Download Counting Atoms

Counting Atoms
Chapter 3.3
Take out your PT, you’ll need it
Quick Review: Structure of
atom
• Where are the protons, neutrons, electrons
located?
• Which 2 subatomic particles make up the
majority of the mass of the atom?
• Which subatomic particle makes up the
majority of the volume of the atom?
• What is the charge of the nucleus?
• What is the overall charge of an atom?
• All atoms of the same element must have
the same number of protons, but not
neutrons
• Atomic number (Z) is # protons in an
element
• Atomic # identifies element
• Atoms are neutral: # protons = # electrons
• Atomic # = # protons = # electrons
• Isotopes
• Atoms of the same element that have different masses
(different # neutrons)
• Mass number= # protons + # neutrons
• ALWAYS A WHOLE NUMBER
• Identifying isotopes
• Hyphen notation
• Nuclear symbol
How many protons, neutrons, and electrons are there in
an atom of chlorine-37?
Atomic mass
• Relative atomic mass- based on mass of carbon-12
atom
• One atomic mass unit, u, is 1/12 mass of carbon12 atom
• Average atomic mass is weighted average of
naturally occurring isotopes of an element
• Number of isotopes
• Atomic Mass of each isotope
• Relative abundance (usually expressed as %)
• This is the mass expressed on the PT
Calculating average atomic
mass
• Need to know:
• Mass of isotope
• Relative abundance (%) of each isotope
• To calculate weighted average:
1. Change relative abundance to decimal (divide by 100 or
move decimal 2 places to the LEFT)
2. Multiply abundance as decimal (result from #1) by atomic
mass of isotope
3. Add together results from #2
4. Include units (u)
• DO NOT DIVIDE AT THE END! IT IS ALREADY A
WEIGHTED AVERAGE (%) SO WE DO NOT NEED
TO FIND AVERAGE AT THE END
There are 2 naturally occurring isotopes of hydrogen. It
consists of 99.9885% Hydrogen-1, with an atomic mass of
1.00785u and 0.0115% Hydrogen-2, with an atomic mass of
2.014102u. What is the average atomic mass of Hydrogen?
• Change % to decimal
• 99.9885% becomes 0.999885
• 0.0115% becomes 0.000115
• Multiply decimal by atomic mass
• 0.999885 x 1.00785u = 1.0077091u
• 0.000115 x 2.014102u = 0.00023162173u
• Add together
• 1.0077091u + 0.00023162173u = 1.007940722u
• In general, average atomic masses are rounded to 2
decimal places so our final answer would be 1.01u