Atoms: The Building Blocks of Matter

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Atoms The Building Blocks of Matter

• Chapter Three

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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 (earths population
5x109 people)
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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

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Aristotle

• rejected Democritus theory
• believed matter to be continuous never reaching
  a basic particle
• a philosopher not a scientist
• set chemistry back 2000 years

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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.
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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
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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
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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

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Daltons 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

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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

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Structure of the Atom
Atom
Consists of 2 regions
Electron Cloud
Nucleus
Where the volume of the atom is outside the
nucleus. Contains the
Small, very dense region located near center of
atom containing both -
Proton
Neutron
Electrons
Positively charged particles (p, 1,
nucleus) Mass 1 amu
Neutral particles (n0, 0, nucleus) Mass 1 amu
Negatively charged particles (e-, -1, electron
cloud) Mass 0 aum
Known as subatomic particles
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J. J. Thomson

• grandfather of atomic structure discovered e-
  w/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

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Cathode-Ray Tube Experiment
Draw and describe
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Robert A. Millikan

• found mass of the e- to be 1/2000 that of a p
• confirmed that e- carries a negative charge
• e- present in all atoms

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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.
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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

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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)
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Another look at Rutherfords experiment
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Composition of Atomic Nucleus
-all nuclei (except H) are composed of p and n0
-very light charge is same magnitude as that of
a p
of p in nucleus determines the atoms identity
(atomic number)
-mass same as a proton (just slightly greater)
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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.
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The Sizes of Atoms
-radius is center of nucleus to outer edge of e-
cloud
-usually expressed in pm
-nucleus has incredibly high density
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Chemical Configuration
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Arrangement of Electrons in Atoms
Chapter Four
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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.
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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
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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)
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• wavelength
• -a repetitive nature
• -?wavelength
• -the distance between corresponding points on
  adjacent waves
• -measured in m, cm, nm

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• 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

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Speed of Light
The speed of light is the most accurately-known
constant in the universe. It must be memorized.
3 x 108 m/s
- 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