nuclear chemistry.doc

7/28/2019 nuclear chemistry.doc

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APC NOTES: Unit 9 NUCLEAR CHEMISTRY A QuickReview

So far this year, we have talked about many chemical processes that involve only

electrons:

o Chemical reactions

o Bond polarity and Molecular polarity

o Molecular geometry

o Bonding

In this unit, we will focus on the spontaneous changes that occur inside the nuclei

of some atoms

o Some spontaneous changes inside the nucleus result in the

emission of particles from the nucleus. These atoms are said to

be radioactive

Some Important Terms

Nucleons - general term for particles in the nucleus (protons and neutrons)

Atomic Number - the number of protons in an atom

o All atoms of the same element have the same atomic number

Mass Number - number of protons + neutrons

Isotopes - atoms of the same element that have different numbers of neutronso So their atomic numbers are the same, but their mass numbers are different!

Radionuclides - nuclei that are radioactive

Radioisotopes - atoms containing radioactive nuclei

Radioactive decay - process whereby unstable nuclei (due to too many protons andneutrons) emits particles spontaneously from the nucleus in order to become more

stable

o Goes from a high energy particle to a lower energy particle

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Types of Radioactive Decay

1. ALPHA EMISSION - nucleus emits an alpha () particle

o An alpha particle is a particle composed of 2 protons and 2 neutrons

In AXZ notation, an alpha particle would be 42So it is the same as a Helium atom 4He2

o Alpha particles are large, slow moving, and easy to stop

They cannot penetrate through a single sheet of paper

o Example

238U92 4He2 + 234Th90

o Example

What product is formed when Ra-226 undergoes decay? 226Ra88 4He2 + 222Rn86

2. BETA EMISSION - nucleus emits a beta (-) particle

o A beta particle is a high speed electron!!!

In AXZ notation, a beta particle would be 0-1

Since it is the same as an electron, it could also be 0e-1

o Beta particles are almost massless, negatively charged, fast moving, and about

100x more penetrating than particlesThey cannot penetrate more than 3mm of aluminum

o Beta particles are formed in the nucleus when a neutron is converted into a

proton and an electronthen the electron is emitted from the nucleus1n0 1p1 + 0e-1

Charge doesnt change because it makes a proton and an electron

o Example

227Ac89 0e-1 + 227Th90Atomic # increases by 1

Remember: ne + p

(then the e is emitted) 3


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3. POSITRON EMISSION - nucleus emits a beta (+) particle

o There are 2 kinds of beta particles:

- which is called a beta particle; 0-1 or 0e-1

+ which is called a positron; 01 or 0e1

o Positrons are almost massless, positively charged particles

o Positron particles are formed in the nucleus when a proton is converted into a

neutron and a positronthen the positron is emitted from the nucleus

1p1 1n0 + 0e1

Positrons are short lived because when they leave the nucleus, they quickly collidewith electrons in the electron cloud to produce gamma rays

0e1 + 0e-1 2 00Therefore, positrons dont really penetrate matter because they dont get a chance

If you lose a proton, why doesnt the charge become negative?

Because when the 0e1 leaves the nucleus, it hits 0e-10e1 + 0e-1 2 00

So you lose a proton and an electron

o Example

11C6 11B5 + 0e1Atomic # decreased by 1

Remember: p n + e(then e is emitted) 4


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4. ELECTRON CAPTURE - nucleus "absorbs" an electron

o During electron capture, an inner shell electron is pulled into the nucleus where it

combines with a proton to form a neutron

0e-1 + 1p1 1n0

So you lose an electron and a proton

o An x-ray is emitted as an electron moves from a higher energy level to a lower

energy level to fill the vacancy left by the captured electron

o Example

87Rb37 + 0e-1 87Kr36Electron written as a reactantAtomic # decreases by 1

No change in mass

5. GAMMA RADIATION - nucleus emits high energy photons (energy!!!)

o Since it is just an emission of energy, there is no change in mass or atomic

number

o Gamma emission accompanies almost every other radioactive emission

o Gamma emission is the most penetrating radiation....10,000X more penetrating

than alpha particles

What Should I Do Now? 1. Read about it more in The Chang 5


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Patterns of Nuclear Stability

The Neutron : Proton ratio

o The nucleus contains all the positively charged protons that should repel each

other, BUT neutrons act as a "glue" to hold them together

o Neutrons exert a force of attraction known as the strong nuclear force (it ismuch stronger than the force of gravity)

o Stable nuclei (up to atomic number 20) contain about equal numbers of protons

and neutrons

Above atomic number 20, more neutrons are required to keep the particles held

together (so the ratio of neutrons to protons goes above 1:1)

The optimum ratio of neutrons to protons is called the "belt of stability"...outside

of this range, atoms are radioactive. Notice how the belt ends at 83 protonsAbove element #83 (bismuth), all nuclei are radioactive...there are so many

protons in the nucleus that no number of neutrons can hold them together due to

the strong positive-positive charge repulsion

*Atoms above the belt of stability (high neutron : proton ratio) emit - particles

1n0 1p1 + 0e-1This will increase the number of protons while decreasing the number of neutrons.

*Atoms below the belt of stability (low neutron : proton ratio) emit + particles or undergo electron capture

1p1 1n0 + 0e1

This will increase the number of neutrons while decreasing the number of protons. 6


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Particle Collisions

Some radioactivity is spontaneous, but some is induced by purposely striking anucleus with another particle (ex. a neutron) and causing the nucleus to change

identity....this is how new elements are made!

Examples 14N7 + 4He2 17O8 + 1H1 27Al13 + 1n0 24Na11 + 4He2 Rate of Radioactive DecayHalf Life = time required for 1/2 of a sample to decay

o Half lives are unpredictable for individual atoms, but work for groups of atoms

Radioactive Decay is a 1st Order kinetic process

Example You have 64 grams of a sample with a half life = 10 years. How much

remains after 40 years?Method #1

Time Amount Left

10yrs 32g20yrs 16g

30yrs 8g

40yrs 4g

Method #3

t1/2 = .693/k

But you have to know the rate constant k

Method #2

Fraction remaining = (1/2)n

Where n = # of half lives 7


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Nuclear Fission

The splitting of heavy nuclei into two lighter ones

An exothermic process (this is an understatement!!!!)

o A small amount of mass gets converted to a large amount of energy as described

by Einsteins equation, E=mc2Used for nuclear power and atomic bombs

Generally accomplished by bombarding heavy nuclei with neutrons

Example 1n0 + 235U92 139Xe54 + 95Sr38 + 21n0 ***It appears that all the mass is

conserved, but remember that protons and neutrons are not exactly equal to1.000000 amu. 235U = 234.9934 amu 139Xe = 138.8891 amu 95Sr = 94.8985 amu 1n = 1.0087amu

Mass of reactants = 236.0021 amu Mass of products = 235.805 amu Mass lost =0.1971 amuIt requires about 3 tons of coal to produce

8 x 107 kJ of energy.

1 gram of fission material can produce about 8 x 107 kJ of energy. 8


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Example Another one of over 200 possible fission reactions of 235U92 1n0 + 235U92

142Ba56 + 91Kr36 + 31n0NOTE: More neutrons are produced in each step. These additional neutrons can then strike other nuclei and

cause a chain reaction of fission as long as there is enough mass of fissionable material present (the minimum

mass needed to sustain a chain reaction is called the critical mass.

http://library.thinkquest.org/17940/texts/fission/fission.htmlExample The atomic bomb dropped on Hiroshima on 8/6/45.What? Why is everybody looking at me like that?

Two subcritical masses of

U-235 were placed in the bomb. When explosives were detonated, the two masses become one mass large

enough to sustain a chain reaction.

Explosives

Subcritical

U-235 wedgeSubcritical

U-235 wedge 9


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Nuclear Fusion

The fusing together of lightweight nuclei

Very exothermic

o This process is used in hydrogen bombs

Happens readily on stars (like our sun)Requires temperature of 40,000,000 Kelvins

"Cold Fusion" is an attempt to fuse nuclei below this temperatureo (It isn't necessarily at a "cold" temperature though!!)

Examples of Fusion 1H1 + 1H1 2H1 + 0e1 *** 1H1 + 2H1 3He2 *** 3He2 + 3He2 4He2 + 21H1

3He2 + 1H1 4He2 + 0e1Time for a sing-a-long...

"The sun is a mass of incandescent gas

A gigantic nuclear furnace

Where hydrogen is turned into heliumAt a temperature of millions of degrees"

A little side note:

We'd like to use fusion as an energy source because...

-lightweight isotopes are readily available

-no radioactive byproducts are producedBut it requires such high energy to overcome the like charge repulsions

-so far we can only achieve this energy requirement on the large scale by initiating it with

an atomic bomb, and I guess its obvious why that's not very practical

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