5 Steps to a 5 AP Chemistry

Nuclear Chemistry  261

of the reaction arrow. Recall that a specific isotope of an element can be represented by the

following symbolization:

In this symbolization A is the mass number (sum of protons and neutrons), Z is the

atomic number (number of protons), and X is the element symbol (from the periodic

table). In balancing nuclear reactions, make sure the sum of all A values on the left of the

arrow equals the sum of all A values to the right of the arrow. The same will be true

of the sums of the Z values. Knowing that these sums must be equal allows one to

predict the mass and atomic number of an unknown particle if all the others are

known.

Consider the transmutation—creation of one element from another— of Cl-35. This

isotope of chlorine is bombarded by a neutron and H-1 is created, along with an isotope of

a different element. First, a partial nuclear equation is written:

The sum of the mass numbers on the left of the equation is 36 =(35 +1) and on the

right is 1 + x. The mass number of the unknown isotope must be 35. The sum of the atomic

numbers on the left is 17 =(17 +0), and 1 +yon the right. The atomic number of the

unknown must then be 16. This atomic number identifies the element as sulfur, so a com-

plete nuclear equation can be written:

Sulfur-35 does not occur in nature; it is an artificially produced isotope.

Natural Radioactive Decay Modes

Three common types of radioactive decay are observed in nature, and two others are occa-

sionally observed.

Alpha Emission

An alpha particle is a helium nucleus with two protons and two neutrons. It is represented

as: or α. As this particle is expelled from the nucleus of the radioisotope that is

undergoing decay, it has no electrons and thus has a 2+charge. However, it quickly acquires

two electrons from its surroundings to form the neutral atom. Most commonly, the alpha

particle is shown as the neutral particle and not the cation.

Radon-222 undergoes alpha decay according to the following equation:

Notice that in going from Rn-222 to Po-218, the atomic number has decreased by 2

and the mass number by 4.

Beta Emission

A beta particle is an electron and can be represented as either This electron

comes from the nucleus, notthe electron cloud, and results from the conversion of a

neutron into a proton and an electron: 01 npe.→+ 11 −^01

−− 1

0

1

βor^0 e.

86

222

84

218

2

Rn→+Po^4 He

2

(^4) He
17
35
0
1
1
1
16
Cl+→ +n H^35 S
17
35
0
1
1
Cl+→+n^1 H
y
x?
Z

AX

KEY IDEA