Electrons

nucleus

We don’t care about the electrons right now…

Remember the easy stuff!!

What particles make up the nucleus?

protons

neutrons

Both protons and neutrons are called NUCLEONS

Recall some stuff about the nucleus:

Notation:

238U92

Atomic number

Mass number

element

Isotopes: nuclei with the same number of protons, but not necessarily the same number of neutrons

Isotopes

• Isotopes have the same number of protons, different number of neutrons

• Another way to show an isotope is to have the mass number follow the name of the element (Carbon-14)

12C and 13C differ in their (a) number of neutrons. (b) number of protons.

(c) number of electrons in orbit around the nucleus. (d) chemical behavior.

(e) all of the above.

What is the charge on a proton? The neutron?

What is the nature of the force between the nucleons?

How does the nucleus hold itself together??!!

The Nucleus is a delicate balance between

[1] The attractive nuclear strong force between all of the nucleons, and

[2] The repulsive electromagnetic force between all of the protons

It turns out that our delicate balance of forces tends to break down when nuclei get large:

The electromagnetic forces eventually overcome the nuclear ones and the nucleus spontaneously breaks apart

This “instability” is called radioactivity:

All nuclei having atomic number larger than 83 (bismuth) are radioactive.

Radioactivity

• Radioactivity is the process in which an unstable atomic nucleus emits charged particles and energy

• Any atom containing an unstable nucleus is called a radioactive isotope or radioisotope for short

•As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger.

•The heavier the nucleus, the more neutrons are required for stability.

•The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass.

Ratio of neutrons to protons in nucleus determines if nuclei are stable

Too many neutrons

Too few neutrons

nuclei with >than 83 protons all unstable

Radioactive decay is how nuclei get to belt of stability

sometimes an unstable nucleus goes through a series of nuclear reactions before it gains stability. This sequence is called a

radioactive series

3 Types of Spontaneous Decay:

1. Alpha particle emission

2. Beta particle emission

3. Gamma ray emissionHigh energy photon (light)

A little bit of History

Lead block

Uranium

Gold Foil

Florescent Screen

Remember this?

This led to the discovery of the nucleus…

Ernest Rutherford’s Other Experiment:

• No study on radiation is complete without giving credit to the Curies (Marie, Pierre and Irene)-

• Their discoveries include radium, polonium and positron.

• Marie-died from Overexposure To radiation

Pierre Curie (1859-1906)

Marie Curie (1867-1934)

Types of Nuclear Radiation

1. Alpha 2.Beta 3. Gamma

alpha particles

and alpha decay

What is an alpha-particle?

It has two protons and two neutrons

4He 2

What is alpha-decay?

It is when a nucleus emits an alpha particle

4He 2

Alpha Decay = Alpha Particle Emission

Nuclear Equations -represent nuclear decay to become stable - they must balance.

1) mass #

2)atomic #

The total number of p+and n0 before a nuclear reaction must be the same as the total number of nucleons after reaction.

1. Alpha emission

Nuclides that undergo alpha decay have too many protons for stability

( >83 protons and mass > 209 )

Alpha Particles ()- They have a mass of 4 and charge of +2

Radium

Ra226

88

88 protons138 neutrons

Radon

Rn222

86

Remember: This is themass number, whichis the number ofprotons plus neutrons

86 protons136 neutrons

+ nnp

p

He)

2 protons2 neutrons

•The alpha-particle is a Helium nucleus.

•It’s the same as the element Helium, with the electrons stripped off !•The alpha particles go in the air collide with air molecules and become Helium – not dangerous.

Example: Write the nuclear equation for the alpha decay of uranium 238:

238U92

234X90

+ 4He 2

Now look at the chart to find this one:

THORIUM

Nuclear Equation Practice:Alpha

DecayHow would alpha decay equations be

written for these atoms:

• 23892 U

• 20984 Po

beta particles

andbeta decay

Beta Decay

Electron!

14C6

14N7

+ electron (beta)

2.Beta emission

A neutron turns into a proton and an electron.

200link

The beta particle is the electron,

written as 0-1 e or 0

-1

Beta Particles ()- are electrons, “no” mass and charge of –1. The decay of Carbon 14

CarbonC14

6

6 protons8 neutrons

NitrogenN14

7

7 protons7 neutrons

+ e-

0Electron -1

(beta-particle)We see that one of the neutrons from the C14

nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus.

In symbolic notation, the following process occurred:

n p + e

Alpha decay of 222Rn:

Beta decay of 3H:

Beta decay of 90Sr

Write out the nuclear equations in each case and identify the daughter nuclei:

86

1

38

Beta Decay

• A beta particle is an electron emitted by an unstable nucleus

• Beta particles can be stopped by a thin sheet of metal such as aluminum

Nuclear Reaction Equation Practice:Beta

Decay

• A beta particle is written 0-1 e or 0

-1

• During beta decay, the mass remains the same and the atomic number increases by one

•21482 Pb

•21884 Po

3890Sr is a radioactive isotope that decays by beta-decay. Its

daughter nucleus is (a) 36

86Kr. (b) 37

88Rb. (c) 37

89Rb. (d) 39

89Y. (e) 39

90Y.

gamma rays

andgamma decay

Gamma decay occurs because the nucleus is at too high an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle. Here's a diagram of gamma decay with helium-3:

Gamma decay is only energy emission, not particle emission.

NeonNe20

10 protons10 neutrons

(in excited state)

10 protons10 neutrons

(lowest energy state)

+

gamma

NeonNe20

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum. It has neither Charge nor mass. They are highly penetrating.

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum. It has neither Charge nor mass. They are highly penetrating.

Gamma particles ()

In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

Gamma decay

• A gamma ray is a penetrating ray of energy emitted by an unstable nucleus

• Gamma rays are energy waves that travel through space at the speed of light

Gamma decay•During gamma decay, the

atomic number and mass remain the same, but the energy of the nucleus decreases

•Gamma rays can be stopped by several centimeters of lead or by several meters of concrete

Other TypesThere are other types of

radioactive decay we will not go into:

- positron emission- neutron emission

Measuring Radiation

The unit of radiation exposure is called the

REM (Radiation Equivalent in Man)

This is a direct measure of the number of damaged cells.

The average dose for one year is about 0.3 Rem

Sudden dose of 1000 rems causes death in 30 days!!

Exposure to ionizing radiation:

alpha

beta

gamma

All of these are IONIZING RADIATION

Ionizing radiation causes cell damage that can lead to DNA damage that can lead to CANCER.

Radiation Doses• Normal

– 1 XRay (medical) 0.04 rem = 40 mrem

– CAT Scan 3 rem

– Portland New York 1 mrem (4 hr flight) (0.001 rem)

– Cosmic Rays 30 – 50 mrem/year

– Food (40K, Ra) ~40 mrem/year

Bad News (acute radiation exposures)• 1 rem

• 25-50 rem

• 50-200 rem

• 450 rem

• 1000 rem

• 5000 rem

More cancer ~ 100 extra cases per 106 people (+0.0001 chance)

Lose white blood cells (+ above)

Sick, low white blood cell count, Get Leukemia in 10-30 years, Genetic mutations in progeny.

50% death in 30 days (with “heroic” medical intervention)

Death, 100%

Can’t shoot back

Detecting Radiation

• Devices used to detect radiation include Geiger counters and film badges

The penetrating power of radiation is really a function of mass.-radiation (zero mass) penetrates much further than -radiation, which penetrates much further than -radiation.

Penetrating Radiation

http://library.thinkquest.org/3471/radiation_effects_body.html

“Background” Radiation

• Background radiation is nuclear radiation that occurs naturally in the environment

– Radioisotopes in the air, water, rocks, plants, and animals all contribute to background radiation

– Cosmic rays (streams of charged particles) from outer space that collide with the Earth’s atmosphere also contribute

– Background radiation levels are low enough to be safe

Half-Life

RATES OF NUCLEAR DECAY

• A half-life is the time required for one half of a sample of radioactive sample of a radioisotope to decay

– Unlike chemical reactions, nuclear decay rates are constant regardless of temperature, pressure or surface area

A very important concept in nuclear chemistry and its applications is the idea of

HALF-LIFE

Half-Life: the TIME it takes for half of a given amount of a specific isotope to decay

This depends on the relative stability of the the isotope in question: semi-stable nuclei last longer than very unstable ones.

Example: 14C (half-life 6,000 years).

How much is left after 6,000 years?

1 Kg

12,000 years?

18,000 years?

24,000 years?

30,000 years?

36,000 years?

Example: 14C (half-life 6,000 years).

How much is left after 6,000 years?

1 Kg

12,000 years?

18,000 years?

24,000 years?

30,000 years?

36,000 years?

There must be an easier way!

We divided by 2 six times

We can do the process in reverse to see how much time it took a substance to decay:

Radioactive Dating:

Applications

Let’s meet some of the unstable isotopes in our neighborhood:• Radon -222

– Forms as part of the Uranium Series– Decomposition of Uranium in many rocks-

granite– Migrates into basement cracks– Dense and collects in lower part of house– Readily inhaled– Its decomposition product is polonium, if it

decays in the lung it emits an alpha particle– The polonium solid decays resulting in lung

disease

Radium 226• 1st radioactive element associated with

biological damage• Some are phosphorescent (glow)• Believed to cause the reaction that

caused leukemia that killed Marie CurrieUranium-238• Used to estimate age of earth• We use its half life to estimated how

long it has been since the rock solidified.

Potassium 40• Light element (one of the few)• Emits positron to form Ar-40• Most of argon in atmosphere is Ar-

40

Artificial Radioactive isotopes-Transmutation

• Artificial radioactive Isotopes are produced by bombarding a target element with nuclei of other elements

• Carried out in cyclotron

• Many new and useful Isotopes have been created.

Practical uses of RadioisotopesH-3 Tritium Archaeological datingAm-241 Americium-241 Smoke detectorsCo-60 Cobalt-60 Cancer treatmentCr-51 Chromium-51 Determination of

blood volumeU-235- Uranium-235 Nuclear reactors and weaponsU-238 Uranium-238 Archaeological

dating

Artificial Radioactive isotopes may be used as tracer materials.

Example: We might like to know how phosphorus in fertilizers is used by plants. By incorporating a small amount of radioactive Phosphorus in the fertilizer, chemists can measure the time it takes for plants to utilize the Phosphorus. They also can track where the P goes.

Dating Archeological samples

Example:U-238 decays slowly through the

uranium disintegration series to lead-206. The half-life of this conversion is 4.5 X 109 years. To determine the age of a rock, we need to determine the amount of uranium present when the rock solidified, No and the amount of uranium in the rock today, Nf

• The amount of U-238 present in the rock today can be measured. However, we can’t calculate the original. The solution lies in the fact that each atom of uranium ends up as an atom of lead. Therefore, if we take a rock sample and determine the number of atoms of U-238 and the number of atoms of Pb-206, we can say that

• No = atoms U-238 + atoms Pb-206• Nt = atoms 238-U• **Errors will exist, the original rock may

have contained some Pb-206 and some of the original U-238 may not end up as Pb-206

Fusion &

Fission

FISSION AND FUSION

• Fission is the splitting of an atomic nucleus into two smaller parts

• Fusion is a process in which the nuclei of two atoms combine to form a larger nucleus

Fission

• A chain reaction is a chain of fission reactions triggered by neutrons released during the fission of a nucleus

• About 20% of the electricity in the US comes from fission reactions

Fission•A

tremendous amount of energy is produced during a fission reaction

Fission

• Advantages to using fission reactions is the lack of air pollution.

• Disadvantages include the risk of exposure and radioactive waste

Nuclear Power PlantFG21_020.JPG

Atomic BombFG21_017.JPG

2 subcritical masses come together when detonated

http://ull.chemistry.uakron.edu/genobc/animations/chain.mov

Fusion

• Fusion reactions can release huge amounts of energy

• Fusion reactions occur in the sun and stars

Nuclear Fission-the process that yields two nuclei of almost equivalent mass.

• It does not occur spontaneously• It is a chain reaction• It is usually used to generate

electricity. Heat is generated in fission reactions.

• Nuclear Fusion- The combination of two nuclei into a larger atom is called nuclear fusion.

• The reactions within the sun are fusion reactions that combine Hydrogen nuclei to form a helium atom

• It takes simple, nonradioactive materials and produces helium, so it should be a clean and inexpensive source of energy.

• Several designs for fusion reactors are being tested. Some success, it will be some time before this cheap, nonpolluting energy source will become a reality.

Nuclear fusion

Tokamak- uses magnetic fields to contain fusion rxn

• Fusion of tritium and deuterium requires about 40,000,000K:

21H + 3

1H 42He + 1

0n

• These temperatures can be achieved in a nuclear bomb or a tokamak.

• A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls. (No known material can survive the temperatures for fusion.)

• To date, about 3,000,000 K has been achieved in a tokamak.

Fusion reaction in sun provides all our energy makes smaller elements

fusion in supernova made elements larger than Fe

Fat ManProduced this cloud after its detonation

at Nagasaki on August 15th, 1945,

killing at least 80,000 civilians

instantly.

140,000 civilians perished in

Hiroshima when Little Boy was

dropped just two weeks earlier.

No attack involving nuclear weapons has

occurred since.

In 1961, when the USSR tested the Hydrogen Bomb “Tsar Bomba”, it was 4000 times more powerful than Little Boy.

Its energy yield was ten times greater then all of the munitions exploded during all of World War 2.

“A 50-mile radius of ground surface has been completely leveled, swept and licked so that it looks like a skating rink ... The same goes for rocks. There is not a trace of unevenness in the ground.... Everything in this area has been swept clean, scoured, melted and blown away."

The USSR concluded that Tsar could not be safely deployed against Western European targets, since the radiation that would have been caused by the blast would spread to the Soviet border. The only cities that would be practical targets for such a device were the New York, Chicago, and Los Angeles metropolitan areas; the use of Tsar was considered overkill for any other target.

The shockwave destroyed buildings and tore roofs off of homes hundreds of miles from ground zero, and windows in Norway and Finland were shattered.

From http://www.atomicforum.org/russia/tsarbomba.html