Download Atoms: The Building Blocks of Matter

Atoms:
The Building Blocks of Matter
Chapter Three
The Atom
-smallest particle of matter that
exhibits properties of that
element
-subatomic particles of
different elements look
exactly the same
-very small ~ 1 x10 -8 cm in diameter
1.0 g Pb = 2.9 x 1021 atoms (earth’s population
~ 5x109 people)
Democritus
• came up with idea of atoms
• matter could be subdivided
until it reached the smallest
“indivisible” particle called
“atomos” which means
“un-cuttable”
• essentially believed in Law
of Conservation of Mass
Aristotle
• rejected Democritus’
theory
• believed matter to be
continuous – never
reaching a basic particle
• a philosopher – not a
scientist
• set chemistry back 2000
years
Law of Conservation of Mass
-mass is neither
destroyed nor
created during
ordinary chemical
reactions or
physical changes
The total mass of a
system remains the
same whether
elements are
combined,
separated, or
rearranged.
Law of Definite Proportions
-a chemical compound contains the same elements in
exactly the same proportions by mass regardless of the
size of the sample or its source
Water is always 2 hydrogens and 1 oxygen: H2O
Law of Multiple Proportions
-if 2 or more different compounds are composed of the same 2
elements, then the ratio of the masses of the 2nd element
combined with a certain mass of the first is always a ratio of
small whole #s
-an easy
example to
remember is
CO and CO2
1 nitrogen
2 nitrogens
John Dalton
• solid sphere atom
(billiard ball)
• number of atoms
large, but # of
different types small
• theories supported law
of definite
proportions, multiple
proportions,
conservation of mass
Dalton’s Atomic Theory
1. All matter is composed of small particles called
atoms
*** 2. All atoms of the same element are identical
*** 3. Atoms cannot be subdivided, created, or
destroyed
4. Atoms of different elements combine in simple
whole-# ratios to form chemical compounds
5. In chemical reactions, atoms are combined,
separated, or rearranged but not destroyed
Modern Atomic Theory
***
• These ideas were not
true
– Isotopes of elements
exist (atoms w/same #
p+ but different # of n0
such as C-12 and C-14)
– Atoms can be divided
into subatomic
particles (p+, e-, n0)
• Modern theory
– All matter is composed
of atoms
– Atoms of any one
element differ in
properties from atoms
of another element
Structure of the Atom
Atom
Consists of 2
regions
Electron Cloud
Nucleus
Small, very dense region
located near center of atom
containing both -
Proton
Where the volume of the
atom is – outside the nucleus.
Contains the
Neutron
Electrons
Positively
charged particles
(p+, +1, nucleus)
Neutral
particles (n0, 0,
nucleus)
Negatively charged
particles (e-, -1,
electron cloud)
Mass = 1 amu
Mass = 1 amu
Mass = 0 aum
Known as subatomic particles
J. J. Thomson
• grandfather of atomic
structure; discovered ew/cathode ray experiment
• e- are very light; mass
considered negligible
when describing “mass”
of atom; weighs ~ 1/1837
that of a proton or neutron
• thought of atom as plum
pudding – all particles
mixed together
Cathode-Ray Tube Experiment
Draw and describe:
Robert A. Millikan
• found mass of the e- to
be ~1/2000 that of a
p+
• confirmed that ecarries a negative
charge
• e- present in all atoms
Now what?
With these discoveries 2 inferences
were made about atomic structure:
1. Because atoms are electrically
neutral, a positive charge is
needed to balance the negative e-.
2. Because e- have negligible
mass, atoms must contain other
particles that account for most of
the mass.
Ernest Rutherford
• performed the gold
foil experiment which
proved the atom was
mostly space and that
all of the positive
charge was located in
a very small central
nucleus
Gold Foil Experiment
+ particles mostly
passed thru the
gold foil. However,
a # were deflected
back at the source
giving evidence to a
+ charged center in
the atom which
must be very small.
-nucleus is very dense and small compared to entire volume of the atom
(marble to a football field)
-e- like planet around the sun (lots of empty space)
Another look at Rutherford’s
experiment…
Composition of Atomic Nucleus
-all nuclei (except
H) are composed of
p+ and n0
-mass ~ same as a
proton (just slightly
greater)
-very light; charge is
same magnitude as
that of a p+
# of p+ in nucleus
determines the atom’s
identity (atomic
number)
Nice to Know
Atoms are always neutral particles since they contain
equal #s of p+ and e-; they have varying #s of n0
depending on the isotope.
What holds it all together?
Very strong forces, known as nuclear forces, between
like particles that are extremely close together hold
the nucleus intact. These are short-range p+- n0, p+p+, and n0- n0 forces.
The Sizes of Atoms
-radius is center of
nucleus to outer
edge of e- cloud
-usually expressed in
pm
-nucleus has
incredibly high
density
Chemical Configuration
Arrangement of Electrons in
Atoms
Chapter Four
The Development of a New
Atomic Model
Much of what we know about the atom
has been learned through experiments
with light; therefore, you need to know
some fundamental concepts of light in
order to understand the structure of the
atom, especially the placement of the
electrons.
Properties of Light
Light actually has a “dual” nature
(or split personality) because there
are instances in which it behaves in
two different ways. We will
examine each of the following:
1. The Wave Description
of Light
2. The Particle
Description of Light
Wave Description
- electromagnetic radiation: a form of
energy that exhibits wavelength
behavior as it travels through space;
examples: visible light, x-rays, UV,
infrared light, microwaves, radio waves
- electromagnetic spectrum: all
the forms of electromagnetic
radiation together; all forms of
this energy travel at a constant
speed (3 x 108 m/s)
-wavelength:
--a repetitive nature
--=wavelength
--the distance between corresponding
points on adjacent waves
--measured in m, cm, nm
-frequency:
-- or f
--the number of waves that pass a
given point in a specific time,
usually one second
--expressed in waves/second which is
called a hertz (Hz)
-- the shorter the wavelength the
greater the frequency
Speed of Light
The speed of light is the most accuratelyknown constant in the universe. It must
3 x 108 m/s
be memorized.
- wavelength units must match (3 x
1017 nm/s)
- formula is C =  where C is the
speed of light,  is the wavelength, and
 is the frequency
- wavelength is inversely proportional to
its frequency: as one increases, the other
decreases