AtomsChapter 4

Protons

•Positively charged = +1

•Approximately 1 amu (atomic mass unit)

•Located in the nucleus

Neutrons

•Neutrally charged = 0

•Approximately 1 amu

•Located in the nucleus

Electrons

•Negatively charged = -1

•Relatively small mass (0.0005 amu)

•Located in the space surrounding the nucleus

Atoms

•Neutral charge

•Same number of protons as electrons to balance the charge

•Atomic number = # protons and # electrons

•Atomic mass = # protons + # neutrons

Isotopes

•Same number of protons (atomic number the same)

•Different number of neutrons (atomic mass different)

Ions•Same number of

protons (same atomic number)

•Same number of neutrons (same approximate atomic mass)

•Different number of electrons (has a charge)

Regular Average•All samples are weighted equally

•Example : If a student took 5 tests each test is worth 1/5th or 20% of the final grade

• Can multiply each test by 1/5th

•OR add all the tests and divide by 5

Weighted Average•Not all samples are weighted equally

•Example: A student takes 5 tests. The first test is worth 40%, the second is worth 30%, and the other are each worth 10%

•Multiply each test by what it is worth

•Add up

•Atomic weight on periodic table

Radioactive DecayChapter 4 and Chapter 25

Radioactive Decay• The tendency of an element to

spontaneously emit radiation until it forms a stable atom

• Unstable nuclei based on ratio of neutrons to protons

• Often results in the formation of a different element

• Radioisotopes: isotopes of atoms with unstable nuclei

Stability

•Ratio between neutrons:protons

•Closer to 1:1 = more stable (elements of atomic number < 20)

•Closer to 1.5:1 = more stable (elements with very large atomic numbers)

Types of Radioactive Decay:

Alpha Particle• 2p+, 2n

• 2+ charge on particle

• Basically an He atom

• Slow moving and not good at penetrating

• Happens to very large atoms

• Lose both proton and neutrons

Types of Radioactive Decay:

Beta Particles• 1e-

• -1 charge

• Move very fast

• Neutron breaks down to a proton and an electron

• Loses a neutron and gains a proton

• Happens to really large atoms

Types of Radioactive Decay: Gamma

Particles•Mostly energy

•Has 0 mass

•No charge

•Accompanies alpha and beta radiation

•Cannot form new atoms

Types of Radioactive Decay: Positron

Emission• Emission of a positron from the nucleus

• Positron = particle the same mass as an electron but the opposite charge

• Proton converted to neutron and positron

• Loses proton, gains neutron

• Happens to very small atoms

Types of Radioactive Decay: Electron

Capture•Nucleus draws in a surrounding electron

•Combines with a proton to form a neutron

•Loses a proton, gains a neutron

•Happens to small atoms

•Also emits a photon

Nuclear Reactions

•Atomic number and atomic mass are conserved

•Reactants = products

•Problems 69 - 72 p. 837

Transmutation

•The conversion of one atom of an element to another element

•Via nuclear reactions

•Can happen naturally (all elements above #83)

•OR can be induced

Half - Life• Radioactive decay can be

measured in half-lives

• Half-life = time required for one half of a radioisotope’s nuclei to decay

• Amount remaining = initial amount * 0.5^n

• n = the # of half-lives

• n = t/T where t = elapsed time, T = half life

Nuclear Reactions

Nuclear Reactions

•Much more powerful than chemical reactions

•Energy released is greater

•Fission and Fusion

Fission• Heavy, unstable atoms fragment into smaller atoms to

increase their stability

• Initiate by hitting with a neutron

• Smaller products form, extra neutrons

• Extra neutrons can trigger more fission reactions = chain reaction

• Atom must be big enough to initiate and maintain a chain reaction = critical mass

Fusion

•Small atoms bind to create more stable atoms

•Release large amounts of energy

•Take really high heat to initiate and maintain reaction (40 million K)

•Thermonuclear reactions

Nuclear Power Plants• Fission Reactions

• Fuel rods contain large atoms (uranium-235)

• Additional rods contain atoms that can absorb extra neutrons

• Positioning can determine how many neutrons are absorbed and the speed of the rxn

Power Generation

•Fission reactions produce a lot of heat

•Heat absorbed by a cooling system of water

•Used to generate steam that drives turbines to produce power

Problems•Tight balance between out of control

chain reactions and producing adequate power

•Continual adjustment

•Some products are extremely radioactive with long half-lives, waste

•Containment structures to shield radioactivity