Defining the Atom > A neutron walks into a bar and asks how much for a beer. Bartender replies...
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Defining the Atom >
A neutron walks into a bar and asks how much for a beer. Bartender replies “For you, no charge”.
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NUCLEAR CHEMISTRY(Sections 4.4, 25.1, & 25.2)
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Review:
Atomic Number tells us: The identity of the element; the number
of protons (and number of electrons in a neutral atom)
Atomic Mass tells us: The average mass in amu of all naturally
occuring isotopes
Mass
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REVIEW
• The number of ______ defines the element.
PROTONS (6 protons = carbon)• Elements with different numbers of
neutrons are called…ISOTOPES
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REVIEW: Isotope Naming
• Element name – mass number• Ex. carbon-12 or neon-22
• Using the element symbol:
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Chemical v. Nuclear Reactions• In a chemical reaction (CHEMICAL CHANGE), atoms gain, lose or share electrons and a new SUBSTANCE is formed.
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Chemical v. Nuclear Reactions• In a nuclear reaction, the nucleus of an atom changes and becomes a new ELEMENT.
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Nucleus Stability• What particles are in the nucleus?
• Protons (+ charge) and neutrons (neutral)
• STRONG NUCLEAR FORCE holds the neutrons and protons together.
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Nuclear Reactions• In nuclear reactions, an unstable
nucleus changes (LOSES MASS and ENERGY) to become more stable.
• Stability of a nucleus depends on the ratio of protons
(p+) to neutrons (n0)
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Radioactivity• No elements have stable nuclei for atomic
numbers above ~83• Unstable if too many or too few neutrons
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Radioactivity• Marie Curie (1867-1934) was a
Polish scientist whose research led to many discoveries about radiation and radioactive elements. She won 2 Nobel prizes.
• In 1934 she died from leukemia caused by her long-term exposure to radiation.
• The element polonium that she helped discover is named after her homeland, Poland
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Radioactivity
• Marie Curie showed that rays emitted by uranium atoms caused fogging in photographic plates.
• Marie Curie named the process by which materials give off such rays radioactivity.
• The penetrating rays and particles emitted by a radioactive source are called radiation.
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Radioactivity
DEFINITIONS:• Radioactivity – the process
by which some substances spontaneously emit energy called radiation. Spontaneously: on its own; no
energy is required to start it
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Radioactivity
DEFINITIONS:• Radiation - The penetrating
rays and particles emitted by a radioactive substance.
• Radioactive Decay – An unstable nucleus spontaneously loses energy by emitting radiation.
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Radioactivity
Three main types of nuclear radiation:• Alpha radiation• Beta radiation • Gamma radiation
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Alpha Radiation
• Alpha particles released• Contains 2 protons and 2 neutrons –
(helium nucleus no electrons)
• symbol: positive charge (2+)
Produced in large nuclei
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Beta Radiation• Beta particle is released.
• A high speed electron leaves the nucleus
• or negative charge (1-)
Occurs in elements with more neutrons than protons: a neutron splits into a proton and electron. The proton stays in the nucleus and the electron is emitted.
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Gamma Radiation• Gamma ray is released.
• High energy radiation• zero charge
Occurs if nucleus is in an excited state and it releases energy (gamma rays).Usually occurs with alpha or beta decay.
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Summary – 3 Types of Radiation
Radiation Type Symbol Mass (amu)
Charge
Alpha 4 2+
Beta ~0 (very small) 1-
Gamma 0 0
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Alpha particles can be stopped by paper. Beta particles can be stopped by wood Most (but not all) Gamma rays can be stopped by
lead or thick concrete.
Radiation
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Alpha particles are the least penetrating. Gamma rays are the most penetrating.
Radiation
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Damage Caused by 3 Types of Radiation
Radiation Type
Exposure Damage
Alpha Stopped by clothing and your outer skin layer.Inhaled or ingested
Lung cancer (i.e., radon gas)
Beta Can penetrate the first few
mm of skin or ingested
Burns, tissue damage, radiation sickness
Gamma Can pass through your
body
Cancer, radiation sickness, tissue
and organ damage
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Radioactive Decay – Nuclear Equations• Nuclear equations written similarly to chemical reactions
• Mass numbers and atomic numbers are conserved
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Alpha Decay• Alpha particles 2+ charge
• Mass number: 238 = 234 + 4• Atomic number: 92 = 90 + 2
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Balancing Nuclear Equations
+ ___• Mass number: 230 = 226 + x
x = 230-226 = 4• Atomic number: 90 = 88 + y
y = 90-88 = 2
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Beta Decay• Beta particles 1- charge
• Mass number: 228= 228 + 0• Atomic number: 88 = 89 + (-1)
+ ___
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Balancing Nuclear Equations
+ ___• Mass number: 239 = 239 + x
x = 239-239 = 0• Atomic number: 93 = 94 + y
y = 93-94 = -1
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Gamma Radiation• Gamma ray zero charge
• Mass number: 230 = 226 + 4• Atomic number: 90 = 88 + 2
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Balancing Nuclear Equations
+ ___• Mass number: 137 = 137 + x
x = 137-137 = 0• Atomic number: 56 = 56 + y
y = 56-56 = 0
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CHECK
1. Certain elements are radioactive because their atoms have
more neutrons than electrons. an unstable nucleus. a large nucleus. more neutrons than protons.
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CHECK
2. An unstable nucleus releases energy by emitting radiation. thermal vibrations. a chemical reaction. giving off heat.
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CHECK
3. Which property does NOT describe an alpha particle?
2+ charge a relatively large mass a negative charge low penetrating power
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CHECK
4. When a radioactive nucleus releases a high-speed electron, the process can be described as
oxidation. alpha emission. beta emission. gamma radiation.
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Half-LifeA half-life (t1/2): the time required for one-half of the nuclei of a radioactive sample to decay.
• After each half-life, half of the atoms have decayed into atoms of a new element.
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Half-Life
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Half-Life
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Half-Life
The ratio of Carbon-14 to stable carbon in the remains of an organism changes in a predictable way that enables the archaeologist to obtain an estimate of its age.
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Half-Life – Sample ProblemCarbon-14 emits beta radiation and decays with a half-life of 5730 years. If you start with a mass of 20,000 g of carbon-14, how long is 3 half-lives? How much mass remains?
3 half lives = 3 x 5730 yrs = 17,190 yrsRemaining mass = 20,000 g x x x = 2500 g
OR: 20,000 g10,000 g 1 half life
5,000 g 2 half lives2,500 g 3 half lives
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Half-Life – Sample ProblemIron-59 has a half-life of 44.5 days. How much of a 200 mg sample will remain after 133.5 days?
Number of half lives: 133.5 days/44.5 days = 3 half lives
Remaining mass = 200. mg x x x = 25.0 mgOR: 200. mg
100. mg 1 half life50.0 g 2 half lives
25.0 g 3 half lives