Transcript Mass number
A QUICK REVIEW OF THE ATOM If you change the It will affect the… number of… Protons Electrons Neutrons And the result will be a… Identity of the atom completely different atom Charge Ion Mass Isotope WHAT ARE ISOTOPES? They are atoms of the same element that have different numbers of neutrons. It’s just a change in the neutrizzle fo shizzle!!! AN EXAMPLE Hydrogen has 3 isotopes There are 3 different “versions” of a hydrogen atom Note how the number of protons never change!!! AN EXAMPLE Hydrogen has 3 isotopes There are 3 different “versions” of a hydrogen atom Having more neutrons increases the mass AN EXAMPLE Hydrogen has 3 isotopes There are 3 different “versions” of a hydrogen atom Mass number = 2 WHY DOES THE ATOMIC MASS ON THE PERIODIC TABLE HAVE SO MANY DECIMAL PLACES? ATOMIC MASS 1 H 1.00797 This atomic mass is the one number that best represents the mass of all three versions of hydrogen. ATOMIC MASS 1 H 1.00797 No atom of hydrogen anywhere in the universe actually has this mass. ATOMIC MASS 1 H 1.00797 This number is a “weighted” average of the masses of every isotope of hydrogen CALCULATING A “WEIGHTED” AVERAGE 1. First, you have to know the mass of each isotope 2. Second, you have to know the percent abundance* of each isotope *see next slide PERCENT ABUNDANCE? Percent abundance is how often that particular isotope occurs in a sample of the element A sample of magnesium collected anywhere in the universe will contain three isotopes of magnesium. Sample of Magnesium PERCENT ABUNDANCE? These are the percent abundances of each isotope of magnesium 78.90% of this magnesium will be the isotope named “magnesium 24” 10.00% of this magnesium will be the isotope named “magnesium 25” Sample of Magnesium 11.10% of this magnesium will be the isotope named “magnesium 26” PERCENT ABUNDANCE? This data will be provided to you in problems or you will be given a table where you can look it up. 78.90% of this magnesium will be the isotope named “magnesium 24” 10.00% of this magnesium will be the isotope named “magnesium 25” Sample of Magnesium 11.10% of this magnesium will be the isotope named “magnesium 26” CALCULATING A “WEIGHTED” AVERAGE The formula (Mass of isotope #1)(percent abundance) 100 (Mass of isotope #2)(percent abundance) 100 (Mass of isotope #3)(percent abundance) 100 CALCULATE THE ATOMIC MASS OF MAGNESIUM FROM THE GIVEN DATA # protons # neutrons Isotope 1 Mass (amu’s) % abundance 12 12 23.985045 78.90% 12 13 24.985839 10.00% 12 14 25.982595 11.10% (magnesium-24) Isotope 2 (magnesium-25) Isotope 3 (magnesium-26) CALCULATING THE ATOMIC MASS OF MAGNESIUM Using the formula for calculating a weighted average 23.985045(.7890) +24.985839(.1000) +25.982595(.1110) 24.306852 amu’s Note how the % abundances were converted to decimals (just divide each by 100) Since its a “weighted” average this number has more influence since this mass occurs 79% of the time It’s NOT just a straight average where you add the masses and divide by 3 TRY ANOTHER EXAMPLE… CALCULATE THE ATOMIC MASS OF CHROMIUM Isotope Mass % abundance #1 49.946046 4.35 #2 51.940510 83.79 #3 52.940651 9.50 #4 53.938882 2.36 CALCULATE THE ATOMIC MASS OF CHROMIUM 49.946046(0.0435) 51.940510(0.8379) + 52.940651(0.0950) 53.938882(0.0236) 51.995926 amu’s VOCABULARY REVIEW Terms associated with isotopes VOCABULARY REVIEW Atomic number = the number of protons It’s the same as the element’s position on the periodic table For example, carbon is the 6th element on the table, so its atomic number is 6 which means it has 6 protons (and 6 electrons if it’s a neutral atom) VOCABULARY REVIEW Atomic mass = the “weighted” average mass of all the different isotopes of an atom This is the “weird” number on the periodic table that usually has a bunch of places past the decimal point VOCABULARY REVIEW Mass number = the sum of protons + neutrons This one kind of makes sense because the electrons essentially don’t have any mass This HAS to be a whole number because you can’t have part of a proton or neutron Mass number is NOT given on the periodic table LET’S PRACTICE THOSE TERMS… 1. 2. 3. 4. 5. What is the mass number of a carbon atom that has 6 neutrons? 7 neutrons? How many neutrons does a nitrogen atom have if its mass number is 14? How many neutrons does a chlorine atom have if its mass number is 36? What is the atomic number of neon? What is the atomic mass of neon? ANSWERS 1. 2. 3. 4. 5. 12 and 13 7 19 10 20.180 amu’s AX Z NOTATION FOR ISOTOPES A shorthand way to write isotopes A SHORT WAY TO WRITE ISOTOPES “A” is the symbol for Mass Number (#protons + #neutrons) “X” will be the symbol of the element AX Z “Z” is the symbol for Atomic Number (# of protons) DIFFERENT FORMS OF AXZ 35Cl17 is the same as 3517Cl Might also see “Cl-35” or “chlorine-35”