Defining the Atom > A neutron walks into a bar and asks how much for a beer. Bartender replies “For you, no charge”.

NUCLEAR CHEMISTRY (Sections 4.4, 25.1, & 25.2)

 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

REVIEW The number of ______ defines the element. PROTONS (6 protons = carbon) Elements with different numbers of neutrons are called… ISOTOPES

 REVIEW: Isotope Naming Element name – mass number Ex. carbon-12 or neon-22 Using the element symbol:

 Chemical v. Nuclear Reactions In a chemical reaction (CHEMICAL CHANGE), atoms gain, lose or share electrons and a new SUBSTANCE is formed.

 Chemical v. Nuclear Reactions In a nuclear reaction, the nucleus of an atom changes and becomes a new ELEMENT.

 Nucleus Stability What particles are in the nucleus? Protons (+ charge) and neutrons (neutral) STRONG NUCLEAR FORCE holds the neutrons and protons together.

 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 (n 0 )

 Radioactivity No elements have stable nuclei for atomic numbers above ~83 Unstable if too many or too few neutrons

 Radioactivity Marie Curie ( ) 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

 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.

 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

 Radioactivity DEFINITIONS: Radiation - The penetrating rays and particles emitted by a radioactive substance. Radioactive Decay – An unstable nucleus spontaneously loses energy by emitting radiation.

 Radioactivity Three main types of nuclear radiation: Alpha radiation Beta radiation Gamma radiation

 Alpha Radiation Produced in large nuclei

 Beta Radiation 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.

 Gamma Radiation Occurs if nucleus is in an excited state and it releases energy (gamma rays). Usually occurs with alpha or beta decay.

 Summary – 3 Types of Radiation Radiation TypeSymbolMass (amu)Charge 42+ ~0 (very small) 1- 00

 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

 Alpha particles are the least penetrating. Gamma rays are the most penetrating. Radiation

 Damage Caused by 3 Types of Radiation Radiation Type ExposureDamage Stopped by clothing and your outer skin layer. Inhaled or ingested Lung cancer (i.e., radon gas) Can penetrate the first few mm of skin or ingested Burns, tissue damage, radiation sickness Can pass through your body Cancer, radiation sickness, tissue and organ damage

Defining the Atom >

>  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

Defining the Atom >  REVIEW: Isotope Naming Element name – mass number Ex. carbon-12 or neon-22 Using the element symbol:

 Radioactive Decay – Nuclear Equations Nuclear equations written similarly to chemical reactions Mass numbers and atomic numbers are conserved

 Alpha Decay

 Balancing Nuclear Equations

 Beta Decay

 Balancing Nuclear Equations

 Gamma Radiation

 Balancing Nuclear Equations

Defining the Atom > Type of DecayParticle emittedChange in mass #Change in atomic # Alpha Decay Decreases by 4 Decreases by 2 Beta DecayNo changeIncreases by 1 Gamma Emission No change

 Balancing Nuclear Equations

Write a balanced nuclear equation for the alpha decay of americium-241

 Balancing Nuclear Equations Write a balanced nuclear equation for the beta decay of bromine-84

Defining the Atom > Write a balanced nuclear equation for the beta decay of bromine-84

1.Certain elements are radioactive because their atoms have  more neutrons than electrons.  an unstable nucleus.  a large nucleus.  more neutrons than protons.

2.An unstable nucleus releases energy by  emitting radiation.  thermal vibrations.  a chemical reaction.  giving off heat.

3.Which property does NOT describe an alpha particle?  2+ charge  a relatively large mass  a negative charge  low penetrating power

4.When a radioactive nucleus releases a high-speed electron, the process can be described as  oxidation.  alpha emission.  beta emission.  gamma radiation.

 Half-Life A half-life (t 1/2 ): 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.

 Half-Life

Defining the Atom >  Radiochemical dating The radioactive decay of elements can be used to estimate the date of older objects.

Defining the Atom >  Radiochemical dating The decay of the carbon-14 isotope is used to estimate the age of formerly living objects up to 50,000 years old.

Defining the Atom >  Carbon has 3 isotopes: C-12, C-13, C-14  The ratio of C-14:C-12 in any living thing is ~ constant.

Defining the Atom > Cosmic Rays (radiation) Collision with atmosphere (N-14) Forms C-14 C-14 combines with oxygen to form carbon dioxide (CO 2 )

Defining the Atom > Once an organism dies, C-14 decays into N half-life2 half-lives 3 half-lives

Defining the Atom > Time = 0C-14 5,730 years 1 half-life N-14 C-14 11,460 years 2 half-lives N-14 C-14 1/2 3/4

Defining the Atom >  Step 1: determine how many half-lives have passed. 100/2 = 50/2 = 25/2 = half-lives  Step 2: calculate the number of years. 3 x 5,730 years = 17,190 years An artifact is discovered by archaeologists. Analysis reveals that 12.5% of the original carbon-14 remains in the artifact. Using carbon dating, what is the approximate age of the artifact? The half-life of C-14 is 5,730 years.

Defining the Atom >  After about 50,000 years, there is only a very small amount of C-14 left.  Scientists use isotopes with much longer half- lives for older objects:

Defining the Atom >  Tyrannosaurus rex fossil U-238 (older than 50,000 years)  A meteorite that landed at MHS U-238 (older than 50,000 years)  An Egyptian mummy C-14 (less than 50,000 years)  An ancient wooden tool found in the mountains in Peru C-14 (less than 50,000 years) Which radioactive isotope should be used to date these objects: U-238 or C-14?

 Half-Life – Sample Problem