Nuclear Chemistry

Nuclear Chemistry• Nucleus of an atom contains

protons and neutrons• Strong forces (nuclear force) hold

nucleus together– Protons in nucleus have electrostatic

repulsion– however, strong force overcomes this

repulsion– Strong force: the interaction that

binds nucleus together– Nuclear force (strong force) is MUCH

stronger than electrostatic force– Strong force increases over short

distances

Nuclear Reaction

Radioisotopes• Nuclear Stability: the larger (more massive) a

nucleus is, the harder it is for it to stay together.

• When a nucleus is RADIOACTIVE, it gives off decay particles and changes from one element to another

Unstable isotope stable

alpha

betagamma

Types of Nuclear Reactions

1. Natural Decay– Also known as Natural Transmutation– Atoms with an atomic number of 1 through 83

have at least one stable (nonradioactive) isotope, but all isotopes of elements with an atomic number of 84 or more are radioactive

Types of Nuclear Reactions2. Nuclear bombardment

– reactions are those in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle.

3. Induced– a change in the nucleus of an atom brought about

by subjecting it (the nucleus) to a) an impact with other nuclei of the same or different

typeb) an impact with, or bombardment by, subatomic

particles or high energy electromagnetic radiations.

4. Electron Capture– During electron capture, an electron in an atom's inner shell is

drawn into the nucleus where it combines with a proton, forming a neutron and a neutrino. The neutrino is ejected from the atom's nucleus.

– Since an atom loses a proton during electron capture, it changes from one element to another.

– Although the numbers of protons and neutrons in an atom's nucleus change during electron capture, the total number of particles (protons + neutrons) remains the same.

– Electron capture is also called K-capture since the captured electron usually comes from the atom's K-shell

5. Nuclear Fission• Reaction splits a large

nucleus apart to form two smaller ones.

• Reaction is unknown in the natural world, is a form of artificial transmutation

• Reaction can take place at any temperature or pressure

• Reaction is currently being used to produce electricity for our use

• Requires mining to extract uranium ore• Produces THOUSANDS of times more energy than conventional chemical explosives• Produces radioactive wastes

6. Nuclear Fusion• Reaction combines two small

nuclei together to form one larger one.

• All stars are powered by nuclear fusion

• Reaction requires temperatures of millions of degrees and vast pressures

• Reaction requires temperatures of millions of degrees and vast pressures

• Hydrogen is the most abundant element in the universe• Produces MILLIONS of times more energy than conventional chemical explosives• Produces essentially no radioactive waste

Common to Both Fission and Fusion

• Both generate their energy the same way by converting small amounts of mass (MASS DEFECT) into extraordinary amounts of energy.

Checking for understandingCompare and contrast nuclear fusion and fission

Radioactive Particles

Characteristics of Some RadiationProperty Alpha

radiationBeta radiation

Gamma radiation

Composition Alpha particle (He nucleus)

Beta particle (electron)

High energy EM radiation

Symbol , 42 He , 0

-1 e

Penetrating power

low moderate Very high

Shielding Paper, clothing

Metal foil Lead, concrete

Atomic number (Z) = # protons in nucleus

Mass number (A) = # protons + # neutrons

= atomic # (Z) + # neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

U23592 U238

92

Mass Number Atomic Number

Element Symbol

Review

Radioactive emission

Unstable parent isotope undergoes radioactive emission to produce daughter isotope.

particle+ daughter

isotope(Nucleoid)

Isotope(Nucleoid)

Alpha Decay (Natural Decay)

𝐻𝑒24

Identify the product formed when uranium-238 alpha decays

𝐻𝑒24 +

Determines which atom from the periodic table

𝑈92238

• The alpha particles released by uranium in the Earth’s crust build up underground in porous rock, where they gain electrons and turn into actual atoms of pure helium.

• This is where we get the helium that is in balloons!

Beta (electron) Decay(Natural Decay) 𝑒−1

0

𝐶614

Identify the product formed when carbon-14 emits beta particle

𝑒−10

+ • A neutron in the nucleus decays to form a proton

(atomic number increases by 1, but mass stays the same) and an electron (the beta particle) which leaves the nucleus at high speeds.

Positron Decay(Natural Decay)

𝑒+10

𝐶614

Identify the product formed when carbon-14 decays positron

𝑒+10

+ • A proton in the nucleus decays to form a neutron

(atomic number decreases by 1, but mass stays the same) and a positron (the antimatter form of an electron) which leaves the nucleus at high speeds.

Gamma Emission γ00

𝐶614

Identify the product formed when carbon-14 releases gamma rays

γ00

+ • This takes the form of a high-energy particle of light that is given

off as the nucleus becomes more stable. • It does not change the identity of the element.• It has no mass or charge, and is so energetic that it can only be

stopped by a 30-cm thick layer of concrete or a 1-foot thickness of solid lead.

• Gamma can be given off by itself, or it can be given off with any of the other types of decay.

Electron Capture 𝑒−10

𝐶614

Identify the product formed when carbon-14 captures an electron.

+ 𝑒− 10

• Electron from electron orbitals captured to

convert proton to neutron.

Deuteron Emission 𝐻12

𝐶614

Identify the product formed when carbon-14 emits deuteron.

𝐻12

+

Proton Emission 𝐻11

𝐶614

Identify the product formed when carbon-14 emits proton.

𝐻11

+

Neutron 𝑁01

𝐶614

Identify the product formed when carbon-14 emits neutron.

𝑁01

+

Band of Stability• Proton/Neutron Ratio: The ratio of n:p

in a stable atom varies with size. Small atoms are stable at a 1:1 ratio.

• As the atom becomes larger, more neutrons are needed for stability, driving the stable n:p ratio as high as 1.5:1.

• This creates a zone of stability, inside of which the isotopes are stable.

• Outside the zone, nuclei either have too many or too few neutrons to be stable, and therefore decay by emitting α, β− or γ particles to bring the ratio back to the zone of stability.

• ALL ISOTOPES OF ALL ELEMENTS ABOVE Bi ARE UNSTABLE AND UNDERGO RADIOACTIVE DECAY.

Radioactive Decay Series• Radioactive decay produces a simpler and more

stable nucleus.• A radioactive decay series occurs as a nucleus

disintegrates and achieves a more stable nuclei• There are 3 naturally occurring radioactive decay

series.• Thorium 232 ending in lead 208• Uranium 235 ending in lead 207• Uranium 238 ending in lead 206

• The radioactive decay series for uranium-238. This is one of three naturally occurring series.

Half - Life

Half- Life• The half-life of a radioactive isotope is defined as the

period of time that must go by for half of the nuclei in the sample to undergo decay.

- Half of the radioactive nuclei/isotope in the sample decay into new, more stable nuclei/isotope

• After one half-life, half (50%) of the original amount of the sample will have undergone radioactive decay.

• After a second half-life, one quarter (25%) of the original sample will remain undecayed.

• After a third half-life, one eighth (12.5%) of the original sample will remain undecayed.

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and turns red. C14 – blue N14 - red

As we begin notice that no time has gone by and that 100% of the material is C14

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

29

The grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

After 1 half-life (5730 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.

30

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red .C14 – blue N14 - red

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

Now 2 half-lives have gone by for a totalof 11,460 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.

31

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

3 12.5% 87.5% 1:7

After 3 half-lives (17,190 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7 32

33

Radioactive Dating

Radioactive Dating

• Radioactive Decay is a RANDOM process. It is not possible to predict when a particular nucleus will decay, but we can make fairly accurate predictions regarding how many nuclei in a large sample will decay in a given period of time.

Radioactive Dating

• used to determine the age of a substance that contains a radioactive isotope of known half-life.

• Step 1: Determine how many times you can cut your original amount in half in order to get to your final amount. This is the number of half-lives that have gone by.

• Step 2: Multiply the number of half-lives by the duration of a half-life

Age of Sample = # Half-Lives X Half-Life DurationSee Reference

chart

Reference Chart

• The oldest rocks on Earth have been found to contain 50% U-238 and 50%Pb-206 (what U-238 ultimate decays into). What is the age of these rocks?

First, find out how many half-lives have had to go by so that you have gone from 100% U-238 to 50% U-238:

100 50 ONE half-life has gone by!

Age of Sample = # Half-Lives X Half-Life Duration = 1 half-life X (4.51 X 109 years) = 4.51 X 109 years old!