atomic weight what does it mean?. the mass of an atom the mass of an atom is measured in atomic mass...

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.