What will be discussed in this chapter?

fundamental particles of the atom Types of forces holding up the atom

and its particles together Nuclear stability Natural radioactivity and types of

radioactive decay Artificial radioactivity Nuclear energy Health hazards

After 1932, physicists viewed

all matter as consisting of only three constituent particles: electrons, protons, and neutrons.

Beginning in 1945, many new

particles were discovered in experiments involving high-energy collisions between known particles. These new particles are characteristically very unstable, and have a very short half-lives, ranging between 10-6 and 10-23 s. So far, more than 300 of these unstable, temporary particles have been catalogued.

The current theory of elementary particles of atoms, the standard model, claims that all matter is believed to be constructed from only two families of particles:

“QUARKS and LEPTONS”

LEPTONS Leptons (from the Greek word

”leptos” meaning, small or light) are group of particles which participate in the weak interaction.

Included in this group are : “electrons (e), muons (μ), and taus (τ).”

They interact only through weak and electromagnetic forces.

There are six types of leptons. Each lepton has its own

antiparticle. A neutrino is associated with

each lepton. Have elementary structure which

means that they don’t seem to break down into smaller units.

LEPTONS

Particle name

symbol

Anti-particle lifetime(s)

Electron e- e+(positron) StableElectron neutrino

νe e Stable

Muon μ- μ+ Unstable

Nutrino muon

νμ μ Stable

Tau τ- τ+ Unsatble

Neutrino tau

ντ τ stable

TYPES OF LEPTONS

The neutrino tau hasn’t been discovered yet but its presence is believed.

Electron is the lepton having the smallest mass.

QUARKS The unusual property of quarks is that they have fractional electronic charges.

Associated with each quark is an antiquark of opposite charge.

There are six types of quarks : up (u), down (d), strange (s), charmed (c), top (t), and bottom (b).

TYPES OF QUARKSName of the particle

symbol charge

Up u + 2/3

Down d -1/3

Strange s -1/3

Charmed c + 2/3

bottom b -1/3

top t + 2/3

Protons and neutrons are

formed as a combination of different types of quarks.

PROTON-- two up [(+ 2/3) + (+ 2/3) ]

& one down quark (-1/3)

NEUTRON-- two down

[(- 1/3) + (-1/3) ] & one up quark (+2/3)

THE FUNDAMENTAL FORCES IN NATURE The strong nuclear forces The electromagnetic forces The weak nuclear forces The gravitational forceThe strength decreases downward.

THE STRONG NUCELAR FORCES These are the strongest forces holding the quarks in protons and neutrons together.

They have the shortest range (10-15 m), meaning that particles must be extremely close before their effects are felt. 

The quarks are considered to be held together by the color force. The strong force between nucleons may be considered to be a residual color force. 

A property of quarks labeled color is an essential part of the quark model.

It has nothing whatever to do with real color provides distinct quantum states. 

THE STRONG NUCELAR FORCES

Analogous to the exchange of photons in the electromagnetic force between two charged particles, Gluons are the exchange particles for the color force between quarks.

The color force involves the exchange of gluons.

THE STRONG NUCELAR FORCES

THE STRONG NUCELAR FORCES

WEAK NUCLEAR FORCES

It is responsible for the radioactive decay of subatomic particles.

Its strength is about 10-5 times the strong forces.

It’s a short range force. If the nucleon number in a nucleus

isn’t too much, the attraction forces between the nucleons ,the strong nuclear forces, counteract the repulsion forces, the weak nuclear forces, between the protons.

If the nucleon number is too much (Z >83), then the distance between the nucleons will be big and the weak nuclear forces will approach zero (because of the increase in the distance between the nucleons). Therefore, the electrical repulsion forces between the protons will be more effective making the nucleus unstable and the nucleus undergoes radioactive decays (e.g., beta decay).

Nuclear Stability

The stability of an atom is the balance of the repulsive and attractive forces within the nucleus.

Nuclear Stability If the attractive strong forces prevail, the nucleus is stable. Stable nuclei don’t undergo radioactive decay.

If the repulsive weak forces outweigh the attraction forces, the nucleus is unstable and undergoes radioactive reactions spontaneously. Such nuclei are called “radioactive nuclei.”

Think about it…

How might a higher number of neutrons change the balance between the repulsive and attractive forces in a nucleus?

How might a lower number of neutrons affect this same balance?

Light nuclei are most stable if they contain an equal number of protons and neutrons, if N=Z.

Nuclear Stability

Heavy nuclei are more stable if the number of neutrons exceed the number of protons (Remember that , as the number of protons increases, the strength of the Coulomb (repulsion) forces increase, which tends to break the nucleus apart).

As a result, more neutrons are needed to keep the nucleus stable since neutrons experience only the attractive nuclear forces.

Nuclear Stability

Nuclear Stability Therefore, the ratio of n0 /p+

determines the stability of a nucleus.

For the stable nuclei, this ratio is close to “1.”

This ratio is “1” for the atoms with atomic number smaller than 20 (though this has exceptions for some isotopes).

As atomic number increases, stable atoms have ratios greater than one, can reach 1.5.

Elements having more than 83 protons do not have stable nuclei. The isotopes of all of these atoms are radioactive.

Nuclear Stability

Nuclear Stability The

shaded cluster is the “band of stability.”

The stable nuclei are present in the band of stability.

The solid line represents a neutron-to-proton ratio of 1:1.

The nuclei which aren’t in the band of stability are radioactive and try to enter the band of stability as a result of some radioactive decays.

Nuclear Stability

Nuclei to the right of the band of stability don’t have enough neutrons to remain stable.

Nuclei to the left of the band have too many neutrons to remain stable.

Nuclear Stability

n/p <1

n/p >1

Stable Nuclei

Nuclei above this belt have too many neutrons.

They tend to decay by emitting beta particles.

Stable Nuclei Nuclei below the

belt have too many protons.

They tend to become more stable by positron emission or electron capture.

Stable Nuclei There are no stable nuclei with an atomic number greater than 83.

These nuclei tend to decay by alpha emission.

Chemical Reactions vs Nuclear ReactionsChemical Reactions Nuclear ReactionsElectrons react outside nucleus.

Protons and neutrons react inside nucleus.

The same number of each kind of atom appear in the reactants and products.

Elements transmute into other kind of elements.

Isotopes react the same. Isotopes react differently.

Mass reactants = mass products.

Mass changes are detectable.

Energy changes equal ~103 kJ.

Energy changes equal ~108 kJ/mol.

Rate of reaction is affected by temperature, pressure, concentration and use of a catalyst

Rate of reaction is not affected by temperature, pressure, concentration and use of a catalyst

Radioactive nuclei are generally classified into two groups:

1) unstable nuclei found in nature, which give rise to what is called “natural radioactivity.”

2) nuclei produced in the laboratory through nuclear reactions, which exhibit “artificial radioactivity.”

NATURAL RADIOACTIVITY In natural radioactivity, the unstable

nuclei undergo the radioactive reactions spontaneously until they reach a stable configuration.

These transformations are accompanied by releases of energy.

These radiations are• alpha radiation(ışıma)• beta radiation• positron emission(yayma)• electron capture

TYPES OF NATURAL RADIOACTIVE DECAYS

1. Alpha Decay (2p+,2no)

Nucleus emits an alpha particle—two protons and two neutrons

• Alpha particle is a helium nucleus.

U23892

Th23490 He2+4

2 +

42He2+

1. Alpha Decay

For an atom which emits rays;

p+ number decreases by two,

Atomic number decreases by two,

n0 number decreases by two,

Mass number decreases by four.

α Th U 42

23490

23892

1. Alpha Decay

Properties of α- raysThey have a fogging effect on the photographic films.Charge: α - particle carry positive charge. Its nuclear

charge is +2.Mass: Mass of each α – particle is 4 times that of a

proton or H-atom.    Penetration power: α - rays have very small

penetration power .They are stopped by a sheet of paper.

Effect of human body: α - rays can be stopped by the skin but very damaging to the skin due to ionization power.

Artificial radioactivity: α - rays can produce artificial radioactivity in certain nuclei.

Ionization capability : They have strong ionizing power because they remove electrons from the atoms of gas through which they pass. With gained electrons, they become He gas.

Velocity: Their velocity range is 3 x 107 m/s to 3 x 106 m/s.

They deflect towards the (-) side in the magnetic field.

(-) end

(+) end

While rays are travelling in an magnetic field,they are deflected towards the (-) end of the field.

2. Beta Decay

• Beta decay is loss of a -particle.

• -particle is an electron having high speed.

0−1 e0−1or

I13153 Xe131

54 + e0−1

n10 p1

1 + e 0- 1

a neutron is converted to a proton.

-

For an atom which emits - rays;*P+ number increases by 1*Atomic number increases by 1*no number decreases by 1*Mass number doesn’t change

2. Beta Decay

ePaTh 01

23491

23490

Properties of β- raysNature: β - rays consist of fast moving electrons.

Charge: β – rays have negative charge. That is - 1.Penatrating power: β – rays have 100 times

greater penatrating power than alpha rays. They are

stopped by a thin sheet of any metal. For example; aluminum

Velocity: Their velocity range is 9 x 107 m/sec to 27 x 107 m/sec. Ionization power: Ionization power of β - rays is

very small. They have a fogging effect on the photographic film.β – rays deflect towards the (+) side in the magnetic

field.

3. Gamma emission (γ) Nuclei seeking lower energy states emit

electromagnetic radiation, which is in the gamma ray region of the electromagnetic spectrum.

Rays are emitted in conjunction with another type of decay (alpha or beta).

An atom sometimes may remain in an excited nuclear state after radioactive decays. decay removes the excess energy and leaves back the atom in the ground state.

Gamma Decay http://ie.lbl.gov/education/glossary/AnimatedDecays/GammaDecay.html

Additional animations: http://ie.lbl.gov/education/glossary/Glossary.htm

00

3. Gamma emission (γ)

UU

UPu23692

*23692

*23692

24094

Properties of γ - raysNature: γ - rays are not particles. They

are electromagnetic radiations with high energy.

Charge: γ – rays have no charge. They are neutral. They don’t deflect in the magnetic field.

Mass: γ – rays have no mass.Velocity: γ - rays travel with the velocity

of light that is 3 x 108 m/sec.Penetration power: Penetration power of

γ – rays is very large. It is about hundred times  larger  than that of β -rays. They are stopped by a thick layer of lead.

WARNING!!!!!If an atom emits number of -s which are twice the number of s,then the isotope of the atom is formed.

01

23492

23892 2 UU

FAJANS RULE:If an atom emits an , it forms an atom that is 2 behind than the atom which emitted the in the periodic table.

If an atom emits a ,it forms an atom which is 1 forward than the atom that emitted the in the periodic table .

4. Positron Emission• It is the loss of a positron.

• Positron is positive electron.

• It is a particle having the same mass as but opposite charge of an electron.

e0+1

C116

B115

+ e01Positron is formed when a proton converts to a neutron.

6. Electron Capture • Addition of an electron to a

proton in the nucleus.• As a result, a proton is

transformed into a neutron.

• Since the nucleus captures an orbital electron from K shell, it is a natural radioactivity.

p11 + e0−1

n10

Be74 e 0

- 1 Li7

3+

AueHg 19779

01

19780

6. Electron Capture

Happens to nuclei with a low neutron:proton ratioA proton becomes a neutron causing a shift up and to the left.  Always results in gamma radiation

5. Neutron Capture

• During neutron capture, isotope of parent nucleus is produced.

This is not a natural radioactive event.

n10

Kr8736

Kr8636n1

0+ + γ

 Which one(s) of the following statements is/are true for atom having following reaction in its nucleus ?

11p → 0

1n + +10β

I. Its mass number increases by 1.II. Its isotope is formed.III. Its netron number decreases by 1.IV. Its atomic number decreases by 1.V. Its number of protons increases by 1.

Solution:In the reaction given above, one proton is converted into one neutron.Thus, atomic number decreases by 1.

So, IV is true

EXAMPLE: 1

EXAMPLE: 2Find X and Y in following reactions.I.  19

38K → 1838Ar + X

II. 80197Hg + Y → 79

197Au

Solution: X is  +1

0β I.

II.

Y is  -10β

EXAMPLE: 3 Find atomic number and mass number of Y in the following reaction.

92234X + β-  + α → Y + γ + 2β+

Solution:

91238Y

EXAMPLE: 4  A, B, C and D elements form compounds AC, A2D and BD. If AC and A2D are radioactive and BD is not radioactive compound, find whether the following compounds are radioactive or not. I. A2

II. A2CIII. C2DIV. BC

Solution:A2   and A2C are radioactive compounds.

C2D and BC are radioactive or not.

  For example 92U238 will go through 8 alpha emissions and 6 beta emissions (not all in order) before becoming 82Pb206

The steps a nuclei follows in becoming stable is called a radioactive series.  The series for 92U238 is shown below as an example.

• Large radioactive nuclei can not stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).