4.4 Unstable Nuclei 4.4 Unstable Nuclei and Radioactive and Radioactive DecayDecay

Radioactive decayRadioactive decay

• In the late 1890s, scientists noticed some substances spontaneously emitted radiation, a process they called radioactivity.

Radioactive DecayRadioactive Decay

• Nuclear reactions can change one element into another element.

Radioactive decayRadioactive decay

Radiation is rays and particles given off.

Reactions in a nucleus is nuclear reactions.

Unstable nucleiUnstable nuclei

• Elements that give off radiation are trying to make themselves more stable.

• They need a form where it does not take effort to exist

Nuclear Nuclear ChemistryChemistry

“Bravo” Test 1954 – 15,000 kilotonsVideo

Alpha ParticlesAlpha Particles

Alpha particles contain 2 p+ and 2 n.

Alpha radiation is alpha particles being given off.

Alpha radiationAlpha radiation

• The atomic number decreases by 2, and the mass number decreases by 4.

Alpha EquationAlpha Equation

Beta RadiationBeta Radiation

Each beta particle is an electron with a 1– charge.

Beta RadiationBeta Radiation

Beta emission converts Beta emission converts a neutron to a protona neutron to a proton

The mass number The mass number remains the same, but remains the same, but the atomic number the atomic number increases by one.increases by one.

Beta EquationBeta Equation

Electric Field Electric Field DeflectionDeflection

Gamma RaysGamma Rays

• Gamma rays are high-energy radiation with no mass and are neutral.

• Gamma rays account for most of the energy lost during radioactive decay.

Characteristics of Characteristics of RadiationRadiation

Unstable atomsUnstable atoms

• Atoms that contain too many or too few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus.

• Few exist in nature—most have already decayed to stable forms.

242He

242 e01 01 00

Types of Radioactive DecayTypes of Radioactive Decay

alpha production (alpha production (He): He): helium nucleushelium nucleus

beta production (beta production (e):e):

gamma ray production (gamma ray production ():):

alpha production (alpha production (He): He): helium nucleushelium nucleus

beta production (beta production (e):e):

gamma ray production (gamma ray production ():):

ThHeU 23490

42

23892

234 234 090 91 1Th Pa e

0023490

42

23892 2 ThHeU

Specifying Specifying IsotopesIsotopes

19

XA

Z

X = the symbol of the element

A = mass number (protons + neutrons)

Z = the atomic number (number of protons)

Nuclear SymbolsNuclear Symbols

Element symbol

Mass number, A (p+ + no)

Atomic number, Z(number of p+)

U23592

Key to Balancing Key to Balancing Nuclear EquationsNuclear Equations

In nuclear reactions, both the atomic number Z and the mass number A must be conserved

Balancing Nuclear Balancing Nuclear EquationsEquations

226 488 2Ra

226 = 4 + ____222

222

88 = 2 + ___86

86

Atomic number 86 is radon, Rn

Rn

Alpha DecayAlpha Decay

Alpha production (Alpha production (): ):

an alpha particle is aan alpha particle is a

helium nucleushelium nucleus

Alpha decay is limited to heavy, radioactive

nuclei

238 4 23492 2 90U Th

4 2 4 22 2He or

Alpha (Alpha (αα) Decay) Decay

E1

P+N

P P-2 + He

4

2 E2

P+N -4

an alpha particle (helium nucleus) is an alpha particle (helium nucleus) is producedproduced

Alpha Alpha RadiationRadiation

Limited to VERY large nucleii.

Example of Alpha Example of Alpha DecayDecay

Ra222

88 86+ He

4

2 Rn

Radium 222 decays by α particle production to Radon 218

218

Beta DecayBeta DecayBeta production (Beta production ():):A beta particle is an A beta particle is an electron ejected from electron ejected from the nucleusthe nucleus

Beta emission converts a neutron to a proton

234 234 090 91 1Th Pa

0 01 1e or

Beta (Beta (ββ) Decay) Decay

Beta emission converts a neutron to a proton

E1

P+N

P P+1 + e0

-1 E2

P+N

Beta Beta RadiationRadiation

Converts a neutron into a proton.

Example of Beta Example of Beta DecayDecay

Notice the mass of the beta particle is zero; it is so small that is must be neglected.

C14

6 7 + e0

-1 N

14

Example of Beta Example of Beta DecayDecay

Th234

90 91 + e0

-1 Pa

234

Thorium 234 decays by β particle production to Protactinium 234(notice: no change in mass number A, and an increase of 1 in atomic

number Z)

Gamma Ray ProductionGamma Ray Production

Gamma ray production (Gamma ray production ():):

Gamma rays are high energy photons produced in association with other forms of decay.

Gamma rays are massless and do not, by themselves, change the nucleus

0023490

42

23892 2 ThHeU

Gamma Ray ProductionGamma Ray Production

Gamma ray production (Gamma ray production ():):

Gamma rays are high energy photons produced in association with other forms of decay.Gamma rays are massless and do not, by themselves, change the nucleus

0023490

42

23892 2 ThHeU