Nuclear Chemistry Just the basics…. By J.M.Soltmann Just the basics…. By J.M.Soltmann

What is Nuclear Chemistry  As its name implies, nuclear chemistry is the study of the nucleus and reactions between nuclei.  Remember that virtually all of the mass of an atom resides in the nucleus, as does all of the positive charge.  Nuclear energy is a much greater form of energy than bond energy.  As its name implies, nuclear chemistry is the study of the nucleus and reactions between nuclei.  Remember that virtually all of the mass of an atom resides in the nucleus, as does all of the positive charge.  Nuclear energy is a much greater form of energy than bond energy.

Radioactivity  While most nuclei are stable, many nuclei are unstable and spontaneously emit particles and electromagnetic radiation.  These nuclei are refered to as radionuclides.

Nuclear Equations  In a nuclear equation, mass numbers and atomic numbers are balanced instead of elements.  The example here to the right depicts a radioactive decay; specifically an alpha decay.  The helium ion is called an alpha particle.  In a nuclear equation, mass numbers and atomic numbers are balanced instead of elements.  The example here to the right depicts a radioactive decay; specifically an alpha decay.  The helium ion is called an alpha particle.

3 Common types of Radioactive Decay  Alpha decay  Beta decay - a ß - particle is a subatomic nuclear particle essentially equivalent to an electron and a ß + particle is a positively charge electron, called a positron.  Gamma decay - high energy photons are emitted which have virtually no mass nor charge.

Nuclear electrons?  Modern theory has shown that a neutron is actually comprised of a proton and an electron.  So, if a nucleus emits an electron, it has really transformed a neutron into a proton.  Also, if a nucleus absorbs an electron, it will convert a proton into a neutron.  Modern theory has shown that a neutron is actually comprised of a proton and an electron.  So, if a nucleus emits an electron, it has really transformed a neutron into a proton.  Also, if a nucleus absorbs an electron, it will convert a proton into a neutron.

Common Particles in Nuclear Reactions  Neutrons ( 1 0 n)  Protons ( 1 1 p or 1 1 H)  Electrons ( 0 -1 e)  Alpha Particles ( 4 2 He or 4 2  )  Beta - Particles ( 0 -1 e or 0 -1  )  Gamma ( 0 0  ) - Gamma radiation consists of high- energy photons, with a mass far too little for consideration.  Positron ( 0 1 e) - A positron is a positively charged electron. It has the mass of an electron but a positive charge.  Neutrons ( 1 0 n)  Protons ( 1 1 p or 1 1 H)  Electrons ( 0 -1 e)  Alpha Particles ( 4 2 He or 4 2  )  Beta - Particles ( 0 -1 e or 0 -1  )  Gamma ( 0 0  ) - Gamma radiation consists of high- energy photons, with a mass far too little for consideration.  Positron ( 0 1 e) - A positron is a positively charged electron. It has the mass of an electron but a positive charge.

Differentiating the Radiations  Alpha emissions are the heaviest and thus have the least penetrating power.  Beta emissions have masses much smaller than protons or neutrons, so they have more penetrating power. In terms of penetrating power,  =100* .  Gamma emissions have essentially no mass, so they are the most powerful. In terms of penetrating power.  = 100* .  Alpha emissions are the heaviest and thus have the least penetrating power.  Beta emissions have masses much smaller than protons or neutrons, so they have more penetrating power. In terms of penetrating power,  =100* .  Gamma emissions have essentially no mass, so they are the most powerful. In terms of penetrating power.  = 100* .

Try this  Write a nuclear equation for the process when mercury-201 undergoes electron capture.

To answer this question:  First we have to understand what mercury- 201 is. Since mercury is always atomic number 80, this isotope is Hg.  Since we are capturing an electron, the electron must be a reactant.  Now we add up mass numbers and atomic numbers. ( = 201 and =79).  Element 79 is gold, so the answer is:  Hg e --> Au  First we have to understand what mercury- 201 is. Since mercury is always atomic number 80, this isotope is Hg.  Since we are capturing an electron, the electron must be a reactant.  Now we add up mass numbers and atomic numbers. ( = 201 and =79).  Element 79 is gold, so the answer is:  Hg e --> Au

Try another  Thorium-231 decays into protactinium-231.  What is the balanced equation?  What other particle(s) is/are involved in the reaction?  Thorium-231 decays into protactinium-231.  What is the balanced equation?  What other particle(s) is/are involved in the reaction?

The answers are:  Th --> Pa e  The extra particle is an electron, but because it is being emitted, it would be called a Beta emission.  Th --> Pa e  The extra particle is an electron, but because it is being emitted, it would be called a Beta emission.

Nuclear Transformations  The first manmade conversion of one nucleus into another was performed by Sir Ernest Rutherford (1919).  Rutherford bombarded a nitrogen-14 atom with alpha particles to produce an oxygen-17 atom plus a proton.  14 7 N He --> 17 8 O H  The shorthand version of this reaction is  14 7 N( ,p) 17 8 OWHY???  The first manmade conversion of one nucleus into another was performed by Sir Ernest Rutherford (1919).  Rutherford bombarded a nitrogen-14 atom with alpha particles to produce an oxygen-17 atom plus a proton.  14 7 N He --> 17 8 O H  The shorthand version of this reaction is  14 7 N( ,p) 17 8 OWHY???

Now try this one:  Write the balanced nuclear equation for the process noted by the shorthand:  Al(n,  ) Na  Write the balanced nuclear equation for the process noted by the shorthand:  Al(n,  ) Na

Now try this one:  Write the balanced nuclear equation for the process noted by the shorthand:  Al(n,  ) Na  Al n --> Na He  Write the balanced nuclear equation for the process noted by the shorthand:  Al(n,  ) Na  Al n --> Na He

Nuclear Stability  Why are some nuclei more stable than others?  To be honest, there are several factors, most of which are beyond the scope of this course.  However, there are a few easy to see indications of nuclear stability.  Why are some nuclei more stable than others?  To be honest, there are several factors, most of which are beyond the scope of this course.  However, there are a few easy to see indications of nuclear stability.

Did you ever wonder…?  We know that like charges repel each other, yet a nucleus can have dozens of positively charged protons held together. Why?  Neutrons are a major reason. All nuclei with 2 or more protons have neutrons. The neutrons and the protons meld by a force of nature, different than gravity or electromagnetism, called the strong (nuclear) force.  Because of the way this force binds the protons and neutrons together, the ratio of protons to neutrons is an issue.  We know that like charges repel each other, yet a nucleus can have dozens of positively charged protons held together. Why?  Neutrons are a major reason. All nuclei with 2 or more protons have neutrons. The neutrons and the protons meld by a force of nature, different than gravity or electromagnetism, called the strong (nuclear) force.  Because of the way this force binds the protons and neutrons together, the ratio of protons to neutrons is an issue.

Smaller Atoms vs Bigger Atoms  In smaller atoms, most stable atoms have neutron to proton ratios of about  As isotopes increase in atomic number, most stable isotopes have increasingly larger ratios of neutrons to protons.  To our knowledge, any isotope with an atomic number greater than or equal to 84 would be radioactive.  In smaller atoms, most stable atoms have neutron to proton ratios of about  As isotopes increase in atomic number, most stable isotopes have increasingly larger ratios of neutrons to protons.  To our knowledge, any isotope with an atomic number greater than or equal to 84 would be radioactive.

Some stability trends  Of the 265 known stable isotopes:  157 of them have even numbers of protons and neutrons.  53 of them have an even number of protons but an odd number of neutrons.  50 of them have an odd number of protons but an even number of neutrons.  Only 5 of them have odd numbers of both protons and neutrons.  Of the 265 known stable isotopes:  157 of them have even numbers of protons and neutrons.  53 of them have an even number of protons but an odd number of neutrons.  50 of them have an odd number of protons but an even number of neutrons.  Only 5 of them have odd numbers of both protons and neutrons.

Magic Numbers  For some reason, nuclei with 2,8,20,28,50 or 82 protons and/or 2,8,20,28,50,82, or 126 neutrons are generally more stable than isotopes without these numbers.  When we think of substances that shield radiation, we tend to think of lead. The most common isotope of lead is Pb; that means it has 82 protons and 126 neutrons.  For some reason, nuclei with 2,8,20,28,50 or 82 protons and/or 2,8,20,28,50,82, or 126 neutrons are generally more stable than isotopes without these numbers.  When we think of substances that shield radiation, we tend to think of lead. The most common isotope of lead is Pb; that means it has 82 protons and 126 neutrons.

Decays and Half-lifes  When a radioactive substance decays, the amount of that particular isotope will decrease.  We call the rate of decay the half-life, because it is the time needed for exactly 1/2 of the isotope to decay.

More on Half-life  If we examine the graph to the right, we see that we started with 50 g of the isotope. Each subsequent point represents half of the previous mass (50 to 25 to 12.5 to 6.25 to to to.78125).  Each point is approximately 24 days apart; The half-life for this substance is 24 days.  If we examine the graph to the right, we see that we started with 50 g of the isotope. Each subsequent point represents half of the previous mass (50 to 25 to 12.5 to 6.25 to to to.78125).  Each point is approximately 24 days apart; The half-life for this substance is 24 days.

Calculations with half-life  Although it is possible to determine the amount remaining of a radioisotope using natural logs { ln(N t /N 0 )=-kt } we do not need to do this.  We only work with whole number increments of the half-life.  Although it is possible to determine the amount remaining of a radioisotope using natural logs { ln(N t /N 0 )=-kt } we do not need to do this.  We only work with whole number increments of the half-life.

For example  The half-life of an isotope is 8 days. If we start with 100 grams of the isotope, how much is present in 32 days?  32 days/(8 days/half-life) = 4 half-lives.  Each half-life divides the previous mass in half.  100g/2 = 50g/2 = 25g/2 = 12.5g/2 = 6.25g  There would be 6.25 g of that isotope left.  The half-life of an isotope is 8 days. If we start with 100 grams of the isotope, how much is present in 32 days?  32 days/(8 days/half-life) = 4 half-lives.  Each half-life divides the previous mass in half.  100g/2 = 50g/2 = 25g/2 = 12.5g/2 = 6.25g  There would be 6.25 g of that isotope left.

You try one  The half-life of Bismuth-211 is 185 years. How much time would it take for a 360 g sample to decay to g?

You try one  The half-life of Bismuth-211 is 185 years. How much time would it take for a 360 g sample to decay to g?  360g/2=180g/2=90g/2=45g/2=22.5g/2=11.25g.  That is 5 half-lives.  5 half-lives*185 years/half-life = 925 years.  The half-life of Bismuth-211 is 185 years. How much time would it take for a 360 g sample to decay to g?  360g/2=180g/2=90g/2=45g/2=22.5g/2=11.25g.  That is 5 half-lives.  5 half-lives*185 years/half-life = 925 years.

Fusion vs. Fission  Fission and Fusion are two types of highly exothermic nuclear reactions, different than the decays covered earlier.  Fusion means to bring two smaller nuclei together to make a larger nucleus.  13 6 C C --> Na p  Fission means to break a larger nucleus into 2 or more smaller nuclei.  U n --> Te Zr n  Fission and Fusion are two types of highly exothermic nuclear reactions, different than the decays covered earlier.  Fusion means to bring two smaller nuclei together to make a larger nucleus.  13 6 C C --> Na p  Fission means to break a larger nucleus into 2 or more smaller nuclei.  U n --> Te Zr n

How much Energy are we talking about?  In fusion and fission, a tiny, almost meaningless mass of each affected nucleus is converted to energy.  Einstein theorized that the amount of energy was dependent on the mass lost and the square of the speed of light. E = mc 2.  In fusion and fission, a tiny, almost meaningless mass of each affected nucleus is converted to energy.  Einstein theorized that the amount of energy was dependent on the mass lost and the square of the speed of light. E = mc 2.

So, how much energy is that?  Well if each uranium atom in a given fission process loses the mass of one electron (9.11x kg):  E = mc 2  E = (9.11x kg) *(3.0x10 8 m/s) 2  E = 8.2x J

But that seems like a small number!  8.2x J is a small amount, but that was for just one atom or uranium. If we had 1 mg of uranium (about the mass of a cystal of salt), that would contain roughly 2.5 x atoms of uranium.  8.2x J/atom * 2.5 x atoms =2.1x10 5 J  That is almost enough energy to handle the electrical needs of this school for a day - from a tiny starting mass.  8.2x J is a small amount, but that was for just one atom or uranium. If we had 1 mg of uranium (about the mass of a cystal of salt), that would contain roughly 2.5 x atoms of uranium.  8.2x J/atom * 2.5 x atoms =2.1x10 5 J  That is almost enough energy to handle the electrical needs of this school for a day - from a tiny starting mass.

Think about this  If we could convert the.25 kg mass of a banana peel (using our Mr. Fusion power supply) into pure energy,  E = mc 2  E = (.25 kg)*(3.0x10 8 m/s) 2  E = 2.25x10 16 J  That’s enough energy to run New York City for a year (with enough energy left over to go Back to the Future)!  If we could convert the.25 kg mass of a banana peel (using our Mr. Fusion power supply) into pure energy,  E = mc 2  E = (.25 kg)*(3.0x10 8 m/s) 2  E = 2.25x10 16 J  That’s enough energy to run New York City for a year (with enough energy left over to go Back to the Future)!