Module 8
1. Module 8
1.9. Page 7
Module 8—Nuclear Decay, Energy, and the Standard Model of the Atom
Lesson Summary
In this lesson you focused on the following questions:
- Which components make up the nucleus of an atom and what keeps them from coming apart?
- What are alpha and beta decay?
- How is the conservation of energy and mass applied to nuclear decay?
The nucleus is very small, only about 10–14 m across, but it makes up nearly the entire mass of the atom. The nucleus is composed of smaller particles called nucleons. The protons and neutrons are both nucleons. The number of protons defines the element. The physical characteristics, such as atomic mass, vary due to the number of neutrons present. Two atoms, each with an identical number of protons but a different number of neutrons, are called isotopes. Each isotope has a unique atomic mass. The atomic mass unit (u) is defined as exactly 
of the mass of the carbon-12 atom (1 u = 1.66 × 10–27 kg).
The nucleus is held together by what physicists call the strong nuclear force, which must be overcome to change the number of nucleons in the atom.
Some nuclei are unstable and decay. This natural change from one substance to another is called transmutation. Alpha decay is defined by the production of an alpha particle ( 
) during the decay of a parent nucleus into a daughter nucleus. The general equation for alpha decay is 
.
Beta decay is defined by the production of a beta particle (
) during the transmutation. The general equation for beta decay is 
. Beta-positive decay is defined by the production of a positron (
) (antimatter electron) during transmutation. The general equation for beta-positive decay is 
.
In all three decay processes, charge and atomic mass number are conserved. Mass itself (atomic mass units, grams or kg) is not conserved, since in each process some mass is converted to energy according to the relationship E = mc2.
All transmutations produce significant amounts of energy in the form of kinetic energy of the emitted particles and sometimes the production of high frequency gamma radiation. Einstein’s mass-energy equivalency (E = mc2) relates the mass defect in transmutations to the amount of energy released. Comparing these values supports the conservation of energy principle.
The alpha, beta, and gamma particles are a form of ionizing radiation. Ionizing radiation is dangerous because it has enough energy to ionize DNA and change chromosomes, which can lead to cancer or, in high doses, radiation sickness and death.
Lesson Glossary
antimatter: a form of matter that has properties opposite to its normal-matter counterpart
antineutrino: a tiny subatomic particle with no charge emitted with in beta decay
alpha particle: two protons and two neutrons bound together to form a stable particle identical to a helium nucleus
atomic mass: the weighted mean atomic mass number of the element’s natural isotopes
This number is given on the periodic table.
atomic mass number (A): the number of nucleons in an atom’s nucleus
atomic number (Z): the number of protons in the nucleus
The atomic number uniquely identifies the element.
beta particle: an electron emitted by the nucleus when a neutron splits into a proton and electron during the beta decay process
daughter element: the element produced by a decay process
isotope: an atom that has the same number of protons but a different number of neutrons and, therefore, a different atomic mass number
nucleon: a proton or neutron
neutrino: a tiny subatomic particle with no charge emitted with a positron in beta-positive decay
neutron: a neutral particle found in the nucleus
parent element: the original element in a decay process
positron: the antimatter to an electron
It is the same type of particle but has an opposite charge. Unlike electrons, positrons are scarce.
proton: a positively charged particle found in all nuclei
transmutation: decay or change into a different element