Nuclear ChemistryDHS Chemistry

Chapters 4 and 25

Little House on the Prarie

Review: Structure of an Atom

Structure of an Atom

An atom consists of three types of subatomic particles, protons, neutrons, and electrons.

Structure of an Atom

• ______ and _______ are located in the nucleus

• Electrons are in a cloud surrounding the nucleus.

• The number of protons is equal to the ____________.

Protons neutrons

atomic number

Structure of an Atom

If the atom is neutral, the number of electrons is equal to the number of ______.

The mass number is equal to number of protons + number of neutrons.

protons

Mass # = p+ + no

Structure of an Atom

• Elements with the same number of protons, but have different numbers of neutrons is called an ______. isotope

Structure of an Atom

Protons Neutrons Electrons

Symbol P+ no e-

Charge + 0 -Location Nucleus Nucleus

Electron cloud

Relative mass

1 amu 1 amu negligible

What’s a chemical reaction?

Chemical Reactions

Why Call it Nuclear Chemistry?

See unitedstreaming.com

Chemical VS. Nuclear • Nuclear reactions are caused from

unstable nuclei becoming stable through radioactive decay.

•Releasing particles and high-energy waves

•Alters the number of nuclear particles (neutrons and protons).

•Nuclear reactions are very different from chemical reactions.

Chemical Reactions

1. Occur when bonds are broken and formed.

2. Atoms remain unchanged, though they may be rearranged.

3. Involve only valence electrons.4. Associated with small energy

changes.5. Reaction rate is influenced by

temperature, pressure, concentration, and catalyst.

Nuclear Reactions

1. Occur when nuclei emit particles and/or rays.

2. Atoms are often converted into atoms of another element. Thus their identity changes.

3. May involve protons, neutrons, and electrons.

4. Associated with large energy changes.5. Reaction rate is not normally affected

by temperature, pressure, or catalysts.

What is an isotope?

Isotope Examples

7

3 Li

7

3 Li

6

3 Li

6

3 Li

Lithium - 6 Lithium - 7

Mass Numbers

Atomic # = protons

Isotopes

• Isotopes are atoms of the same element that vary in their number of neutrons, thus they have different mass numbers.

•The convention for writing isotope names is first the element name dash and then the mass number.

For example: Carbon-14, carbon-12

carbon-13How many protons and neutrons does each of

the isotopes of carbon have?

• Carbon – 148 neutrons, 6 protons• Carbon – 126 neutrons, 6 protons• Carbon – 137 neutrons, 6 protons

Mass Number

(protons + neutrons)

More on Isotopes•When an isotope is unstable it is called a radioisotope.

•To gain a more stable configuration, the nuclei emit radiation.

•The resulting stable atom is called the daughter product.

•This is called radioactive decay.

Strong Nuclear Force• The dense nucleus has two different

kinds of nuclear particles (protons + neutrons) closed packed together.

•The protons are positively charged nucleons.

•The neutrons are neutral nucleons.•All of the protons in the nucleus repel each other and cause an electrostatic force that pushes the nucleus apart.

•However, there is a force holding the nucleus together.

•The strong nuclear force is a force that acts only on subatomic particles that are extremely close together.

•If the strong nuclear force overcomes the electrostatic force, the nucleus stays together.

Neutron to Proton Ratio

• The strong nuclear force is not always strong enough to overcome the electrostatic force. When this happens the nucleus breaks apart. The stability of the nucleus can be determined by the ratio of neutrons to protons.(n/p)

Neutron to Proton Ratio• Elements with low atomic numbers

are most stable when the neutron to proton ratio is 1:1.

• As the atomic number increases, more and more neutrons are needed to overcome the electrostatic force.• Thus, the stable ratio of neutron to

proton increases as the atomic number increases.

• 1.5:1 is the largest ratio for a stable nucleus.

• The graph above plots the neutrons versus protons. The band created is called the band of stability. Anything that falls outside of that band is radioactive.

• All elements with an atomic number greater than 83 are radioactive.

The Band of Stability

Nuclear StabilityNeutron : Protons

Stable if: • Smaller than Fe

~1 1• Between Fe & Pb

1-1.5 1• Bigger than Pb

1.5 1

Extra Practice

• Calculate the neutron to proton ratio, and determine if the isotope is stable.

1.

2. U- 293

2:2 1:1

201:92 2.18:1

stable

unstable

Example: Determine the neutron to proton ratio for

Lead-206. Is it stable?

Lead – 20682 protons

124 neutrons (206 – 82)

124 = 1.51 stable 82 1 ratio

III. Radioactivity

Geiger Counter

Radioactive Substances

• Bananas• Atomic fire ball candies• Colored gemstones (blue

topaz)• Fiesta Ware

• Table ware that contained unsafe amounts of radioactive Uranium

• Uranium for color glaze. Up to 14% can be Uranium

Types of Radiation

Types of Decay

• Nuclear reactions change an atom of one element to an atom of another element. This process is called transmutation. In a nuclear reaction there are three common types of radiation that are emitted: alpha, beta, and gamma. The first two are involved in transmutation, changing the identity of the atom.

Properties of Alpha, Beta, and Gamma Radiation

Property Alpha(α) Beta (β) Gamma (γ)

CompositionAlpha

particlesBeta

particles

High-energy electromagneti

c radiation

Description of Radiation

Helium Nuclei

Electrons Photons

Charge 2+ 1- 0

Relative Mass heaviest lightest 0

Relative Penetrating

Power

Blocked by paper

Blocked by metal foil

Not completely blocked by lead

or concrete

The experiment

• Emit means to give off or release

Alpha Particles• Alpha particles contain the same

composition as a helium nucleus. Out of all of the radiation particles, alpha particles move the slowest and are the least penetrating. As a result of alpha decay, the mass number decreases by 4 and the atomic number decreases by 2.

He42 42or

Alpha decay

Beta Particles• Beta particles are similar to an electron

except they come from the unstable nucleus of the atom. Beta particles are formed and ejected when a neutron decays to a proton and an electron. The proton stays in the nucleus and the electron is the beta particle. Beta emission is a constant flow of quick moving electrons that can be stopped by a metal foil. As a result of beta decay the atomic number increases by one. The mass number does not change.

• Gamma rays are short wavelengths (photons) that move the quickest of all the types of decay, and the most harmful! They are very high-energy electromagnetic radiation. These rays often are released at the same time as an alpha or beta particle. The ray is the energy lost in the reaction. Gamma emission does not affect the atomic number or the mass number of the isotope.

Gamma Rays

Gamma Rays

Are very dangerous!!

(write that down)

C. Gamma ( ) Emission

• usually occurs along with other forms of radiation.

• Gamma particle is emitted• No change in mass number• No change in atomic number

Practice Problems

1. Why is radiation given off?

2. What is the most penetrating particle?

3. What is a main difference between a nuclear reaction and a chemical reaction?

4. Is Carbon-14 radioactive? Why or Why not?

III. Balancing Nuclear Reactions

Balancing Nuclear Reactions

• In nuclear reaction equations we account for all of the changes in the mass number and atomic mass that occur through the decay of the nucleus.

• To verify this, we include the mass number and atomic mass of every particle or atom involved in the reaction.

Mass number

Atomic number

24He

Mass number

Atomic number

10

Solving Problems• When solving/balancing a nuclear

reaction, •Look to find the difference of the mass numbers and atomic numbers between the reactants and the products.

•This will indicate the particle that was released or the atom that was formed.

•Make sure you have the same total mass number and atomic number on both sides of the equation.

Nuclear Equations

Nuclear Reactions

Practice

Fill in the blank with the proper radiation particle or isotope

1)

2)

3)

4)

120 55

Cs 12056 Ba + ____

120 55

Cs 12056 Ba + ____

15064Gd 146

62 Sm + _____

15064Gd 146

62 Sm + _____

24195 Am _____ + 4

2 He

24195 Am _____ + 4

2 He

_____ 21081 Tl + 4

2 He

_____ 21081 Tl + 4

2 He

Ex 1: Write a balanced equation for the alpha decay of polonium-210

210

84 Po 206

82 Pb + 4

2 He

210

84 Po 206

82 Pb + 4

2 He

Ex 2: Write a balanced equation for the beta decay

of carbon-14

Examples

• Ex 1. Write an equation for the alpha decay of Protactinium-231

• Ex. 2

83211Bi ____ 81

207Tl

231Pa91 4He2 + 227 Ac89

4He2

1.

2.

eH 01

31 ___

_____Pr14459

14458 Ce

3He2

0e-1

B. Carbon Dating

Carbon Dating• Carbon dating is a specific type of

radiochemical dating• All living organisms have the same

ratio of carbon-14, carbon-13, and carbon-12 as the in atmosphere.

• However, once an organism dies and there is no new carbon intake, the unstable carbon-14 starts to break down.

More on Carbon dating• Scientists calculate the ratio of

carbon-12 and carbon-13 to carbon-14 in dead organisms.

• Then they compare the ratio to the atmospheres ratio and determine how many half-lives have passed.

• With that information they can calculate how long the organism has been dead.

IV. Nuclear Energy

Radioactive Decay Rates

• We measure radioactive decay in terms of half- lives.

• A half life is the time it takes for half of a radioactive sample to decay.

A. Fission

Fission

• Fission means to break apart. Nuclear fission occurs when a nucleus splits apart into different fragments.

• This generally occurs with atoms that have a mass number heavier than 60.

• The nuclei do not always split the same way. Scientists have found 200 different products from the fission reaction of Uranium-235.

More on Fission• Another important factor of fission

reactions is that they cause a chain of reactions.

• The products of the initial reaction can collide with other molecules and cause a new fission reaction to occur.

• This domino affect could go on for a long time. This is how an atomic bomb works.

Fission Reaction

Nuclear Power• Nuclear power plants harness the

energy released in fission reactions and turn it into electricity.

• One of the main issues the power plant has to deal with is keeping the chain reactions going, but not letting them speed out of control. To this date there have been two large nuclear accidents.

Nuclear Power• Nuclear power plants have to be very

precise in their regulations of the reactions. Some of the products of the fission reaction are extremely radioactive.

• To ensure safety of all living things, the waste must be properly stored.

• It can take up to twenty half-lives for such radioactivity to reach levels safe enough for exposure.

• For some waste products this can be thousands of years.

Fusion

Fusion

• Fusion means to come together. • Nuclear fusion is the combining

of atomic nuclei. • In fusion reactions, scientists

bring together nuclei of atoms that have mass numbers less than 60.

Fusion

• This kind of a reaction also releases large amounts of energy.

• It useful to know, that the sun is powered by fusion reactions.