## Understanding Oxidation Numbers

### Dale Peterson

#### Description

This lesson will help students grasp the relationships among protons and electrons, locations of various elements, electron energy levels, and oxidation numbers.

#### Objectives

The student knows that matter is mostly neutral, but that particles can attain a charge by the gain or loss of electrons.

#### Materials

-Dry erase board or chalk board
-Markers or chalk
-Period tables
-Optional Practice Problems (See Associated File)

#### Preparations

1. Ensure students have ready access to a periodic table, either in a textbook or individual copies.
2. A transparency with concentric circles already drawn is very helpful, but not required.
3. Make student copies of optional practice problems. (See Associated File.)

#### Procedures

This lesson assumes knowledge of the periodic table and how it is organized, specifically, families or groups, periods or rows, transition elements, representative elements, alkali metal family, alkaline earth metal family, halogens, noble gases, atomic number, and mass number. It's also necessary to know the atoms consist of subatomic particles: protons, neutrons, and electrons.

An oxidation number is defined as the number of electrons that an atom gains, loses, or shares when bonding with another atom. Understanding electron energy levels is very helpful when trying to learn about oxidation numbers.

1. First, review some facts:
(a) The atomic number, regardless of the element, indicates how many protons one atom of an element contains. Protons always carry positive charges.
(b) Atoms, to remain stable, must have an equal number of positive charges and negative charges. Therefore, an element with an atomic number of 10, has 10 protons and 10 electrons.
(c) Practice with the students. Identify several elements on the periodic table. Ask them to identify the number of protons and electrons, and then ask why the positive and negative charges are the same. (Answer = stability)

2. Explain that every atom of every element has electrons arranged in energy levels. The energy levels directly correlate to the period or row that an element is located. For example, sodium (Na) is located in the alkali metal family, the third period or row. Since sodium is located in the third period, its electrons are located in three energy levels.

3. Now draw an energy level diagram for sodium. Sodium has an atomic number of 11, so each atom has 11 protons and 11 electrons.

(a) Draw three concentric circles around the nucleus of a sodium atom. Off to one side, write 11P and 11E to represent the protons and electrons.
(b) On the innermost circle, point out that a maximum of two electrons are represented, which directly correlates to the number of representative elements in the first period. Draw two circles or x's on the energy level circle to represent two electrons.
(c) On the next circle, moving away from the nucleus, point out that a maximum of eight electrons are represented, directly relating to the number of representative elements in the second period. Draw eight circles or x's on the energy level circle to represent eight electrons.
(d) Remind the students that, so far, 10 electrons have been accounted for, but since each sodium atom has 11 electrons, one more energy level is needed. This third energy level correlates with the third period or row. Draw one circle or x to represent one electron.
(e) Point to the electron(s) in the outer energy level and inform the students that for a chemical reaction to occur, this energy level must be completely full or completely empty. Since this energy level can hold up to eight electrons (because it is in the third period and the third period has eight elements), and it now contains one electron, tell the students that it must give up one electron or gain seven electrons from another atom for a successful chemical reaction.
(f) Ask which appears to be easier: giving up one electron or gaining seven electrons. The correct answer is giving up one electron.
(g) Draw a line through the 11E which was written earlier, and write 10E to represent the number of electrons after one is given up.
(h) Compare the current numbers of positive charges with the number of negative charges. There is one more positive charge after an electron is given up. Explain that this represents the oxidation number.

4. This process is fairly easy to understand (steps a-h above) through the first 36 elements, where the energy levels are 2, 8, 8, and 18, respectively.

5. Repeat this process with an element in the same family. Lithium, for example, has an atomic number of three. It is located in the second period, has two electrons in the first energy level and one electron in the second energy level.

(a) Ask the students if they notice any similarities between Lithium's and Sodium's outer energy levels. (Both have only one electron.)
(b) Tell the students that every member of the alkali metal family has one electron in its outer energy level, so each has an oxidation number of one.
(c) Invite the students to practice drawing electron energy level diagrams for Beryllium and Magnesium. Ask what similarities they discover in the outer energy levels. (Each has two electrons.) Since they each have two electrons, they'll give up two electrons to form chemical bonds, so the oxidation numbers equal two. Point out that this is true for the entire family.
(d) Have the students practice drawing electron energy level diagrams for the halogen family (#17) and the oxygen family (#16).
(e) Members of the halogen family have seven electrons in their outer most energy levels. It is easier to fill this energy level by gaining one electron than by giving up seven electrons. The oxidation number for all members of the halogen family is ?-1?.) Members of the oxygen family have six electrons in their outer energy levels. It is easier to fill this energy level by gaining two electrons than by giving up six, so the oxidation number for the elements is -2.

6. In the associated file are practice problems as a guide for the novice chemistry teacher/student. They include a lot of detail and are intended as a formative assessment tool to help guide the students' learning. Every representative element could be evaluated using these questions.

#### Assessments

Formatively assess thoughout the lesson as individual students are asked questions. Specifically, ask about the number of protons and electrons an element has, its atomic number, what period is the element in, what family is the element in?

Individually assess students using the following questions/tasks as best suited to the needs of your class. Answers can be oral, written on paper or on the board:
(a) Why are the number of protons and electrons of an element the same?
(Because the positive and negative charges must be balanced for stability)
(b) Demonstrate, through use of an electron energy diagram, why the oxidation numbers within a family are the same. (The electron energy
diagrams within a family of elements all have the same number of eletrons in the outer energy level.)
(c) List the subatomic particles of an atom. (Proton, neutron, electron)
(d) What are the charges of each subatomic particle? (Positive, neutral, negative)
(e) Define oxidation number. (The number of electrons that an atom gains, loses, or shares when bonding with another atom.)

#### Extensions

Less able students may be provided paper which has concentric circles already printed and a paper copy of the periodic table on which to write.

Advanced students may be challenged with writing the chemical equation for NaCl and then demonstrating their understanding of oxidation numbers by drawing electron energy levels for sodium and chlorine. The final step is to identify which electrons move where (from which energy level to which energy level) when the NaCl bond forms.