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Chapter 3
Atoms: The Building Blocks of
Matter
I. The Atom
*Matter: anything that has
mass and volume
*Indirect Evidence:
evidence you get without
actually seeing or touching
the object
-Greek Model (Democritus)
*atom: smallest particle of an
element that has the prop of
the element
~atomos: meaning not to
be cut
-Dalton’s Atomic Theory
1. All elements are composed
of atoms
2. Atoms of the same element
are exactly alike
3. Atoms of diff. elements
are diff.
4. Compounds are formed
by joining of atoms of 2 or
more elements
5. In Chem Rxn’s, atoms are combined,
separated, or rearranged.
-Are exceptions:
1. Atoms are divisible
2. Given elements can have atoms
with diff masses (isotopes)
*Law of Conservation of Mass: pg 66
*Law of Multiple Proportions: pg 66
*Law of definite proportions: pg 66
II. The Structure of the Atom
-Thomson’s Model:
~Plumb Pudding
*Electrons: - charge
~Cathode Ray (pg 70)
*Cathode: conn to the neg
*Anode: conn to the pos
-Rutherford’s Model
~Gold Foil Experiment
~Inferences made about atomic
struct
1. e- small
2. neg
3. must have a + charge to make an
atom neutral
4. Atom mass must come from
somewhere else??
~the atom was mostly empty space
with a dense, positively charged
nucleus in the center.
*Nucleus: the center of the atom
*Nuclear Force: hold nuclei together
 Like charges “+/+”
*Electrostatic (Electromagnetic): holds
opposite charges together “e-/+”
-Bohr Model
~electrons move in definite
orbits around the nucleus
-Wave Model (Today accepted
model)
~electrons don’t move about
in a definite path
~probable location is based
on how much nrg the
electron has
-Subatomic particles
*smaller than the atom
• Electrons
• Electrons orbit around
an atom. In this
picture, they are the
small yellow bits.
• They have a negative
charge.
• They are lighter than
protons or neutrons.
• They are positively
charged.
• Are located in the
nucleus.
• In this picture, the
protons are the
blue pieces in the
center of the atom.
• Neutrons are neither
positive nor negative.
• Neutrons are in the
nucleus of an atom.
• In this picture,
neutrons are the
purple pieces in the
center of the atom.
mass (amu)
charge location
~proton:
1
~neutron: 1
~electron: 1/1836
+ nucleus
+/- nucleus
e- cloud
SEE HISTORY OF THE ATOM WEBSITE
http://web.neo.edu/rjones/Pages/1014new/Le
cture/chemistry/chapter_8/pages/history_
of_atom.html
http://www.emsb.qc.ca/laurenhill/science/mo
dels.html
http://www.absorblearning.com/chemistry/d
emo/units/LR301.html#J.J.Thomson
III. Counting Atoms
*Atomic Mass Unit (amu):
measures mass of subatomic
part.
-?? Where does it come
from??
1/12 mass of a C-12 atom
-Atomic #
*number of + in the nucl
~identifies the element
~never chngs
-Mass # and Atomic Mass
*sum of the + and neutrons in its
nucleus
-Isotopes
*same # of +, but diff # of
neutrons(same elem, diff mass)
*Nuclide: any isotope of any element
EX: table 3-3 pg 77
-Isotopes of H (pg 76)
1. Protium: most common
1+, 1e-, 0+/- mass# = 1
2. Deuterium:
1+, 1e-, 1+/- mass# = 2
3. Tritium:
1+, 1e-, 2+/- mass# = 3
isotope notation
1.
mass #
H : Nuclear symbol
atomic#
pg: 77
2. Hydrogen - mass #
pg 80
*Aver Atomic mass: aver of the
masses of all the atoms in the
sample.
(that’s why Atomic mass is not a
nice numb)
~mass # - atomic# = #of neutrons
*Mole: # of particles in 12 grams of C12
pg 81
*Avogadro’s #: # of particles in 1 mole
of substance.
6.022 X 1023 atoms
mole
*Molar Mass: mass of 1 mole of pure
substance
units---- g/mole
-Gram/Mole Conversion Examples