Atomic Weight

What does it mean?

The Mass of an Atom

• The mass of an atom is measured in atomic mass units (amu) aka Dalton.

• The atomic mass of an atom is the sum of the mass of its protons and neutrons

• Each proton and neutron weigh @1 amu

• By international agreement the Carbon-12 isotope (6 protons + 6 neutrons) is the standard by which every other type of atom is measured.

Atomic Weight (Mass)

• You will notice each element’s mass in the periodic table is NOT an even number.

• Atomic weight is actually the ratioed average mass of all the known isotopes of each element.

• The proportions are based on each isotope’s natural abundance.

• Ex: the atomic mass of Carbon is 12.011amu (atomic mass units)

• The majority of carbon isotopes are Carbon-12, but Carbon-13 and Carbon-14 exist in the same proportion everywhere. The average atomic mass of carbon is 12.011

Formula Weight

• A formula tells us:• Which elements are present• How many of each element is present

• An ionic compound does not exist as itself, but as part of a matrix

• They are not expressed as molecules but as a formula unit

• The sum of the atomic weights of a formula unit of an ionic compound gives the formula weight

• Just Add up the atomic mass of each type!

Examples

• NaCl

• Na = 22.98amu

• Cl = 35.45amu

• NaCl =

• 58.43amu

• NH4NO3

• N = 14.01 X 2 = 28.02

• H = 1.00 x 4 = 4.00

• O = 16.00 x 3 = 48.00

• NH4NO3 =

• 80.02amu

Molecular Weight

• The formula weight of one unit of a molecular compound, i.e., covalent compounds

• Water H2O H: 2 atoms @ 1.00amu

+ O: 1 atom @ 16.00amu

= 18.00amu

• Octane: C8H18

• C = 8 x 12.01 = 96.08amu

• H = 18 x 1.00 = 18.00amu

C8H18 = 114.08amu

Assignment

• Atomic, Formula, and Molecular weight problem set

• Due Tuesday Jan 13.