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How atoms differ

How atoms differ. L. Atomic Number. Moseley discovered that each element had a unique charge (+) in its nucleus Number of p + Atomic Number = # of p + = # of e -. Isotopes. All atoms of an element have the same number of: p + e - Number of n 0 changes

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How atoms differ

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  1. How atoms differ L

  2. Atomic Number • Moseley discovered that each element had a unique charge (+) in its nucleus • Number of p+ Atomic Number = # of p+ = # of e-

  3. Isotopes • All atoms of an element have the same number of: • p+ • e- • Number of n0 changes • Isotopes – the same element with a different number of n0 but the same number of p+ and e- • Thus, they have a different mass

  4. Mass number • To identify isotopes • Number after element’s name • Ex: • Carbon-14 • Potassium-39 • Short hand: • 3919K • 146C • Top number is sum of n0 and p+ • Bottom number is the atomic number

  5. Finding the number of n0 n0 = mass # - atomic #

  6. Mass of individual atoms • p+ = 1.67 x10-24g • n0 = 1.67 x10-24g • e- = 1/1840th of 1.67 x10-24g • So small scientist came up with Atomic mass units (amu) • Gave c-12 an exact mass of 12.0000 • 1 amu = 1/12th the mass of C-12

  7. Practice

  8. Atomic mass • Weighted average of the isotopes of that element Average atomic mass = (isotope mass1 x %1) + (isotope mass2 x %2) + (isotope mass3 x %3)…. • Cl • 75% Cl-35 • 25% Cl-37 • (35 x 0.75) + (37 x 0.25) = 35.5 26.25 + 9.25

  9. Hydrogen • H-1 Abundance 99.9851% • H-2 Abundance 0.0151% • Helium • He-3 abundance 0.0001373% • He-4 abundance 99.9998633%

  10. Application of average atomic mass • The closer an element is to a whole number for its average atomic mass the more likely it is that it’s most abundant isotope has that atomic mass • Ex: F has an atomic mass of 18.998amu • F-19 (99%) • There are exceptions • Br has an atomic mass of 79.904amu • Br-79 (51%) & Br-81 (49%) • Br-80 doesn’t exist

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