Find the number of neutrons, given the atomic mass and atomic number

• # of neutrons = atomic mass - atomic number

• Example - Neon-20 has an atomic number of 10

  • # of neutrons = 20 - 10

    = 10 neutrons

Find the average atomic mass, given the abundances of isotopes

• Multiply each mass by the abundance as a decimal

• Add each product together

• Use the formula:

  • Avg. amu = m₁p₁ + m₂p₂ +… m_n-1p_n-1 + m_np_n , where n is the number of isotopes, m is the mass, and p is the percent of abundance

• Example - Copper’s abundance is made of Copper-63 (62.9296 amu, 69.17%) and Copper-65 (64.9278 amu, 30.83%). What is the average atomic mass?

  • Avg. amu = (62.9296 amu)(0.6917) + (64.9278 amu)(0.3083)

  • = (43.5284 amu) + (20.0172 amu)

  • = 63.5456

Find the abundance of each isotope, given the masses and average atomic mass

• Use the formulae:

  • Avg. amu = m₁p₁ + m₂p₂ and p₁ + p₂ = 1

  • All of the abundances should equal 1 (or 100%

  • Substitute the known masses and average amu

  • Use substitution to isolate for one abundance, and then solve for the other