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Atomic Mass and Isotope

Atomic Mass and Isotope. Objectives: SWBAT calculate the average atomic mass of an element when given the relative abundance of the isotopes. Bellringer: What does the atomic mass tell you about the atom?. RELATIVE MASSES. Chemists more than 200 years ago used a

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Atomic Mass and Isotope

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  1. Atomic Mass and Isotope Objectives: SWBAT calculate the average atomic mass of an element when given the relative abundance of the isotopes. Bellringer: What does the atomic mass tell you about the atom?

  2. RELATIVE MASSES Chemists more than 200 years ago used a relative scale to compare weights of atoms to each other. Dalton assigned a H atom a mass of 1. According to his scale a helium atom has a relative mass of 4 because it is 4 times as heavy.

  3. RELATIVE MASSES • Using Dalton’s scale a carbon atom has a relative mass of 12 because a carbon atom is twelve times heavier than a hydrogen atom

  4. Comparing Masses • In 1961 Dalton’s method of comparing masses of atoms was replaced by IUPAC- • International Union of Applied Physics and Chemistry.

  5. IUPAC RELATIVE MASSES IUPAC decided that the most common isotope of C which is 12C would be used as a reference standard and assigned an atom of 12C a mass of 12 exactly. Using this scale the helium isotope is assigned a relative mass of 4 Comparing a helium atom to a carbon atom The He atom is 3 times lighter

  6. RELATIVE ATOMIC MASSES A Krpton atom that is given a relative mass of 36 A Kr atom is three times heavier than a 12C atom

  7. Atomic Symbols • Show the mass number and atomic number • Give the symbol of the element mass number 23 Nasodium-23 atomic number11

  8. Isotopes • Atoms with the same number of protons, but different numbers of neutrons. • Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 1717 chlorine - 35 chlorine - 37

  9. RELATIVE ATOMIC MASSES All isotopes of elements are given a relative isotopic mass compared to the 12C isotope. There are 3 isotopes of Mg 24Mg 25Mg 26Mg These 3 atoms are different because they have different numbers of neutrons

  10. Abundances of Isotopes In a sample of pure Mg you will find the isotopes of Mg always occur in the following quantity 78.7% 24Mg 10.13% 25Mg 11.17% 26Mg Like Magnesium most elements exist as a mixture of isotopes. Eg 1H, 2H and 3H

  11. Guided Practice 1 Calculate the average relative atomic mass of Magnesium given: Isotope Relative Mass Abundance 24Mg 23.99 78.7% 25Mg 24.89 10.13 % 26Mg 25.98 11.17% Assume we have 100 atoms of Mg. mass contributed by the 24Mg isotope is 23.99 x 78.7 mass contributed by the 25Mg isotope is 24.89 x 10.13 mass contributed by the 26Mg isotope is 25.98 x 11.17 Total mass of 100 Mg atoms = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17)

  12. Finding Ar Total mass of 100 Mg atoms = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17) Ar(Mg) = (23.99 x 78.7) + (24.89 x 10.13) + (25.98 x 11.17) 100 Ar(Mg) = 24.3 This is not the true mass of a Mg atom but its relative mass compared to a 12C atom.

  13. Finding Ar The general rule is: Ar = Σ(relative isotopic mass x abundance) 100

  14. Guided Practice 2 • Find the relative atomic mass of Chorine. Isotope Relative Mass Abundance 35Cl 34.969 75.80% 37Cl 36.966 24.20% Ar(Cl) = (34.969 x 75.8) + (36.966 x 24.2) 100 Ar(Cl) = 35.45

  15. Find Ar(O) Isotopes Relative Isotopic Mass Abundance 16O 15.995 99.76% 17O 16.999 0.04% 18O17.999 0.20% Ar(O) = (15.995 x 99.76) + (16.999 x 0.04) + (17.999 x 0.2) 100 Ar(O) = 16

  16. Atomic Mass of Magnesium IsotopesMass of Isotope Abundance 24Mg = 24.0 amu 78.70% 25Mg = 25.0 amu 10.13% 26Mg = 26.0 amu 11.17% Ar(Mg) =(24 x78.7) + (25 x10.13) + (26 x 11.17) 100 Ar(Mg) = 24.3

  17. HW: Practice • Quiz tomorrow on: • Atom History • Atomic Structure • Drawing Bohr Models • Finding Relative Abundance (?)

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