1 nuclear chemistry why do some atoms undergo nuclear changes?

1

NUCLEAR CHEMISTRY

Why do some atoms undergo nuclear

changes?

2ATOMIC COMPOSITIONATOMIC COMPOSITION

• ProtonsProtons– + electrical charge+ electrical charge– mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g– relative mass = 1.007 atomic mass units relative mass = 1.007 atomic mass units

(amu)(amu)• ElectronsElectrons

– negative electrical chargenegative electrical charge– relative mass = 0.0005 amurelative mass = 0.0005 amu

• NeutronsNeutrons– no electrical chargeno electrical charge– mass = 1.009 amumass = 1.009 amu

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RadioactivityRadioactivity• One of the pieces of evidence for the One of the pieces of evidence for the

fact that atoms are made of smaller fact that atoms are made of smaller particles came from the work of particles came from the work of

Marie CurieMarie Curie (1876-1934). (1876-1934). • She discovered She discovered radioactivityradioactivity, ,

the spontaneous disintegration of the spontaneous disintegration of some elements into smaller pieces.some elements into smaller pieces.

4

RadioactivityRadioactivity Emission of particles and/or

energy due to a change in the nucleus of an atom.

Nuclear Radiation also called Ionizing radiation

Measure with Geiger Counter

5Geiger Counter: Geiger Counter:

Radiation detectionRadiation detection

6IsotopesIsotopes• Hydrogen:

– 11H, protium

– 21H, deuterium

– 31H, tritium

(radioactive)

• Helium, 42He

• Lithium, 63Li

and 73Li

• Boron, 105B

and 115B

• Except for 11H the mass

number is always at least 2

x atomic number.

• Repulsive forces between

protons must be

moderated by neutrons.

7IsotopesIsotopes

• Same element (Same element (ZZ) but different mass ) but different mass number (number (AA).).

• Boron-10Boron-10 has has 5 p5 p and 5 n: and 5 n: 101055BB

• Boron-11Boron-11 has has 5 p5 p and 6 n: and 6 n: 111155BB

10B

11B

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Why do unstable isotopes undergo nuclear reactions?

Unstable isotopes undergo nuclear reactions so that they may be changed, or

transformed, into stable isotopes.

9Stability Stability of of NucleiNuclei

• Heaviest naturally Heaviest naturally

occurring non-radioactive occurring non-radioactive

isotope is isotope is 209209Bi with 83 Bi with 83

protons and 126 neutronsprotons and 126 neutrons

• There are 83 x 126 = There are 83 x 126 =

10,458 possible isotopes. 10,458 possible isotopes.

Why so few actually exist?Why so few actually exist?

10Band of Stability Band of Stability and Radioactive and Radioactive DecayDecay

Isotopes with a n/p ratio, below or

above the band of stability decay, by

various nuclear reactions.

11Stability Stability of of NucleiNuclei

• Out of > 300 stable isotopes:

EvenEven OddOdd

OddOdd

EvenEven

ZZNN

157157 5252

5050 55

31311515PP

191999FF

2211H, H, 66

33Li, Li, 101055B, B, 1414

77N, N, 1801807373TaTa

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Types of RadiationTypes of Radiation

Types of Radiation

What are three types of nuclear radiation?

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Types of RadiationTypes of Radiation

Radiation is emitted during radioactive decay.

Three types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.

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Types of NUCLEAR Types of NUCLEAR RadiationRadiation

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Types of RadiationTypes of RadiationAlpha Radiation

Some radioactive sources emit helium nuclei, which are also called alpha particles.

• Each alpha particle contains two protons and two neutrons and has a double positive charge.

• An alpha particle is written He or .42

– The electric charge is usually omitted.

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Alpha Radiation

The radioisotope uranium-238 emits alpha radiation and is transformed into another

radioisotope, thorium-234.

U23892

Uranium-238

Th +23490

Thorium-234

He ( emission)42

Alpha particle

Radioactivedecay

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Alpha Radiation

When an atom loses an alpha particle, the atomic number of the product is lowered by two and its mass number is lowered by four.

• In a balanced nuclear equation, the sum of the mass numbers (superscripts) on the right must equal the sum on the left.

• The same is true for the atomic numbers (subscripts).

U23892 Th +234

90 He42→

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Alpha Radiation

Because of their large mass and charge, alpha particles do not travel very far and are not

very penetrating.

• A sheet of paper or the surface of your skin can stop them.

– But radioisotopes that emit alpha particles can cause harm when ingested.

– Once inside the body, the particles don’t have to travel far to penetrate soft tissue.

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Beta Radiation

An electron resulting from the breaking apart of a neutron in an atom is called a

beta particle.

• The neutron breaks apart into a proton, which remains in the nucleus, and a fast-moving electron, which is released.

n10

Neutron

p +11

Proton

e 0–1

Electron(beta particle)

→

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Beta Radiation

• The –1 represents the charge on the electron.

• The 0 represents the extremely small mass of the electron compared to the mass of a

proton.

n10

Neutron

p +11

Proton

e 0–1

Electron(beta particle)

→

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Beta Radiation

• The nitrogen-14 atom has the same mass number as carbon-14, but its atomic number has increased by 1.

• It contains an additional proton and one fewer neutron.

C146

Carbon-14 (radioactive)

N +147

Nitrogen-14 (stable)

e ( emission)0–1

Beta particle

→

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Beta Radiation

A beta particle has less charge than an alpha particle and much less mass than an alpha

particle.

• Thus, beta particles are more penetrating than alpha particles.

– Beta particles can pass through paper but are stopped by aluminum foil or thin pieces of

wood.

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Beta Radiation

Because of their opposite charges, alpha and beta radiation can be separated by an electric field.

• Alpha particles move toward the negative plate.

• Beta particles move toward the positive plate.

• Gamma rays are not deflected.

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Gamma Radiation

A high-energy photon emitted by a radioisotope is called a gamma ray.

• The high-energy photons are a form of electromagnetic radiation.

• Gamma rays are emitted during radioactive decay.

Ra +22688

Radium-226

Th23090

Thorium-230

He + 42

Alpha particle

Gamma ray

→

Pa +23491

Protactinium-234

Th23490

Thorium-234

e + 0–1

Beta particle

Gamma ray

→

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Types of RadiationTypes of Radiation

Gamma rays have no mass and no electrical charge.

• Emission of gamma radiation does not alter the atomic number or mass number of an atom.

Gamma Radiation

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Gamma rays are extremely penetrating making them dangerous.

• Gamma rays pass easily through paper, wood, and the human body.• They can be stopped, although not completely, by several meters of

concrete or several centimeters of lead.

Gamma Radiation

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Penetrating AbilityPenetrating Ability

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Gamma rays can be dangerous because of their penetrating power. What property determines the relative penetrating power of electromagnetic radiation?

CHEMISTRY & YOUCHEMISTRY & YOU

The wavelength and energy of electromagnetic radiation determine its relative penetrating power. Gamma rays have a shorter wavelength and higher energy than X-rays or visible light.

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Characteristics of Some Types of Radiation

Type Consists of Symbol ChargeMass (amu)

Common source

Penetrating power

Alpha radiation

Alpha particles (helium nuclei)

, 2+ 4Radium-226

Low (0.05 mm body tissue)

Beta radiation

Beta particles (electrons)

, 1– 1/1837Carbon-14

Moderate (4 mm body tissue)

Gamma radiation

High-energy electromagnetic radiation

0 0 Cobalt-60Very high (penetrates body easily)

He42

e0–1

Interpret DataInterpret Data

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Which process involves a radioactive nucleus releasing a high-speed electron?

A. oxidation

B. alpha emission

C. beta emission

D. gamma radiation

C

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RadioactivityRadioactivity

How do nuclear reactions differ from chemical reactions?

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RadioactivityRadioactivity

Unlike chemical reactions, nuclear reactions are NOT

affected by changes in temperature, pressure, or the

presence of catalysts.

Nuclear reactions of a given radioisotope cannot be slowed

down, sped up, or stopped.

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Nuclear ReactionsNuclear Reactions

• Ernest Rutherford found Ra forms Rn gas

when emitting an alpha particle.

• 1902—Rutherford and Soddy proposed

radioactivity is the result of the natural

change of the isotope of one element into

an isotope of a different element.

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Nuclear ReactionsNuclear ReactionsorTransmutationsorTransmutationsNatural Decay

Spontaneous breakdown of unstable nuclei.

Called Radioisotopes

35

NATURAL Decay:NATURAL Decay:

DECAY = Fall apartAlpha DECAYBeta DECAY

Positon DECAY

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RadioactivityRadioactivity

Radioactive decay is a spontaneous process that does not require an input of energy.

• If the product of a nuclear reaction is unstable, it will decay too.

• The process continues until unstable isotopes of one element are changed, into

stable isotopes of a different element.

• Stable isotopes are not radioactive.

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Nuclear ReactionsNuclear Reactions• Alpha emissionAlpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons are rearranged but conserved

Nucleons = PROTON OR NEUTRON (particle in the nucleus)

38

Nuclear ReactionsNuclear Reactions• Beta emissionBeta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

How does this happen?

39Other Types of Nuclear Other Types of Nuclear ReactionsReactions

Positron (Positron (00+1+1): a positive electron): a positive electron

K-capture:K-capture: the capture of an electron from the first or K shell

An electron and proton combine to form a neutron.0

-1e + 11p --> 1

0n

207 207

40Radioactive Decay SeriesRadioactive Decay Series

41

Writing Nuclear Reactions Parent element = Reactant

Daughter element = Product

Radioactivity = radiation produced

42

Writing Nuclear Reactions

Law of Conservation of Mass

L. of C. of Charge Isotopic Notation Mass # and Nuclear Charge

146C = 14

7N + 0-1e

43

44

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Key ConceptsKey Concepts

Unlike chemical reactions, nuclear reactions are not affected by changes in temperature, pressure, or the presence of catalysts. Also, nuclear reactions of a given radioisotope cannot be slowed down, sped up, or stopped.

Three types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation.