Transcript Unit_-_Atomic_Theory

Atomic Theory
•Matter (400 BC) - The Greeks were the first to
study matter.
•Greeks believed that matter was composed of
indivisible particles
called atoms.
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2 original theories on matter:
˙1) Matter is continuous - can be chopped up and
never reach a base particle (disliked due to
crystals).
2) Matter is discontinuous - if
you chop matter up you would
eventually hit a base particle =
atom.
•Atom - the smallest part of
an element that still retains
the properties of that element
= base particle.
•With the invention of the balance matter was studied
quantitatively.
•From this several laws were formulated:
1) Law of Conservation of Mass - Lavoisier.
2) Law of Definite Composition - Proust.
3) Atomic Theory - Dalton.
Dalton’s Atomic Theory
1) All matter is composed of tiny indivisible
particles called atoms.
Incorrect - due to fission - a nuclear process that
breaks down the atom into 3 subatomic particles:
1) neutron (neutral) in the nucleus (center of the
atom).
2) proton (positive) in the nucleus.
3) electron (negative) in the electron cloud (the
space surrounding the nucleus.
2) All atoms of the same element have the same
mass.
•Incorrect due to isotopes - atoms of the same
element with different numbers of neutrons.
A = atomic mass = #n + #p
•Atomic mass is the number of protons (for the
same element this number is the same) and the
number of neutrons,but if the number of neutrons
changes then the mass changes.
3) Atoms of different elements have different
masses.
•True - different elements have different masses
because they have a different number of protons
and the atomic mass is the sum of the protons and
the neutrons.
4) Atoms unite and disunite in definite proportions
of atoms.
True!
(ex) 2H2 + O2 ------> 2H2O
Law of Multiple Proportions - The ratio of
masses of one element that combines with a
constant mass of another element (in different
compounds) can be expressed as a small whole
number ratio.
(ex)
Mass of Sn Mass of O
SnO
119 g
16 g
SnO2
119g
32g
Divide both by the smallest for the ratio of atoms.
16g/16g = 1 32g/16g = 2
Therefore the ratio of O atoms is 1:2
History of the Atom
1) J. J. Thomson - cathode (discharge) ray tube
(put diagram on the board)
Results: could get the negative particles to pop off
the negative plate and migrate to the positive plate
(opposites attract), but could not get positives to
come off and migrate to the negative plate.
• Got a discharge of electricity which seemed to be
made of very tiny negative particles - discovered
the electron!
2) Robert Millikan - oil drop experiment.
(put diagram on the board)
Results: He calculated the charge on the oil
droplet. He always got some multiple of a base
number. Concluded that this base number must be
the charge of an electron = -1.6 X 10-19 C
• SI unit for electrical charge is the Coulomb = C.
• Using the data of both Thompson and Millikan it
was possible to calculate the mass of an electron =
9.1 X 10-31 kg.
•Protons were later discovered in a device similar to
Thomson’s discharge tube. Found particles
traveling in the opposite direction of the electrons
which had a positive charge.
• Protons have the same amount of electrical charge
as the electron, but opposite in sign = 1.6 X 10-19 C.
• A proton’s mass is approximately 1836 times more
massive than an electron.
• The mass of a proton and a neutron are
approximately the same.
• At this point in time 2 theories tried to explain and
predict the atom’s construction:
1) Dalton - solid, indivisible - WRONG!
Not indivisible!
2) Thomson (plum pudding model) - solid blob of
positive charge (evenly distributed throughout) with
negative electrons embedded on the surface. This is
what was accepted at this time.
3) Radioactivity (late 1800, early 1900’s)
- Becquerel and the Curies
•Realized that some atoms fall apart on their own.
When they did so they emitted one of three types
of particles:
1) gamma rays (neutral in charge) - also called
x -rays - have very high energy.
2) alpha rays (positive in charge).
3) beta rays (negative in charge) - similar to
Thomson’s electron, but higher in energy.
• ALPHA PARTICLES
• The alpha particle is the heaviest. It is
produced when the heaviest elements
decay. Alphas are rays not waves. The
alpha particle is an helium atom and
contains two neutrons and two
protons. The alpha particles is
relatively large and heavy. A sheet of
paper or a 3-cm layer of air is
sufficient to stop them. The alpha
particle emitter will not penetrate the
• outer layer of our skin, but is
dangerous if inhaled or swallowed.
The delicate internal workings of the
living cell forming the lining of the
lungs or internal organs, most
certainly will be changed (mutated)
or killed outright by the energetic
alpha particle. The number of lung
cancer cases among uranium miners
from inhaled and ingested alpha
sources is much higher than those of
the public at large.
• BETA PARTICLES
• Beta rays are much lighter energy particles. The beta
particle is an energetic electron given off by the
nucleus of unstable isotopes to restore an energy
balance. They can be stopped, for instance, by an
aluminium sheet a few millimetres thick or by 3
meters of air. Although the beta particle is around
8000 times smaller than the alpha particle, it is
capable of penetrating much deeper into living
matter. Each encounter with a living cell, and there
may be many before the beta energy is dissipated, is
likely to dam age some of the chemical links
between the living molecules of the cell or cause
some permanent genetic change in the cell nucleus
• GAMMA RAYS
• The next "particle" is the very high energy "X-ray"
called the gamma ray. It is capable of damaging
living cells as it slows down by transferring its
energy to surrounding cell components. Lead
shields are required to stop it from penetrating the
skin. The gamma has the highest penetrating
ability and is most harmful to us.
4.) Rutherford’s Experiment
- Used alpha particles.
- Tested Thomson’s model of the atom.
- Put diagram on the board.
Rutherford’s Results:
1) Most alphas passes straight through the foil and
hit the detectors straight out in front.
2) A few alphas were deflected at slight angles.
3) 1 out of 20,000 hit the foil and bounced straight
back toward the box.
• Rutherford believed that the atom consists of a
small dense central core where all of the positive
charge is concentrated. This core he called the
nucleus. He believed that the rest of the atom was
mainly empty space with negative electrons
spread throughout. He called this part of the atom
the electron cloud.
•Rutherford explains his results using the above
model for his atom:
•1) Most alphas passed straight through because
they passed through the vast empty space of the
electron cloud. Most passes straight through
because the empty space was so big.
2) A few alphas (+) were defected at slight angles
because they got close to the (+) nucleus. Since
like charges repel, they were deflected at slight
angels. It happened only occasionally because both
the nucleus and the alphas are so small that the
chance of them coming close to one another was so
small.
3) 1/20,000 alphas (+) directly hit the (+) nucleus
head on and since like charges repel they were
deflected back toward the box. So few were
deflected back because both the alphas and the
nucleus were so small that the chances of a direct
hit were so small.
• Thomson tries to explain Rutherford’s results:
1) Most alphas passed straight through because
there is not enough positive charge at the point of
impact.
2) Could not explain why some alphas were
deflected at slight angles.
3) Could not explain why 1/20,000 bounced back
toward the box.
•Rutherford’s results disproved Thomson’s theory
of the atom!
5) Neils Bohr
Planetary model of the atom - he compared the
movement of the electrons around the nucleus to
the movement of the planets around the sun.
6) Chadwick (1932) - showed the existence of
the neutron (neutral).
•Put diagram on the board.
Things to know:
•The electron cloud gives the atom most of its
volume and the least of its mass.
•The electron cloud keeps two atoms from
occupying the same space (like charges repel).
•Electrons have a negative charge and a mass of
9.11 X 10-28 g.
•The mass of a proton and a neutron are about the
same and are both much more massive than an
electron.
•The electron has so little mass that it is not
considered when determining the mass of an
atom.
•All electrons are identical except for the amount of
energy they possess.
•Nucleus contains n + p.
Atom = neutral
#p(+) = #e-(-)
Cation (+) - lost e(-), therefore more p than eAnion (-) - gained e-, therefore more e- than p.
• If like charges repel, how can so many protons
(+) exist in the small dense central core of the
nucleus? Wouldn’t they repel?
• When a proton and a neutron are very close
together a strong attraction occurs between them.
• Proton - proton and neutron - neutron
attractive forces exist when such pairs are very
close together.
• These short range p-n, n-n, and p-p forces hold
the nucleus together = nuclear forces.
Isotopes and Atomic Number
Atomic number (Z) = whole number on P. Table =
tells us the # protons.
Atomic mass (A) = decimal # on P. Table = tells
us the # of protons and neutrons.
Mass number (A) = # of protons and neutrons =
rounded off atomic mass.
Nuclear notation
A
(Z = p)
Z
X
(A = p + n)
# n = A -Z
(ex)
40
Ca
20
(ex) (82Pb207)4+
(ex) (53I 127)1-
• Nucleons - particles that make up the nucleus
(p + n).
• Isotope - atoms of the same element with
different # of n.
• Since the # of p for an element remains the same
and the atomic mass is the # of n + p, when the
number of n changes so does the atomic mass of
that element.
Isotopes of hydrogen
1) protium - the most common for hydrogen
nucleus contains 1p surrounding by 1 e-.
Nuclear notation = 1
Z = 1p
N = A - Z = 1 - 1 = 0n
Atom = neutral
1p = 1e-
1
H
2) deuterium (heavy H)
1
2
H
Z = 1p
#n = A -Z = 2 -1 = 1n
Atom = neutral
1p = 1e-
3) tritium (radioactive) - long 1/2 life
3
H
Z=1p
#n = A - Z = 3 - 1 = 2n
Atom = neutral
1 p = 1 e1
•Series - elements in the same row
1st series - H, He
2nd series - Li - Ne
3rd series - Na - Ar
Nuclear Reactions
•Nuclear change = nucleus changes
Nuclear Reactions
•Decay, makes isotopes, fission, fusion
4)
1) alpha decay - He nucleus (2He
Alpha decay
(ex) 84Po210 ----->
84
210
----->
Po
210
----->
Po
84
Alpha decay
(ex) 92
2
4
He
2
4
+
He
82
206
Pb
238
----->
U
238 -----> He4
U
92
2
238 -----> He4
U
92
2
+
234
Th
90
Alpha decay
(ex) 84Po212 ----->
84
210
----->
Po
84
210
----->
Po
2
4
He
2
4
+
He
82
208
Pb
2) Beta decay = -1
0
B (high
energy e-)
1
----->
n
(ex) 0
1
----->
0n
1 ----->
n
0
-1
0
B
0
B
-1
+
1
H
1
234
----->
Th
(ex) 90
234 ----->
Th
90
234
----->
90Th
0
B
-1
0
-1B
+
91
234
Pa
234
----->
Pa
(ex) 91
234
----->
0
Pa
B
91
-1
234
----->
0
Pa
B
91
-1
+
92
234
U
Half life - the time it takes for half the
original pile to decay.
(ex) Es99 - 320 days
No102 - 3 seconds