-24 23.985 78.70% Mg-25 24.986 10.13% Mg-26 25.983 11.17% Step 1:(Mass of each isotope)(% abundance as a decimal) = Step 2: Sum the calculations for all isotopes Mg-24: (23.985)(0.7870) = 18.876 2.531 Mg-25 (24.986)(0.1013) = 2.902 Mg-26 (25.983)(0.1117) = Weighted average is the total:24.309 amu

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