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```					Atomic Structure

Section 18.2
The Nucleus

   Nucleus- contains most of the mass of an atom
–   Protons & Neutrons contains most of an atom’s mass
–   Electrons- 1/2000th of the mass of a proton
   Grams are not used to measure mass of atoms
because atoms are too small
–   Amu = atomic mass unit
   Mass of proton = mass of neutron = 1amu
Atomic structure

   Amu – 1/12th the mass of a carbon-12 atom,
which contains 6 protons and 6 neutrons
   Protons- determine the identity of an atom
–   Every atom has a different # of protons
   Atomic number- tells us the number of
protons
   Mass number- total number of protons &
neutrons
   # of neutrons = mass # - atomic #
Atomic Structure

   Electrons- determine physical and chemical
properties of atoms
   Charges of atoms- indicates # of electrons
gained or lost
   Cation- ion with a positive charge- lost
electrons
   Anion- ion with a negative charge- gained
electrons
Atomic Structure

   Isotope- Atoms of the same element with different
numbers of neutrons
   To indicate the identity of an isotope, write the element
name – mass #
–   Carbon-12 Carbon-14
   Average atomic mass- a weighted average of the masses
of the isotopes of an element
–   Average atomic mass = (% x mass) + (% x mass) + (% x
mass)….
–   Reason it is an average- there are several isotopes for each
element
Example of average atomic mass for
carbon

mass number 13 exact weight          percent
abundance
12                12.0000            98.90
13                13.003355          1.10

To calculate the average atomic weight, each exact atomic
weight is multiplied by its percent abundance (expressed as
a decimal). Then, add the results together and round off to
an appropriate number of significant figures.
This is the solution for carbon:
(12.000000) (0.9890) + (13.003355) (0.0110) = 12.011

   C has an average atomic mass of 12.011
amu.
   This is closer to 12 than to 13, which means
most isotopes must be carbon-12.
   This matches our known data, which say that
98.90% of carbon atoms are C-12.

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