All of the processes discussed in this section are examples of
nuclearreactions, which are different from ordinary chemical reactions.Ordinary chemical reactions involve the exchange and sharing ofelectrons, while nuclear reactions involve alterations in the very coreof an atom; that dense nucleus made up of protons and neutrons.
Radioactivity
You will need to be familiar with severaltypes of nuclear reactions and terms related to them to be fullyprepared for the
SAT II Chemistry test, and in this section we’llreview everything you’ll need to know. The first concept we discuss isradioactivity. Strictly speaking,
radioactivity is thespontaneous disintegration of an unstable atomic nucleus and thesubsequent emission of radiation. But what makes atoms radioactive tobegin with, and what makes them undergo radioactive decay? It turns outthat there is a stable ratio of protons to neutrons for each element;for the first 20 elements on the periodic table (hydrogen throughcalcium), this ratio is 1 proton to 1 neutron, for example. Protons andneutrons in excess of this stable number can be emitted radioactively.Below we have listed examples of the important types of radioactivedecay.
Alpha decay occurs when the nucleusemits an alpha particle. Alpha particles have a positive charge and areequivalent in size to a helium nucleus, and so they are symbolized as
.Alpha particles are the largest radioactive particle emitted. This typeof radioactivity results in a decrease in the atomic number by 2 and adecrease in the atomic mass by 4. The equation below shows uranium-234undergoing alpha decay:
Beta decay occurs when the nucleusemits a beta particle. Beta particles have a negative charge and aremuch smaller than alpha particles. They’re equivalent to high-speedelectrons and are symbolized by
or
.This type of radioactivity causes an increase in the atomic number by 1but no change in mass number. The equation below represents uranium-233undergoing beta decay.
How does a nucleus, which is composed ofonly protons and neutrons, eject an electron? A neutron is composed ofa proton and an electron fused together. In beta emission, the electronis emitted from the nucleus, while the proton part remains behind, thusincreasing the atomic number by 1.
Example
Complete the balanced equation by determining the missing term.
Explanation
Remember, the sum of the atomic numbers and themass numbers must be equal on both sides of the equation. We arelooking for a component that has mass number of 80 and an atomic numberof 34 (34 protons). Using this information and the periodic table, wecan identity the element produced by this beta decay as Se, orselenium. The missing term is
Se. And the completed equation is:
Gamma decay consists of the emissionof pure electromagnetic energy; no particles are emitted during thisprocess, and it is symbolized by equation;00g. After beta, positron, oralpha decay, the nucleus is left in a high-energy state, and at thispoint it will often emit gamma rays, which allows it to relax to itslower-energy ground state. Since gamma rays do not affect charge ormass, they are often not included in nuclear equations.
Positron emission occurs when an atom becomes more stable by emitting a
positron 01
e,which is the same size and mass as an electron but has a positivecharge. This process converts a proton into a neutron; the positron isemitted and the neutron remains behind in the nucleus, decreasing theatomic number by 1.
Often the emission of an alpha or a betaparticle creates another radioactive species, which undergoes furtherradiation/emission in a cascade called a
radioactive series.Notice that in the course of all of these types of radioactive decay,neither protons nor neutrons are either created or destroyed: this isdue to what’s known as the
law of conservation of matter, whichstates that mass is neither created nor destroyed. So when you seeradioactivity equations on the SAT II Chemistry test, one of the mostimportant things to remember is that the sum of the mass numbers andthe sum of the atomic numbers must both be equal on both sides of theequation.
Example
Write the equation for the alpha decay of radium-221.Write the equation for the beta decay of sulfur-35.
Explanation
The radium-221 atom has atomic number (
A) = 88 and mass number (
Z)= 221. When an alpha particle is emitted, the atomic number is reducedby 2 and the mass number is reduced by 4. The atomic number of theresulting atom is 86, so the element created as a result of thisradioactive decay is radon-217.
The sulfur-35 atom has an atomic number of16 and a mass number of 35. When it undergoes beta decay, the atomicnumber is increased by 1 and the mass number remains the same. Theatomic number of the atom created is 17, so the atom is chlorine-35.
Fission and Fusion
There are two main types of nuclear reactions: fusion and fission. In
fusion reactions, two light nuclei are combined to form a heavier, more stable nucleus. In
fissionreactions, a heavy nucleus is split into two nuclei with smaller massnumbers. Both processes involve the exchange of huge amounts of energy:about a mil
lion times more energy than that associated with ordinarychemical reactions. In either case, if the new particles contain morestable nuclei, vast quantities of energy are released.
Nuclear power plants rely on fission to createvast quantities of energy. For example, U-235 nuclides can be bombardedwith neutrons, and the result is
lots of energy, threeneutrons, and two stable nuclei (Kr-92 and Ba-141). The three neutronsformed can collide with other U-235 atoms, setting off a chain reactionand releasing tons of energy.
Example
Is the following process an example of fission or fusion?
Explanation
This is an example of fission. Fission occurswhen a large nucleus is bombarded by a small particle, such as aneutron. The result is two smaller nuclei and additional neutrons, anda chain reaction process begins.
