North Salem High School

Regents Chemistry – Mrs. Ermann Nuclear Chemistry

1 2 Quick Isotope Review!

Isotopes are atoms with the same number of protons, but differing numbers of neutrons.

Symbols for isotopes:

An isotope can also be written as: sodium-25. The “25” indicates the atomic mass of the isotope.

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What is Nuclear Chemistry?

Nuclear chemistry is the branch of chemistry that studies reactions that happen within an atom's nucleus. We're only going to touch on a few of the basics. We'll start by defining radiation then look at how some elements undergo radioactive decay. We'll learn about and practice how to calculate half-life problems. Are all elements radioactive? Nope. Most of the 88 or so naturally occurring elements have pretty stable nuclei since neutrons keep the protons from repelling or pushing each other apart. Once you get past atomic number 82, however, all elements have radioactive isotopes. These isotopes will break apart or "decay," turning from one element into a different one - over and over again - until the nucleus is relatively stable. (By the way, there are a few lightweight isotopes like Carbon-14 that are radioactive, too. It's just more common among the bigger ones). The process of changing an unstable nucleus into a stable one is called "radioactive decay." You'll learn the details about how this works under "Types of Decay", but basically elements give off particles and/or rays that change the ratio of protons and neutrons in the nucleus.

These resources will help you understand the basics of nuclear chemistry:  http://www.chem4kids.com/files/atom_nucleus.html  http://www.lbl.gov/abc/Basic.html 3 ------Types of Radioactive Decay

There are three main types of radioactive decay: alpha, beta, and gamma. Let's pause here a minute to define "decay." When an element decays the parent element's nucleus changes - it will actually decay to turn into a different daughter element altogether! How is this possible? Because during radioactive decay the number of protons in the nucleus can change.

Alpha Radiation/Decay During Alpha decay an atom spits out two protons and two neutrons from its nucleus. This little bundle is called an "alpha particle."  The symbol looks like Helium because Helium-4 has the same number of protons and neutrons as an alpha particle (no electrons, though).  Since Alpha particles have two protons and no electrons, they have a net charge of 2+.  During Alpha radiation an atom's proton count drops by two, and we know what that means - a NEW element is formed!  Alpha radiation can be stopped by PAPER.

Beta Radiation/Decay Remember we said a neutron is a proton with an electron attached? In beta decay a neutron sends its electron packing, literally ejecting it from the nucleus at high speed. The result? That neutron turns into a proton!  Beta decay increases an atom's electron count by 1 (notice the 1- in the symbol).  During Beta radiation an atom's proton count grows by one. Once again, NEW element!  Beta radiation can be stopped by WOOD. 

Gamma Radiation/Decay Gamma rays is electromagnetic radiation similar to light. Gamma decay does not change the mass or charge of the atom from which it originates. Gamma is often emitted along with alpha or beta particle ejecton.  Gamma radiation can be stopped by LEAD. The diagram below shows what materials can block each type of radiation. Notice it's easy to block alpha radiation (paper will do!) but tough to block gamma radiation (you'll need a lead vest).

4 NOTES FROM: http://edtech2.boisestate.edu/lindabennett1/502/Nuclear %20Chemistry/Nuclear_start.html

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SYMBOLS USED IN NUCLEAR CHEMISTRY

Write the Symbol(s) Corresponding Words, Meaning, or Description

Example: or Alpha particle

(alpha radiation, helium nucleus)

Electron (show atomic number and mass number in symbol form)

Beta particle (fast moving electron)

Proton (hydrogen nucleus)

Gamma ray (gamma radiation)

5 Neutron

1. Fill in the blanks to identify the missing reactants or products in these nuclear reactions. Recall that the sums of the atomic numbers and mass numbers must be conserved on either side of the nuclear equation. Fill in the type of decay (alpha, beta, gamma) it is. a. Np  Pu + ______Type:

c. C  + e _____

d. K  e + _____

e. Pu He + _____

f. U  Th + _____

g. K  e + _____

h. ______ Ir + He

i. K  Ca + _____

j. Pu  He + _____

k. Ga  e + _____

l. Ge  Ga + _____

m. Li  He + ______

n. B  + He _____

o.  e + Mg _____

p. Rn He + _____

2. Using your knowledge of nuclear chemistry, write the equations for the following processes:

a. Alpha decay of radon-198

6 b. Beta decay of uranium-237

c. Iridium-174 undergoing alpha decay

d. Platinum-199 emitting a beta particle

e. Alpha decay of plutonium-244

f. Beta decay of antimony-125

3. For each decay example, calculate the new atomic number, mass number, and Isotope symbol.

Atomic Number Mass Number of New Isotope Original Isotope of New Isotope New Isotope Symbol

18-(-1) = 19 42

7 Half Life Nuclear half life is the time it takes one half of the atoms in a current sample to decay. Sample size doesn't matter - whether you have one gram or one ton of something, half of the sample will decay during the first half life. The sample will continue to shrink by half during each successive half-life until the atom becomes stable. Half-lives vary from less than a second to billions of years! Keep in mind that any sample or measurable size will contain billions of atoms, and there is no way to predict which specific atoms will decay next. You just know one half of them will decay during each half life.

8 How to calculate half life You will have to know how to do to types of half-life calculations: 1. Determine the half life of a radioisotope 2. Determine how much time has elapsed (how much time has gone by – an example of this is using Carbon data to determine the age of keletons and other artifacts). The following are examples of how to do this.

Determining half-life: Question: 100 grams of a radioisotope decayed to 12.5 grams after 90.7 years. What is the half life of the radioisotope? 1. Start with the initial amount given: 100 grams. 2. Divide the initial amount in half until you have reached the final grams (12.5 g).

3. The question tells us that this process took 90.7 years. To find one half life, divide 90.7 by the half lives that have occurred (3).

Answer: One half life is 30.2 years. Determining time elapsed: In this calculation, we are trying to determine how long it takes an amount of radioisotope to decay based on its half life. Question: How many days are required for 200. grams of Radon-222 to decay to 50 grams if the half life is 3.82 days? 1. Start with the initial amount (200. grams) and keep on dividing by two until you reach the final amount.

2. For the sample to decay to 50 grams, two half lives must pass. 3. Multiply the half life of Radon-222 by the two half lives to determine how much time has passed.

Answer: 7.64 days.

1. After 42 days a 2.0 g sample of phosphorus-32 contains only 0.25g of isotope. What is the half-life of phosphorus-32?

2. What is the half life of Sodium-25 if 1.00 gram of a 16.0 gram sample remains unchanged for 237 seconds?

3. A 50.0 gram sample of Nitrogen-14 decays to 12.5 grams in 14.4 seconds. What is its half life?