Download CHM 103 Lecture 6 S07

Announcements & Agenda (01/22/07)
You should currently be reading Ch 4!
Quiz on Wednesday over Ch 3!
Today:
 Radiation (Ch 9 – only responsible for notes)
 Electron energy levels (3.7)
 Periodic trends (3.3, 3.8)
1
Last Time: All Atoms of the Same
Element Have the Same # of Protons!!!
11 protons
Symbol
11
Na
Also, the # of protons = the # of electrons for
a neutral atom
2
Last Time: Nuclear Symbols & Isotopes
• represents a particular isotope of an element.
• gives the mass number in the upper left corner
and the atomic number in the lower left corner.
Example: An atom of sodium with atomic
number 11 and a mass number 23 has the
following atomic symbol:
mass number
23
protons + neutrons
Na
atomic number
11
3
Last Time: The Atomic Mass is NOT
the Same as the Mass Number
 Listed on the periodic table
Na
22.99
 Gives the mass of “average” atom of each element
compared to 12C
 Average atom based on all the isotopes and their
abundance %
 Atomic mass (!unlike mass #!) is not a whole #
4
Why Do We Care About Isotopes?
Answer: Nuclear Stability!
“belt of stability”
Unstable nuclei


Radioactive
Can decay to
release alpha
particles, beta
particles and
positrons.
5
Radiation Primer: Subatomic
Particles Revisited
Radiation comes from nucleus of an atom
Unstable nucleus emits a particle or energy

protons and/or neutrons typically lost from nucleus
changing identity of element
 alpha (particle)
 beta (particle)
 gamma (pure energy)
6
Half-Life of a Radioisotope
The time for the radiation level to fall (decay)
to one-half its initial value
decay curve
initial
1
half-life
8 mg
4 mg
2
2 mg
3
1 mg
7
Examples of Half-Life
Isotope
C-15
Ra-224
Ra-223
I-125
C-14
U-235
Half life
2.4 sec
3.6 days
12 days
60 days
5700 years
710 000 000 years
8
Medical Uses of Radiation
Nuclear medicine has two main arenas
Diagnostic methods
Small amounts of radioisotopes
administered to help image an organ or
follow a physiological process
Therapeutic methods
Larger radiation doses to deliver fatal
punch to diseased tissue
9
Diagnostic Methods
Radioisotopes behave chemically the
same as stable isotopes of the same
atom
Thus, can use to target an organ or a
physiological process
Usually -emitters because radiation
has to emerge from the body if the
imaging equipment is to see it
10
Some Radio-Imaging Isotopes
P-32
Cr-51
Fe-59
Se-75
I-131
Hg-197
Eye tumors
Spleen shape and GI disorders
Bone marrow function
Pancreas scan
Thyroid malfunction
Kidney scan
11
PET Scans
Positron Emission Tomography
Good for following physiological processes
Patient given short-lived +-emitter
11
6
- +
C
11
5
B +
0
1
e+
t1/2
20 min
The + hits an electron in tissues
12
PET Scans
With annihilation event, a pair of gamma
rays are emitted in opposite directions
0
1
e+
+
0
-1
e-
2
Very clear images because signal
distinguished from background radiation
(only seen in one direction)
13
A PET scan instrument places patient inside a ring
of detectors to see the paired, 180o gamma rays
14
PET Scans
Can incorporate C-11 into a number of
organic compounds to follow
Blood flow
Glucose metabolism
Oxygen uptake
Find brain areas associated with epilepsy
Find hard to spot tumors
15
http://www.breastcancer.org/testing_pet.html
Abnormal
lymph
nodes
imaged by
PET scan
Also:
http://www.ldcmri.com/html/pet_scans.html
16
Therapeutic Radiation
Selective destruction of pathological
cells and tissues
Rapidly dividing cells most vulnerable thus it targets cancer
Used when cancer is not well localized
Co-60 is common source of x-rays and
-rays
17
Therapeutic Radiation
Can also deliver a radiation dose with
an internal administration of selected
isotopes
I-131 targets thyroid cancer - thyroxine
contains iodine
I-125 crystals implanted in prostate
gland to deliver continuous radiation (t1/2
= 60 days)
Y-90 implanted in pituitary to slow tumor
growth everywhere
18
Radio-Tracer Compounds
Huge use of radioisotopes to study
chemical reactions
Can see 10-19g/L - almost individual
atoms
Photosynthesis - carbon in glucose
comes from CO2
Calcium - uptake is 90% efficient in
children; 40% efficient in adults
Zinc - uptake by trees in winter 2 ft/day
19
Shifting Gears…
Electron Energy Levels
20
Characteristics of Electrons
 Extremely small mass
 Located outside the nucleus
 Moving at extremely high speeds
roughly in a sphere
 Form the “glue” that holds
compounds together
 Have specific energy levels
21
Atomic Spectra
Atoms can absorb and emit radiation
Absorption
 Usually, a source of white light passes
through a sample, and the atoms absorb only
specific frequencies of light.
Emission
 When excited atoms emit photons, the
frequencies of the photon are specific.
 “quantization of energy”
COOL DEMO…
22
ABSORPTION
EMISSION
23
A ball on a staircase shows some properties of
quantized energy states.
24
Explanation for Discrete
Energies: the Bohr Model
First model of the electron structure of atoms
Gives levels where an electron is most likely to
be found
Incorrect today, but a key in understanding the
atom
25
Line Spectra & the Bohr Model
Bohr Model
• Bohr noted the line spectra of certain elements
and assumed the electrons were confined to
specific energy states. These were called orbits.
26
Quantum Mechanics
Describes the arrangement of electrons in
atoms in terms of:
 Main or principal energy levels (n)
 Can describe electrons with “quantum
numbers”
 Energy subshells
 Orbitals (space occupied within the atom)
27
Electron Levels (Shells)
 Contain electrons that are similar in energy
and distance from nucleus
 Low energy electrons are closest to the
nucleus
 Identify by numbers 1, 2, 3, 4, 5, 6…..
 The first shell (1) is lowest in energy, 2nd level
next and so on 1<2<3<4
28
Number of Electrons
Maximum number of electrons in any
electron level = 2n2
n =1
n =2
n =3
2(1)2
2(2)2
2(3)2
=
=
=
2
8
18
29
Order of Electron Filling
All electrons in the same energy level have similar
(BUT NOT IDENTICAL) energy.
Shell 1
Shell 2
Shell 3
2 electrons
8 electrons
18 electrons
(8 first,10 later)
Order of filling for the first 20 electrons
Shell
1
2e
2
8e
3
8e
4
2e
30
Electron Configuration
 Lists the shells containing electrons
 Written in order of increasing energy
Element
Shell
1
2
3
He
2
C
2
4
F
2
7
Ne
2
8
Al
2
8
3
Cl
2
8
7
31
Learning Check
A. The electron configuration for sulfur
1) 2,6
2) 8,2,6 3) 2, 8, 6
B. The element in period 3 with two electrons
in the outermost energy level
1) Mg
2) Ca
3) Be
32
Orbitals: “Locations” of the Electrons
• three-dimensional spaces around a nucleus
where an electron is most likely to be found.
• have shapes that represent electron density
(not a path the electron follows).
• each orbital can hold up to 2 electrons.
33
s Orbitals
An s orbital
• has a spherical shape around
the nucleus.
• is found in each energy level.
n=3
n=2
n=1
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Publishing as Benjamin Cummings
34
p Orbitals
A p orbital
• has a two-lobed shape.
• is one of three p orbitals in each energy level from n = 2.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
35
Electrons in Energy Levels n = 1- 4
Energy
Level
1
2
3
4
Orbitals
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
Maximum
No. of Electrons
2
2
6
2
6
10
2
6
10
14
Total
Electrons
2
8
18
32
36
Organization of the Periodic Table
s1 s2
1
2
3
4
5
6
p1 p2 p3 p4 p5 p6
d1 - d10
f1 - f14
37
Periodic Law
All the elements in a group have the same
electron configuration in their outermost shells
Outermost electrons are called valence electrons
Elements with same # of valence electrons
display similar chemical & physical properties!!!
Example: Group 2
Be 2, 2
Mg 2, 8, 2
Ca 2, 2, 8, 2
38
Groups and Periods
39
Periodic Table
Note: Two methods for numbering; we will use
1A, 2A, etc.
40
Metals, Nonmetals, and Metalloids
The heavy zigzag line
separates metals and
nonmetals.
• Metals are located to
the left.
• Nonmetals are located
to the right.
• Metalloids are located
along the heavy
zigzag line between
the metals and
nonmetals.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
41
Metals, Nonmetals, & Metalloids
Metals
• shiny and ductile
• good conductors of heat and electricity
Nonmetals
• dull, brittle, and poor conductors
• good insulators
Metalloids
• better conductors than nonmetals, but not as
good as metals
• used as semiconductors and insulators
42
More Periodic Trends
How Atomic Orbital filling affects:
Atomic Size (Radius)
Ionization Energy

Definitions….
Atomic Radii: Distance between center of
nucleus and outer electron shell
Ionization Energy : Cost of removing an efrom a neutral atom
43
Atomic Radius Within A Group
Atomic radius increases
going down each group
of representative
elements.
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Publishing as Benjamin Cummings
44
Atomic Radius Across a Period
Going across a period left to right,
• an increase in number of protons increases attraction
for valence electrons.
• atomic radius decreases.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
45
Ionization Energy In a Group
Going up a group of
representative elements,
• the distance decreases
between nucleus and
valence electrons.
• the ionization energy
increases.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
46
Ionization Energy
• Metals have
lower
ionization
energies.
• Nonmetals
have higher
ionization
energies.
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Publishing as Benjamin Cummings
47