 Atoms of the same element are NOT exactly similar  They may have different masses ◦ Vast majority of mass is from protons and neutrons  Element is defined as # of protons ◦ ∴ protons cannot be different  Electrons must balance protons in neutral atom ◦ ∴ electrons cannot be different  # of neutrons may be different between atoms ◦ These are isotopes of the same element  Isotopes are atoms of the same element with different masses due to different number of neutrons

 View P. 113 in textbook  Same Element, different mass due to neutrons P-7 N-7 Mass = 14 P-7 N-8 Mass = 15

 ◦ A = mass number ◦ Z = atomic number ◦ X = symbol ◦ Example:  Full  Example: Sodium-24  Short Hand  Example: Na-24  Mixed  Example: 24 Na

 The atomic mass unit has 2 common symbols ◦μ◦μ ◦ amu  1 amu Defined as 1/12th the mass of a carbon-12 atom  The size of the atomic mass unit is arbitrary  Could have used ◦ 1/24th the mass of a carbon atom or ◦ 1/10th the mass of the iron or…….  Three reasons for using 1/12th the mass of a C-12 isotope are: ◦ Carbon is a very common element. ◦ It results in nearly whole-number atomic masses for most other elements. ◦ Lightest element, hydrogen (H), has a mass of approximately 1 amu.  The mass of C-12 is EXACTLY 12amu (important for sig-figs)

 Mass on Periodic table is the average of isotopes naturally found ◦ No Silicon isotope with a mass of amu  This does tell us that likely MOST Silicon isotopes have a mass of close to 28 amu  Some elements have 2-3 common isotopes  Others have 5-6 common isotopes ◦ Tend to be heavier elements  Some isotopes are highly radioactive

 Example: ◦ What is the average of the following? (3 Sig-Figs)  Cl-35  Cl-36 ◦ 35.4amu  Add all masses, divide by the number of atoms

 Example: ◦ Find the average mass of the following? (3 sig-figs)  10.8amu IsotopeNumber of Atoms B B B-1275 TOTAL: 545

 Example ◦ One of the dietary sources of potassium is the banana: 93.1% of the potassium atoms are potassium-39 (20 neutrons), 6.88% are potassium-41, and only a trace are potassium-40. Find average mass. ◦ Pretend % is # of atoms and the total # of atoms is 100  39.1amu Isotope% Abundance K % K % K %

 Example ◦ Elemental boron is a combination of two naturally occurring isotopes: boron-10 has a relative abundance of 19.78% and boron-11 has a relative abundance of 80.22%. Find average mass.  10.80amu IsotopeMeasured Mass (amu)% Abundance B % B %

 Read Textbook P  Textbook Questions ◦ P.112 #’s ◦ P.113 #’s ◦ P.116 #’s ◦ P.117 #’s  Handout ◦ Calculating Average Atomic Mass  Due Next class