COMPOSITION OF MATTER
1. Atoms- building blocks of
matter (smallest particles)
- “ atomos” (uncut, indivisible/
indestructible, can’t be divided)
2. Molecules – “little quantity”
- composed of 2 or more
kinds of atoms
Discoveries about Atoms
1. Democritus
“Matter is composed of small
particles which are indivisible called
atoms”.
2. Aristotle
“ Matter is divided into smaller
particles without limit”.
Early Greek Theories
 400 B.C. - Democritus thought matter could
not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
Democritus
earth
Aristotle
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
John Dalton
 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
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All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Adding Electrons to the Model
Materials, when rubbed, can develop a charge
difference. This electricity is called “cathode rays”
when passed through an evacuated tube (demos).
These rays have a small mass and are negative.
Thomson noted that these negative subatomic
particles were a fundamental part of all atoms.
1) John Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Joseph John Thomson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Ernest Rutherford model “Nuclear Atomic
Model” (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
Ernest Rutherford (movie: 10 min.)
 Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
4.Neils Bohr’s model (Solar System Model)
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher
shell if hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra &
line spectra. It’s when electrons fall back down that
they release a photon. These jumps down from
“shell” to “shell” account for the line spectra seen in
gas discharge tubes (through spectroscopes).
5. Electron Cloud Atomic Model
 Electron cloud-a space
(n-energy level) in
which electrons are
likely to be found.
 Electrons whirl about
the nucleus billions of
times in one second
 They are not moving
around in random
patterns.
 Location of electrons
depends upon how
much energy the
electron has.
Electron Cloud:
 Depending on their energy they are locked
into a certain area in the cloud.
 Electrons with the lowest energy are found in
the energy level closest to the nucleus
 Electrons with the highest energy are found in
the outermost energy levels, farther from the
nucleus.
1st energy level (n-1) = 2 e–s
2nd energy level (n-1) = 8 e–s
3rd energy level (n-3) = 18 e–s
4th energy level (n-4) = 32 e–s
nth energy level (n-5 ----) = 32 e–s
Atomic numbers, Mass numbers
 There are 3 types of subatomic particles. We already
know about electrons (e–) & protons (p+). Neutrons (n0)
were also shown to exist (1930s).
 They have: no charge, a mass similar to protons
 Elements are often symbolized with their mass number and
atomic number
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E.g. Oxygen: 16 O
These values are given on the periodic table.
For now, round the mass # to a whole number.
These numbers tell you a lot about atoms.
# of protons = # of electrons = atomic number
# of neutrons = mass number – atomic number
Calculate # of e–, n0, p+ for Ca, Ar, and Br.
Atomic
Mass
p+
n0
e–
Ca
20
40
20
20
20
Ar
18
40
18
22
18
Br
35
80
35
45
35
Bohr - Rutherford diagrams
 Putting all this together, we get B-R diagrams
 To draw them you must know the # of protons, neutrons,
and electrons
 Draw protons (p+), (n0) in circle (i.e. “nucleus”)
 Draw electrons around in shells
He
p+
2
2 n0
Li
Li shorthand
p+
3
4 n0
3 p+
4 n0
2e– 1e–
Draw Be, B, Al and shorthand diagrams for O, Na
Be
B
Al
4 p+
5 n°
O
5 p+
6 n°
13 p+
14 n°
Na
8 p+ 2e– 6e–
8 n°
11 p+ 2e– 8e– 1e–
12 n°
Molecular Theory of
Matter
1. All matter is made up of
molecules.
2. Molecules are constantly
in motion. (Diffusion,
Brownian Movement)
a. Diffusion- spreading of
molecules in a container or any
available space.
Solid- fast Liquid – faster
Gas – fastest
b. Brownian Movement –
zigzag/random movement of
molecules
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3. There are spaces between
molecules. (Intermolecular
Spaces)
Intermolecular Spaces –
spaces in between
molecules
Solid- wide Liquid – wider
Gas- widest
4. Molecules attract one
another. (Surface tension,
capillarity –cohesion &
adhesion)
Intermolecular Attraction –
attraction of molecules
a. Capillarity – rising of liquid
into a narrow tube
Cohesion- attraction between
same kind of molecules
Adhesion- attraction between
different kinds of molecules
b. Surface Tension- thin,
elastic sheet on the surface