AtomicAtomic StabilityStability

IsotopesIsotopesIsotopes are atoms of an element that Isotopes are atoms of an element that have have different numbers of neutronsdifferent numbers of neutrons in in their nucleus.their nucleus.

Cu Cu63

29

65

29

Copper – 63

OR

Copper - 65

Stable IsotopesStable IsotopesNucleons are held together by strong Nucleons are held together by strong forces which act over short distances.forces which act over short distances.

The larger the nucleus, (the more protons The larger the nucleus, (the more protons and neutrons it contains) the greater the and neutrons it contains) the greater the distance is between protons and therefore distance is between protons and therefore the force needed to hold them together the force needed to hold them together isn’t as strong. isn’t as strong.

Neutrons .

Atomic number (Z)

(protons)

= 1For the most stable nuclei (those with small atomic numbers), the ratio is 1.

N

Z= 1.5

For elements with large atomic numbers, the most stable nuclei are those with a ratio of 1.5

X - nuclideMass # A

Atomic Z number

To predict nuclear stability, compare the number of protons to neutrons.

RadioactivityRadioactivity

Radioactivity is the Radioactivity is the spontaneousspontaneous breakdownbreakdown of unstable nuclei to produce of unstable nuclei to produce particles or energyparticles or energy..

TransmutationTransmutation – when an unstable – when an unstable nucleus of one element emits a particle nucleus of one element emits a particle and and becomes a different elementbecomes a different element that is that is more stable.more stable.

How does a nucleus How does a nucleus emit a particle?emit a particle?

Types of emissionsTypes of emissions

1.1. Beta particleBeta particle – – ββ – a neutron in an – a neutron in an unstable nucleus may emit a high-unstable nucleus may emit a high-energy electron called a Beta particle energy electron called a Beta particle and changes to a proton. This is called and changes to a proton. This is called Beta decay. Beta decay.

1 1 0

n → p + e

0 1+ 1-

It increases the protons in the nucleus by 1 and becomes a different element, The mass remains the same.

Types of emissions, Types of emissions, cont.cont.

2.2. Electron CaptureElectron Capture – – this is done by nuclei that this is done by nuclei that has too many protons. The nucleus absorbs an has too many protons. The nucleus absorbs an electron from an orbital. electron from an orbital. It changes a positive It changes a positive proton into a neutron (+ and - = neutral), and proton into a neutron (+ and - = neutral), and decreases the atomic number by 1. The mass decreases the atomic number by 1. The mass number remains the samenumber remains the same..

1 0 1

p + e → n

1+ 1- 0

51 0 51 Cr + e → V + energy 24 1- 23

When the nucleus stabilizes, it releases energy in the form of a gamma ray (γ)

Types of Emissions, Types of Emissions, cont.cont.

OR . . .

Positron Emission: a proton emits a positron. The mass number stays the same, but the atomic number decreases by 1.

1 0 1

p → e + n

1+ 1+ 0

38 38 0 K → Ar + e 19 18 1+

Types of Emissions, Types of Emissions, cont.cont.

3. Alpha Particles – α – unstable nuclei that have N/Z ratios much larger than 1.5 can decay by emitting a particle. Many of the elements that have atomic numbers greater than 83 and masses greater than 209 decay by emitting alpha particles.

238 234 4 U → Th + He 92 90 2

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

Nuclear equations must be balanced!

The sum of the numbers on the right must equal the sum on the left.

Artificial Artificial RadioactivitRadioactivit

yy

Nuclear FissionNuclear Fission

A very heavy nucleus splits into two A very heavy nucleus splits into two smaller nuclei, each is more stable than smaller nuclei, each is more stable than the original nucleusthe original nucleus

Most happens artificially by bombarding Most happens artificially by bombarding the nucleus with neutronsthe nucleus with neutrons

Chain ReactionChain Reaction

Chain reaction – the neutrons emitted by Chain reaction – the neutrons emitted by the fission of one nucleus can cause the the fission of one nucleus can cause the fission of another nucleus.fission of another nucleus.

Chain fission reactions can produce a Chain fission reactions can produce a large amount of energy and are used to large amount of energy and are used to generate electrical energy in nuclear generate electrical energy in nuclear power plants from U-235 and Pu-239power plants from U-235 and Pu-239

Critical mass – the minimum amount of fissionable material required to sustain a chain reaction.

Nuclear Fusion Nuclear Fusion

Small nuclei fuse together to form a larger Small nuclei fuse together to form a larger more stable nucleus and energy is more stable nucleus and energy is released. released.

In stars, like our Sun, hydrogen atoms In stars, like our Sun, hydrogen atoms combine to produce helium atoms.combine to produce helium atoms.

Hydrogen bombs utilize the fusion Hydrogen bombs utilize the fusion process. process.

Mass Energy Conversions in Mass Energy Conversions in Nuclear ReactionsNuclear Reactions

Unlike chemical reactions, the total mass Unlike chemical reactions, the total mass of the reactants is not equal to the total of the reactants is not equal to the total mass of the products in a nuclear reaction. mass of the products in a nuclear reaction. The difference is called The difference is called mass defectmass defect..

The mass which appears to be lost is The mass which appears to be lost is converted to energy. converted to energy.

E = mCE = mC22

E = mCE = mC2 2 can be used to find out how much can be used to find out how much energy is produced for a given mass.energy is produced for a given mass.

Since fusion reactions involve the greatest Since fusion reactions involve the greatest mass defect, they produce the greatest mass defect, they produce the greatest amount of energy.amount of energy.

Benefits and UsesBenefits and Uses

Radioactive Dating – C-14Radioactive Dating – C-14

Industry – tracing the path of chemical Industry – tracing the path of chemical processes processes

Energy – nuclear power plantsEnergy – nuclear power plants

Medicine – cancer therapy Medicine – cancer therapy Co-60 frequently used for radiation treatmentsCo-60 frequently used for radiation treatments I-131 used to detect thyroid functionI-131 used to detect thyroid function

Half-lifeHalf-life

Radioactive samples decay at a constant Radioactive samples decay at a constant rate. This rate of decay is called “half-life.”rate. This rate of decay is called “half-life.”

Since the half-life of an isotope is a Since the half-life of an isotope is a constant value and is not influenced by constant value and is not influenced by outside factors such as temperature or outside factors such as temperature or pressure, it is used to determine the age of pressure, it is used to determine the age of an object – radioactive dating. an object – radioactive dating.

Half life continuedHalf life continued

C-14 half-life is 5715 years. After that C-14 half-life is 5715 years. After that amount of time, only half of the original amount of time, only half of the original material remains. After another 5715 material remains. After another 5715 years, only half of that material remains.years, only half of that material remains.

QuestionQuestion

Do you want to have isotopes that are Do you want to have isotopes that are used in cancer treatments to have long or used in cancer treatments to have long or short half lives?short half lives?

SHORT!

Half-life examplesHalf-life examples80 mg of a sample decays to 10 mg in 30 minutes. What is the half-life?

80 mg → 40 mg → 20 mg → 10 mg

1 2 3

Each arrow represents a half-life.

10 minutes

Half-life examplesHalf-life examples

What is the mass of K-42 that remains in a 16 g sample after 37.2 hours?

16 g → 8 g → 4 g → 2 g

37.2 h/3 = 12.4 h