Nuclear Chemistry

(Topic for Regents exam, SAT II exam and AP exam)

Video Animations

• Online resources from our TB for ch 21 (discovery: alpha, beta and gamma radiation)

• Chem tours chapter 20 of Gilbert’s book see: http://www.wwnorton.com/college/chemistry/gilbert2/contents/ch20/chemtours.asp

• Modes of radioactive decay, Balancing nuclear reactions and Fusion of Hydrogen

The Nucleus

• Remember that the nucleus is comprised of protons and neutrons.

• The number of protons is the atomic number.• The number of protons and neutrons together is

the mass of the atom.

Isotopes

• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238

Stable Nuclei• The shaded region in the

fig. shows what nuclides would be stable, the so-called belt of stability.

• It is the ratio of neutrons to protons that determines the stability of a given nucleus.

Radioactivity• It is not uncommon for some nuclei to be

unstable, or radioactive.• There are no stable nuclei with an atomic

number greater than 83.• Radioisotopes = isotopes that are unstable

and thus radioactive• There are several ways radionuclides can

decay into a different nuclide • (a nuclide is a nucleus with a specified number of protons and

neutrons (TB, p. 902) .

Predicting the mode of radioactive decay.

In general:

•neutron-rich nuclei tend to emit beta particles •proton-rich nuclei tend to either emit positrons or undergo electron capture •heavy nuclei tend to emit alpha particles.

•The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus.

Radioactive Series

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

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

• Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

Types of Radioactive Decay Alpha Decay

= Loss of an -particle (a helium nucleus)

He42

U238

92 Th234

90 He42+

Atomic # decreases by 2Mass # decreases by 4# of protons decreases by 2# of neutrons decreases by 2

Which element undergoes alpha decay to form lead-208?

• Analyze• Plan• Solve

• Check

Types of Radioactive Decay Beta Decay

= Loss of a -particle (a high energy electron)

0

−1 e0

−1or

I131

53 Xe131

54 + e0

−1

Atomic # increases by 1# of protons increases by 1# of neutrons decreases by 1Mass # remains the same

Types of Radioactive Decay Positron Emission

= Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C11

6 B11

5 + e01

Atomic # decreases by 1# of protons decreases by 1# of neutrons increases by 1Mass # remains the same

Electron capture

• Capture by the nucleus of an electron from the electron cloud surrounding the nucleus (effectively converts a proton to a neutron).

• Ex: Rubidium-81 is converted to Krypton-81 by this process (Atomic numbers: Rb = 37, Kr = 36)

• Nuclear equation:

Types of Radioactive Decay Gamma Emission

= Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations)

00

Artificial Transmutation= done by bombarding the nucleus with high-energy particles (such

as a neutron or alpha particle), causing transmutation

4020Ca + _____ -----> 40

19K + 11H

9642Mo + 2

1H -----> 10n + _____

**Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**

Nuclear Fission• Nuclear fission is the type of reaction carried out in

nuclear reactors.• = splitting of large nuclei into middle weight nuclei and

neutrons

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• This process continues in what we call a nuclear chain reaction.

• = the combining of light nuclei into a heavier nucleus

• 21H + 2

1H 42He + energy

• Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.

Nuclear Fusion

Half-Life

= the time it takes for half of the atoms in a given sample of an element to decay

- Each isotope has its own half-life; the more unstable, the shorter the half-life.

- Table T Equations:

fraction remaining = (1/2)(t/T)

# of half-lives remaining = t/T

Key: t = total time elapsed T = half-life

Sample Half-Life Question 1A - RegentsMost chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days?

Step 1: Determine how many half-lives elapse during 168 days.

Step 2: Calculate the fraction remaining.

Sample Half-Life Question 1B- Regents (Hint:1st use Regents tables to find half-life)

(a) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?

Step 1: Calculate the mass remaining:

mass remaining = fraction remaining X original mass

(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each

half-life.)

Sample Half-Life Question 2- Regents

How much was present originally in a sample of Cr-51 if 0.75gremains after 168 days?

Step 1: Determine how many half-lives elapsed during 168 days.

Step 2: Multiply the remaining amount by a factor of 2 for each half-life.

Equations to learn for calculations based on half-life (AP)

• Radioactive decay is a first order process;

• ln (Nt/No) = -kt

• Nt = No e-kt

• k = 0.693/t1/2

• or t1/2 = 0.693/k

• (kinetics topic)• k = ? (rate

constant or decay constant)

• N = ?For time interval t

Energy changes in Nuclear Reactions E =mc2

• Einstein E =mc2

• mass defect• For nuclear

reactions ∆E =c2∆m

• E = energy in Joules (J = kg•m2/s2)

• m = mass in kg• C = speed of light • (2.9979 x 108 m/s)

Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical

applications, nuclear power plants)

Medical Uses• 60Co (cobalt-60) used in

cancer treatments and used to kill bacteria in food products

• 226Ra (Radium-226) used in Cancer treatment

• 131I diagnosis and treatment of thyroid disorders

• 11C Positron emission tomography (PET scans)

Other Uses• 14C archaeological dating (of

once living things) and radiolabelled organic compounds

• 238U archaeological dating (U-238 to Pb-206 ratio)

• 241Am (Americium-241) smoke detectors

• 235U nuclear reactors and weapons

Activities and Problem set 5TB ch. 21 – sections 21.1

and 21.4 most impt for AP exam

POGIL activity on nuclear chemistry

Lab activity: Paper lab on nuclear decay

Concept map (Group)PHET dating game

http://phet.colorado.edu/en/simulation/radioactive-dating-game

• Ch 21 Problems TO DO• all GIST, sample & practice

exercises, Visualizing concepts, • Ch 21 end of ch. Red

ex:21.7,9,11,13,15,19,23,25,27,29,31,33,41,43,45,57,

65,70, (21.70 requires graphing)

(problems are from eText; most of them are identical to those in your textbook)