Nuclear
Reactions
All of the processes
discussed in this section are examples of nuclear reactions,
which are different from ordinary chemical reactions. Ordinary chemical
reactions involve the exchange and sharing of electrons, while nuclear
reactions involve alterations in the very core of an atom; that dense nucleus
made up of protons and neutrons.
Radioactivity
You will need to be
familiar with several types of nuclear reactions and terms related to them to
be fully prepared for the SAT II Chemistry test, and in this section we’ll
review everything you’ll need to know. The first concept we discuss is
radioactivity. Strictly speaking, radioactivity is
the spontaneous disintegration of an unstable atomic nucleus and the subsequent
emission of radiation. But what makes atoms radioactive to begin with, and what
makes them undergo radioactive decay? It turns out that there is a stable ratio
of protons to neutrons for each element; for the first 20 elements on the
periodic table (hydrogen through calcium), this ratio is 1 proton to 1 neutron,
for example. Protons and neutrons in excess of this stable number can be
emitted radioactively. Below we have listed examples of the important types of
radioactive decay.
Alpha decay occurs
when the nucleus emits an alpha particle. Alpha particles have a positive
charge and are equivalent in size to a helium nucleus, and so they are
symbolized as
. Alpha particles are the largest radioactive particle
emitted. This type of radioactivity results in a decrease in the atomic number
by 2 and a decrease in the atomic mass by 4. The equation below shows
uranium-234 undergoing alpha decay:
Beta decay occurs
when the nucleus emits a beta particle. Beta particles have a negative charge
and are much smaller than alpha particles. They’re equivalent to high-speed
electrons and are symbolized by
or
. This type of radioactivity causes an increase in the atomic
number by 1 but no change in mass number. The equation below represents
uranium-233 undergoing beta decay.
How does a nucleus, which
is composed of only protons and neutrons, eject an electron? A neutron is
composed of a proton and an electron fused together. In beta emission, the
electron is emitted from the nucleus, while the proton part remains behind,
thus increasing the atomic number by 1.
Example
Complete the balanced
equation by determining the missing term.
Explanation
Remember, the sum of the
atomic numbers and the mass numbers must be equal on both sides of the
equation. We are looking for a component that has mass number of 80 and an
atomic number of 34 (34 protons). Using this information and the periodic
table, we can identity the element produced by this beta decay as Se, or
selenium. The missing term is
Se. And the completed equation is:
Gamma decay consists
of the emission of pure electromagnetic energy; no particles are emitted during
this process, and it is symbolized by equation;00g. After beta, positron, or
alpha decay, the nucleus is left in a high-energy state, and at this point it
will often emit gamma rays, which allows it to relax to its lower-energy ground
state. Since gamma rays do not affect charge or mass, they are often not
included in nuclear equations.
Positron emission occurs
when an atom becomes more stable by emitting a positron01e, which is the same size and mass as an electron
but has a positive charge. This process converts a proton into a neutron; the
positron is emitted and the neutron remains behind in the nucleus, decreasing
the atomic number by 1.
Often the emission of an
alpha or a beta particle creates another radioactive species, which undergoes
further radiation/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 is due to what’s known as the law of conservation of matter, which states that mass
is neither created nor destroyed. So when you see radioactivity equations on
the SAT II Chemistry test, one of the most important things to remember is that
the sum of the mass numbers and the sum of the atomic numbers must both be
equal on both sides of the equation.
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 reduced by 2 and the mass number is reduced by 4. The atomic number
of the resulting atom is 86, so the element created as a result of this radioactive
decay is radon-217.
The sulfur-35 atom has an
atomic number of 16 and a mass number of 35. When it undergoes beta decay, the
atomic number is increased by 1 and the mass number remains the same. The
atomic 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 mass numbers. Both processes involve the exchange of huge
amounts of energy: about a million times more energy than that associated with
ordinary chemical reactions. In either case, if the new particles contain more
stable nuclei, vast quantities of energy are released.
Nuclear power plants rely
on fission to create vast quantities of energy. For example, U-235 nuclides can
be bombarded with neutrons, and the result is lots of energy,
three neutrons, and two stable nuclei (Kr-92 and Ba-141). The three neutrons
formed can collide with other U-235 atoms, setting off a chain reaction and
releasing tons of energy.
Example
Is the following process an
example of fission or fusion?
Explanation
This is an example of
fission. Fission occurs when a large nucleus is bombarded by a small particle,
such as a neutron. The result is two smaller nuclei and additional neutrons,
and a chain reaction process begins.
Social Plugin