5 Steps to a 5 AP Chemistry 2019

Nuclear Chemistry ❮ 261

the left and right of the reaction arrow. Recall that a specific isotope of an element can

be represented by the following symbolization:

ZAX

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:

(^3517) Cl+→ 01 nH 11 +yx?
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 + y on the right. The atomic number
of the unknown must then be 16. This atomic number identifies the element as sulfur,
so a complete nuclear equation can be written:
1735 Cl+→ 01 nH 11 + 1635 S
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: 24 He or a. 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:
(^22286) Rn→+ (^21884) Po 24 He
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 −− 10 βor^01 e. This electron
comes from the nucleus, not the electron cloud, and results from the conversion of a
neutron into a proton and an electron:^10 np→+ 11 − 10 e.
KEY IDEA