Unit 7 Lesson 2: Energy Release

a particle composed of 2 protons and 2 neutrons often involved in nuclear processes

the number of protons in an atom of an element

the sum of the protons and neutrons that make up the nucleus of an atom

the principle that mass is equivalent to concentrated energy

the conversion of an atom of one element into an atom of a different element through nuclear changes

During a chemical change, the chemical bonds between atoms of elements break. New bonds then form to join those same atoms in different arrangements. Chemical reactions involve changes in the arrangement of electrons but not changes to the nuclei of atoms. Recall that an atom is composed of a dense nucleus of protons and neutrons with electrons located outside the nucleus.

The atomic number is shown to the lower left of the element’s symbol on the periodic table. It is the number of protons in an atom of an element. The number of protons determines the identity of an element.

The mass number is shown to the upper left of the element’s symbol. It is the sum of the protons and neutrons that make up the nucleus of an atom. To determine the number of neutrons in the nucleus, subtract the atomic number from the mass number.

As the name implies, a nuclear change involves changes in the nucleus of an atom. Thus, the numbers of protons and neutrons that make up a nucleus can change.

Because the identity of an atom is determined by the number of protons in its nucleus, any change to the number of protons will cause the atom to change to a different element. The process of transmutation is the conversion of an atom of one element into an atom of a different element through nuclear changes, such as the one represented in the model shown.

Its emission always results in transmutation because of the change in protons of the large nucleus.

When this occurs, the combined masses of radon and the alpha particle are slightly less than the mass of the original radium. It may seem that some small amount of mass was lost, but the law of conservation of mass states that mass cannot be created or destroyed.

In this model, it is easy to note that the total number of protons and neutrons (the mass number) is the same before and after the change. Also, the total nuclear charge, as noted by the atomic numbers, is the same before and after the change. During nuclear changes, the relative numbers of protons and neutrons can change, but the total number of protons and neutrons remains the same.

The energy released during a nuclear change can be a million times greater than the energy released during a chemical change. The energy released during a nuclear process is so great that there is a measurable difference in the masses of the reactants and the products as matter is converted to energy.

A chemical equation is balanced according to the number of atoms of each element before and after the change. However, the nuclear equation is balanced according to mass number and charge. It cannot be balanced according to the number of atoms of each element because the identities of the atoms can change.

Mass-energy equivalency states that mass and energy are related according to the equation E=mc2 . In this equation, the variable E is an amount of energy, the variable m is an amount of mass, and the variable c is the speed of light. The amount of energy released after a nuclear fusion or fission process will be equal to the product of the difference in the mass and the speed of light squared.

Protons and neutrons must be represented in a model of the nucleus of an atom.

An atomic number tells me the number of protons in the nucleus.

The lower number is the atomic number, which shows the alpha particle has 2 protons. The upper number is the mass number, which shows the alpha particle has a total of 4 protons and neutrons. Thus, there are 2 neutrons in the particle. “ He ” is the atom symbol and represents the element helium. An alpha particle is also a helium atom.

Protons must always be represented. Neutrons must be included in models where neutrons are present in the atom.

Mass-energy equivalency states that mass and energy are related by the equation E=mc2 . During the nuclear transmutation process, a small amount of mass is converted to energy and released as a product of the reaction. The amount of energy released is equal to the change in mass times the speed of light squared.

In a model of a nuclear change, the total mass number and total charge must remain the same. The number and types of atoms of elements can change.

You know that the total mass number and the total charge (as shown by the atomic numbers) must remain the same before and after the change. Thus, for this change, the mass number of the emitted particle must be 223−219=4 , and the atomic number of the emitted particle must be 88−86=2 , which is the atomic number of helium. The model can be completed as shown. Ra88223 → Rn86219 + He24

The atomic number is the number of protons in the nucleus. The mass number is the atomic number plus the number of neutrons in the nucleus.