Atomic Structure

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Atomic Structure
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Atom consists of 3 main subatomic particles
proton (p)
neutrons (n0)
electrons (e-)
neutral
0
1 a.m.u
1 a.m.u.
1/12 of the mass of a carbon-12 atom (reference
element)
Atomic Mass Unit
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General Structure of the Atom
nucleus
composed of protons and neutrons
very small, dense, charged
surrounded by the electron cloud
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Atomic Structure History 1800s to present
Idea of atoms that elements are composed of
tiny, identical particles that couldnt be
divided into anything smaller
John Dalton
J.J. Thomson
plum pudding model of the atom
discovered the electron
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Cathode ray tube
opposite charges attract
(proton and electron)
like charges repel
proton/proton or electron/electron
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Discovery of the nucleus
Ernest Rutherford
Gold Foil Experiment
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Rutherford concluded that atoms have a
very small, dense and () charged nucleus
most of the atom is empty space
Gold atom
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Niels Bohr
Planetary model of the atom
described electrons electrons can only be found
in certain regions called energy levels
Atomic Emission Spectrum (hydrogen)
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Series of lines of colored light that is unique
to each element (fingerprint)
Emission Spectrum
Iron
carbon
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All wavelengths of light
White light
prism
single element
only a few wavelengths of light produced
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What does this have to do with electrons?
energy levels
atom
Atom absorbs a definite amount of energy
(quantum)
electrons are excited to a higher energy level,
farther away from nucleus (unstable)
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Electron falls back to the lower energy level
emitting the same quantum of energy as a
specific color of light
Ground state
All electrons are in the lowest possible energy
levels (stable)
No in between states !!!
Bohrs model of the atom
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Erwin Schrodinger
Quantum Mechanical model of the atom
electron as a wave
electron does not follow a definite path can
only tell where an electron is likely to be found
electron cloud or orbital
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different orbital shapes
spherical
s orbital
1 orientation
p orbital
dumb bell shape
3 orientations
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d orbitals
5 orientations
f orbitals
1 example
7 orientations
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Schrodingers thought experiment
hydrogen cyanide
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number of protons in the nucleus of the atom
Atomic Number
determines identity of the atom
sum of the protons and neutrons (whole number)
Mass Number
Symbol notation
Ex Carbon-12
mass number
mass number
12C 6
6 p, 6 e-, 6n0
atomic number
19
56Fe 26
26
26
30
8
8
10
Oxygen-18
137 Cs 55
Cesium-137
55
19
19
23
Potassium-42
210 Pb 82
Lead-210
82
14 C 6
6
6
8
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Isotopes
atoms that have the same number of protons but a
different number of neutrons same atomic
number, different mass number
23 Na 11
24 Na 11
sodium-23
sodium-24
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12 C 6
13C 6
14 C 6
Carbon isotopes
All have 6 protons
6 n0
7 n0
8 n0
2 H 1
3 H 1
1 H 1
Hydrogen isotopes
All have 1 proton
0 n0
1 n0
2 n0
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Average Atomic Mass
weighted average of the mass numbers of the
isotopes of an element
C-12 C-13 C-14
12.011
The average is closest to the mass number of
the Most abundant isotope
H-1 H-2 H-3
H-1 is most abundant
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Calculating average atomic mass
Ex An element has two isotopes with mass
numbers 10 and 11 if the percent abundances are
20.0 and 80.0 respectively, what is the
average atomic mass of the element. Also identify
the element.
AAM
(10)(0.200)
(11)(0.800)
2.00
8.80
AAM 10.80 amu, boron
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Ex 77.35 of the atoms of an element have a
mass number of 35. The rest of them have a mass
number of 37. Calculate the average atomic
weight and identify the element. Also sketch its
mass spectrum.
AAM
(35)(0.7735)
(37)(0.2265)
27.0725
8.3805
AAM 35.45 amu, chlorine
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Ex Gallium has two isotopes Ga-69 and Ga-71
If the average atomic mass of gallium is 69.72
amu, calculate the percent abundance of each
isotope.
69.72
(69)(x)
(71)(1-x)
69.72 69x 71 -71x
-71
-71
-1.28 -2x
X 0.64 or 64 of Ga-69
-2
-2
36 of Ga-71
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The Nucleus
Radioactive decay
Process by which an unstable nucleus emits
particles and energy turns into an atom of a
different, more stable element
Marie Curie
Henri Becquerel
14 C 6
12C 6
unstable, radioactive
stable, not radioactive
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Types of Decay
Alpha, a
Nucleus emits 2 protons and 2 neutrons
4He 2
causes atomic to decrease by 2 and mass to
decrease by 4
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Beta, ß
nucleus emits an electron
n0 ? p e-
emitted as beta particle
e-
stays in the atom causing the atomic to
increase by 1
mass is unchanged
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High energy rays (like x-rays) from unstable
nucleus
Gamma rays, ?
accompanies alpha or beta
not particles no mass, no charge
Ex 60 Co ? 60 Ni 0 e ? 27
28 -1
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complete these nuclear decay equations
222 Rn ? 4 He 86
2
218 Po 84
137 Cs ? 0 e 55 -1
137 Ba 56
131 I ? 0 e 53 -1
131 Xe 54
243 Am ? 4 He 95
2
239 Np 93
42 K 19
214 Po ? 210 Pb 84
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4 He 2
? 42 Ca 0 e
20 -1
? 32 S 0 e
16 -1
? 234 Th 4 He
90 2
238 U 92
32 P 15
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some uses for radioactive isotopes (radioisotopes)
nuclear energy and weapons
medical imaging
Tc-99
cancer treatment
U-235
Co-60
I-131
dating
Pu-239
C-14
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Nuclear Power
core
nuclear fission
nucleus is split, releasing energy
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fission
Ba
U-235
Kr
Fusion
small nuclei come together releases energy
fuels the sun and stars
2 H 3 H ? 4 He 1 n 1 1
2 0
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rate of radioactive decay Half Life
Half Life
the time it takes for 50 of a sample of
radioactive atoms to decay
t1/2
Ex C-14 t1/2 5730 yr
100 g
50 g
25 g
12.5 g
(2)(5730) 11,460 yr
(3)(5730) 17,190 yr
5730 yr
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decay curve
Fraction remaining
of half lives
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half lives Time (T)
half life (t1/2)
Half Life problems
Ex Iodine-131 has a half life of 8 days. What
fraction of a sample would be left after 24 days?
24 3 H.L.s 8
1 ?
½ ?
¼ ?
1/8
Ex Strontium-90 is a radioactive isotope
produced from atomic bomb explosions. Its half
life is 28 yrs. How much of a 160 gram sample
will be left after 112 years?
112 4 28
160 g ?
80 g ?
40 g ?
20 g ?
10 g
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Ex An old bone contains ¼ the amount of
carbon-14 as similar living tissue. If the half
life of carbon-14 is 5730 yrs, how old is the
bone?
1 ?
½ ?
¼ ?
2 half life periods
(5730)(2)
11, 460 yrs.
Ex Radon-222 is a radioactive gas that seeps
into homes and can eventually cause lung cancer
has a half life of 3.82 days. How long would it
take fro 40 mg to decay down to 2.5 mg?
40 ?
20 ?
10 ?
5 ?
2.5
4 H.L. s
(3.82)(4)
15 days
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Ex Cesium-137 is used in the atomic clock (t1/2
30.23 yrs.) How long would it take for
100 grams to decay down to 12.5 grams?
100 ?
50 ?
25 ?
12.5
3 H.L. s
(30.23)(3)
91 yrs
A sample of carbon-14 weighs 0.01mg after 22,920
yr. What was the mass of the original sample?
(t1/2 5730 yr)
22,920 5730
4 H.L.s
0.01 ?
0.02 ?
0.04 ?
0.08 ?
0.16 mg