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Chemistry
Chapter 4
Notes #2 (sec. 3)
Periodic Table
 All
atoms are composed of smaller
particles including equal numbers of
protons (+) and electrons (-)
 Atomic Number = # of protons

Distinguishes one element from another
 Elements
are in sequence by atomic
number
 The table also reflects the arrangement of
electrons....
Periodic Table
 Periods


7 Horizontal Rows (across)
By increasing atomic number
 Groups/Families


Columns (up and down)
Have elements with similar properties
Periodic Table

Chemical Name
 Chemical symbol –
letters to rep. element
 Atomic Number – # of
protons in nucleus
 Atomic Mass – The
mass of an atom
measured in amu’s
(atomic mass units)
How atoms differ….

For now, electrons and protons will be equal
 # of neutrons can be different


Atoms with the same number of protons and different
numbers of neutrons are called isotopes
Mass Number = #protons + #neutrons
#p+
#n0
#e-
Hydrogen-1
1
0
1
Hydrogen-2
1
1
1
Hydrogen-3
1
2
1
Isotopes
Isotopes
 Abundance
of isotopes is constant in
nature.

Potassium (K)….K-39, K-40, and K-41
93.25% have 20 neutrons, Mass # = 39
6.7502% have 22 neutrons, Mass # = 41
.0117% have 21 neutrons, Mass # = 40

# neutrons = mass number – atomic #



Mass of Atoms
 Mass
of an atom is in the nucleus
 Protons and Neutrons = 1.67 x 10-24 g
 Electrons = about 1/1840 of a p+ or n0
 Chemists method for measuring mass of
atoms – easier numbers


Atomic standard = carbon 12 atom (6 p and
6n) = 12 atomic mass units
One amu (atomic mass unit) = 1/12 carbon 12
atom
Mass of Atoms

Mass is close to the mass of protons, but not
exact.


Masses of subatomic particles in amu’s




Ex. Silicon 30 = 29.974 amu
Electron = 0.000549
Neutron = 1.00865
Proton = 1.004276
Atomic Mass of an element is the weighted
average mass of the isotopes of that element
Mass of Atoms
#15 page 104
Isotope
Boron 10
Boron 11

Abundance
19.8 %
80.2%
Mass (amu)
10.013
11.009
(0.198 x 10.013) + (.802 x 11.009)
 = 10.81 amu
 Notice it is closer to 11 amu than 10. Why?

THE END!