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.
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
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
. 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
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
. 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.
Complete the balanced equation by determining the missing
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
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 positron 01e, 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.
Write the equation for the alpha decay of radium-221.Write
the equation for the beta decay of sulfur-35.
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
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 fission reactions,
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.
Is the following process an example of fission or fusion?
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.