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Transcript
Relative atomic mass
The relative atomic mass of an
element (Ar) is the average mass of
an atom of the element, taking into
account all of its isotopes and
their relative abundance, compared
to one atom of carbon-12
The atom
0 atomos – Greek word meaning “indivisible”
0 Elements are made up of atoms
0 Atoms
0 Are the smallest unit of an element
0 Are made up of subatomic particles
0 Cannot be broken down by chemical reactions
0 All matter is built from ~100 elements
The atom
If you grind down gold into dust, each dust particle
still has the properties of gold!
With the right equipment you could keep grinding
until you reach a particle that can not be divided
any further and still retains the properties of gold–
the atom!
Scanning Tunneling
Microscope
Sub-atomic particles
Relative
charge
+
Location
Notes
Proton
Relative
mass
1
Nucleus
= Atomic #
• Defines type of atom
• Makes up part of
atomic mass
Electron
0.0005
-
Electron cloud
= Atomic # in a
neutral atom
Neutron
1
0
Nucleus
• Makes up part of
atomic mass
Particle
Notation
Atomic Mass Number
Atomic Number
Definitions
0 Atomic number (Z): Tells us the type of element
0 Defined by the number of protons
0 Mass number (A): The number of protons + neutrons
0 The mass of the atom is centered in the nucleus
Atomic Number (Z)
0 Z = The number of protons
0 This number is unique for
each element
0 This number never varies…
Carbon always has 6 protons
Atomic Number (Z)
0 Carbon has 6 protons
0 The atom is neutral
0 How many electrons does
it have?
Ions
0 An atom that has gained or lost electrons
0 Cation- An ion with a positive charge
0Atom has lost electrons
0 Anion- An ion with a negative charge
0Atom has gained electrons
Ions
0 Determine the number of electrons present in an oxygen ion
that has a negative 2 charge (O-2)
0 Z = the # of protons = 8
0 In a neutral atom, oxygen has 8 protons
(8 negative charges to balance the 8 positive charges that the protons carry)
0 the -2 charge indicates that we have 2 more negative charges
(therefore 2 more electrons)
0 10 total electrons
Atomic Mass Number
0 If the relative mass of a protons and neutrons is 1,
why is the atomic mass = 12.01?
0 Different kinds of carbon exist!
0 Average atomic mass is the
average of all types of carbon
that exist
Isotopes
0 Isotopes- atoms of the same element
with different numbers of
neutrons
0 They still have the same # of
protons! Z is always the same
0 The number of neutrons can vary
0 Same chemical properties
Isotopes
0 Isotopes are identified by their mass #:
Subatomic
Particles
Potassium- 39
Potassium-40
Potassium-41
Protons
19
19
19
Neutrons
20
21
22
Electrons
19
19
19
The differing masses leads to different physical properties (bp & mp)
Isotopes
0 The relative abundance of each isotope is constant
Subatomic
Particles
Potassium- 39
Potassium-40
Potassium-41
Protons
19
19
19
Neutrons
20
21
22
Electrons
19
19
19
Calculating Atomic Mass
0 Weighted average atomic mass
= sum of all mass contributions
0 Mass Contribution
= (Atomic Mass)(Percent Abundance)
Calculating Atomic Mass
0 Boron has two naturally occurring isotopes:
boron-10 (abundance 19.8%, mass = 10.013 amu) and
boron-11 (abundance 80.2%, mass = 11.009 amu).
Calculate the atomic mass of boron.
Mass contributions:
Boron-10
(10.013)(.198)= 1.98
Boron-11
(11.009)(.802)= 8.83
Weighted average atomic mass: 1.98 + 8.83 = 10.81 amu
Announcements
0 Turn in take home test – Today
0 Turn in % correct – Today
0 Turn in solution labs if you haven’t already
0 Atomic Structure Quiz- turn in work
0 Field trip – tomorrow
0 Report to cafeteria before the 1st bell
0 Bring lunch or buy from cafeteria lunch 11-12pm
0 Sunny 70 degrees…. Bring appropriate clothing…wear
clothes that can get dirty…wear tennis shoes
0 Take pre-survey – check edmodo
0 Presentation dates 22nd (1-3) 23rd (4-6) in F108 at 11:50
0 IJournal #10 by Wednesday
0 Finish packet by Thursday
Radioisotopes
0 The stability of the nucleus depends on a balance between
protons and neutrons
0 If there are too many or too few neutrons, changes will
occur to the nucleus to create a more stable environment
0 These changes give out radiation in the form of
0 Alpha particles
0 Beta particles
0 Gamma rays
0 Nuclear Reaction: a reaction that involves a change
to an atoms nucleus
0 A change to an atoms nucleus changes its identity
0 Radioactive atoms undergo changes that alter their
identities
0 Radioactivity is a process that emits radiation
0 Tracers used to detect structure and function
of organs
0 Medical Diagnosis – PET scan
0 Cancer treatment
0 Food preservation
0 Sensors in Smoke Detectors
0 Nuclear fuel for power plants
0 Detected with a Geiger Counter.
(When ions strike the cylinder of the Geiger counter, it emits
an audible click.)
0 Dosimeter – measures the total amount of
radiation that a person has received.
0 Usually is worn like a badge.
0 The film is later developed and the exposure to
radiation can be measured.
α
0 Composition: Alpha particles
0 Charge: +2
0 Radiation: Helium Nuclei
0 Penetrating Power: Blocked by paper
0 Cannot penetrate skin, not dangerous
4He
2
α
4He
2
0 Helium nucleus released by nucleus
0 Mass decreases by 4
0 Atomic number decreases by 2
α
4He
2
U  Th  He
238
234
4
92
90
2
0 Uranium has changed into a new element: Thorium
0 Alpha particle is released
β




Composition: Beta Particles
Radiation: Electrons
0
1
e
Charge: -1
Penetrating Power: Blocked by metal
foil, more dangerous
0
β
1
e
0 Neutron changes into a proton
0 Electron released (do not need to worry about the
mechanism of how this happens)
0 Mass unchanged (the mass of a neutron is roughly
equal to the mass of a proton)
0 Atomic number increases by 1
β
0
1
14
6
e
C 7 N  e
14
0 Carbon is changed into nitrogen
0 Atomic # increases
0
1
γ

Composition: Electromagnetic Radiation
(Rays)

Radiation: Photons

Charge: Neutral

Penetrating Power: not completely blocked by
lead or concrete – dangerous!
γ
0 Pure energy released from nucleus
0 Mass unaffected
0 Atomic number unaffected
γ
87
87
Sr* Sr
38
38

0 Pure energy released from nucleus
0 No new element formed
0 Protons and neutrons change relative positions in the nucleus
Carbon-14 dating
0 14C has 8 neutrons (too many to be stable)
0 The atom can become stable by beta decay
0 1 neutron will be changed into a proton
0 1 electron will be ejected
14
6
C 7 N  e
14
0
1
0 Carbon dating:
0 Used to measure the age
of artifacts that were once
part of a living organism
0 Every 5730 years the 14C to 12C
ratio falls by 50% (1 half life) after a
living organism dies
• Can determine the age of organic
matter with 14C
(Up to 45,000 yrs)
Cobalt- 60
0 Radiation therapy (radiotherapy) – the treatment of
cancer with radiation
0 Emits gamma rays
0 Protons and neutrons change positions in nucleus
0 Damages normal and
malignant cells
0 Knocks off electrons
making it impossible
for cells to grow
Iodine-131
0 Used as a medical tracer
0 Iodine is an essential element that
enables the thyroid gland to
produce thyroid hormones
0 Radioisotopes travel through the body just like the isotopes that are naturally in
our body (same chemical properties)
0 Positions in the body can be monitored by detecting radiation levels
0 Emits beta particles and gamma rays
0 Short half life of 8 days – quickly eliminated from body
Iodine-125
0 Treatment of prostate cancer
0 Inserted into gland (isolated treatment)
0 Emits beta radiation
0 Longer half life of 80 days
Mass Spectrometer
0 Sample introduction
0 Direct insertion
0 MALDI plates
commonly used
0 Direct injection
0 If already in gaseous state
Mass Spectrometer
0 Vaporization0 Sample vaporized
(if not already in gaseous state)
0 Allows individual atoms to
be analyzed
0 Ionization0 atoms hit with high energy
electrons
0 electrons knocked out
0 produces positively charged
ions
0 no effect on mass
X(g) + e-  X+(g) + 2e-
Mass Spectrometer
0 Acceleration
0 Positive ions
attracted to
negatively charged
plates in machine
0 The ions accelerate
due to electrical
field
Mass Spectrometer
0 Deflection
0 positive ions deflected by a
magnetic field
0 cause position changes
0 Amount of deflection based on
charge/mass ratio:
0 ions with smaller mass are
deflected more than
heavier ions
0 ions with higher charges are
deflected more than lower
charges
Mass Spectrometer
0 Detection
0 Positive ions are detected
and sent to a recorder
0 Strength of signal
dependent on the
number of ions with a
particular charge/mass
ratio
Uncharged particles removed
by vacuum pump
Mass Spectrometer
0 Relative atomic mass
0 Real mass numbers are very difficult to use
0 Relative masses provide a frame of reference with more
straight forward numbers
0 Carbon-12 isotope
0 Common element (in all living things)
0 Solid
0 Easy to transport and store
0 Chosen as the standard in 1961
0 Given a relative mass of exactly 12
Mass Spectra/Spectrum
0 X axis- mass/charge (m/z) ratio
0 m/z ratio- always assume +1 charge
0 Y axis- % abundance
Mass Spectra/Spectrum
0 Mass Spectra for Boron:
0 100 ions have a mass of 11
0 23 ions have a mass of 10
0 Total ions in sample: 123
0 Relative atomic mass:
0 81.3% have a mass of 11
0 18.7% have a mass of 10
0 (.813)(11) + (.187)(10) =
0Ar = 10.81
Electromagnetic Spectrum
c = ʄ λ c = 3 x 108 m s-1
All waves travel at the same speed – can be
distinguished by different wavelengths (λ)
Electromagnetic Spectrum
Wavelengths are
measured peak to
peak
Frequency is the # of waves
in a given period
Line Spectra
ΔEelectron= Ephoton
0 Line spectrums act like fingerprints to identify elements
0 Electrons absorb energy and move into an excited state
0 The excited state is very unstable, so the electrons fall back to
their ground state or lowest energy level.
0 Energy is then lost by the electron in the form of photons (light)
Hydrogen Spectra
Convergence
Equations
c=ʄλ
E = hf
c = 3 x 108 m s-1
h = 6.63 x 10-34 J.s
h = planks constant
c= speed of light
Energy levels
Ionization energy:
- minimum energy needed to remove
an e- from its ground state
Types of transitions 
Electron Arrangements
0 Need to be able to write electron arrangements for
atoms through Z= 20 (Calcium)
0 Simpler than the method you learned in honors chem!
Energy
level
# e- it can
hold
n= 1
n= 2
n= 3
n= 4
2
8
8
8
Z= 20 Ca
e- arrangement:
2, 8, 8, 2
Energy levels are separated
by commas & # of e- are
shown
Z= 12 Mg
e- arrangement:
2, 8, 2
Z= 5 B
e- arrangement:
2, 3
Z= 1 H
e- arrangement:
1
e- arrangements
0 Valence electrons  electrons in outermost energy level
0 Easiest to remove = lowest ionization energy
Z= 5 B
e- arrangement:
2, 3
3 valence e-
Z= 12 Mg
e- arrangement:
2, 8, 2
2 valence e-
Z= 20 Ca
e- arrangement:
2, 8, 8, 2
2 valence e-
Z= 1 H
e- arrangement:
1
1 valence e-