Are You Charged?

Rosemary WilsonSanta Rosa District Schools

Description

This lesson is a teacher-directed study of the charges on ions with an easy method of remembering charges based on elements' locations on the periodic table.

Objectives

The student knows that the number and configuration of electrons will equal the number of protons in an electrically neutral atom and when an atom gains or loses electrons, the charge is unbalanced.

Materials

-Student copies of Ions & Their Charges Worksheet (See Associated File)
-Laser pointer
-Classroom periodic wall chart
-Chalkboard and chalk or an overhead projector and marker

Preparations

1. Make copies of the Ions & Their Charges Worksheet found in the associated file, one copy per student.
2. Acquire a laser pointer.

Procedures

Note:
A. The students should be well versed in the structure of atoms in that every atom is composed of a certain number of protons, the atomic number of the element, a certain number of electrons, the same as the number of protons for a neutral atom, and various numbers of neutrons giving rise to different isotopes of an element.

B. The students should also have a basic understanding of electron configurations of elements, i.e. that there is a maximum of two electrons in an “s” shell, a maximum of six electrons in a “p” shell, a maximum of ten electrons in a “d” shell, and a maximum of fourteen electrons in an “f” shell. (For a lesson concerning electron configurations of elements, see Beacon Lesson Order, Order All Electrons.)

C. You might take a few minutes to review the addition and subtraction of numbers that have positive and negative signs to clear any misconceptions.

Steps #1 - #34, 20 minutes
1. Begin the lesson by reviewing the numbers of protons, electrons and neutrons in an element of your choice. You might call on one student to choose an element for student involvement. For this lesson, the example of calcium will be used first and oxygen last.

2. Explain to the students that an atom can differ in its number of neutrons and still be the same element, differ in its number of electrons and still be the same element, but cannot differ in its number of protons. If an atom differs in its number of electrons, the atom's charge is not “balanced” but becomes “unbalanced” and results in either a positive or a negative ion. Explain that an ion is just an atom with a charge.

3. Explain to the students that in order for an atom to be electrically neutral, the number of electrons of an atom must equal the number of protons in that atom.

4. Highlight the position of the example element on the periodic wall chart. Calcium is number 20, in group IIA, the second column from the left.

5. Explain to the students that calcium has twenty (20) protons and twenty (20) electrons. You can also tell the students that the most abundant isotope of calcium has twenty neutrons but this lesson is unconcerned with the numbers of neutrons of atoms since the neutrons have no charges and do not affect the charges of ions of elements.

6. Explain to the students that calcium has two “s” electrons in its first period, two “s” electrons and six “p” electrons in its second period, two “s” electrons and six “p” electrons in its third period and two “s” electrons in its fourth period.

7. Tell the students that elements will lose, gain or share electrons in order to achieve a completed outer shell of electrons, commonly called a Perfect Octet, eight electrons.

8. Explain that this completed outer shell is eight electrons in all periods except for the first period where the completed outer shell is two.

9. Explain that elements will lose, gain, or share any type of electrons, “s,” “p,” “d,” or “f,” but for the most part, the electrons involved in forming ions will be “s” and “p” electrons of the last level added on an atom.

10. Highlight the example element on the periodic wall chart, calcium being the example used for this lesson. Ask the class how many “s” and “p” electrons calcium has in its outer level.

11. Solicit answers from several students to check if they answer two “s” electrons. Explain that calcium does not have any “p” electrons in its outer shell because it is in group IIA.

12. Pose this question to the class using your example element: “Would it be easier for calcium to lose two electrons and rely on the completed outer shell of the third period or would it be easier for calcium to gain six electrons in order to achieve a completed outer shell of eight electrons in the fourth period?”

13. The answer from any students who offer answers should be that it would be easier for calcium to lose two electrons.

14. Place the number +20 for twenty protons with positive charges on a chalkboard or an overhead projector. Remember to use your example element's atomic number for this illustration.

15. Draw a circle around the number. On the circle, write the number -20 for twenty electrons with negative charges. Make a mark through the twenty on the circle and write –18 (20 minus 2) for a loss of two electrons.

16. Ask the students what the resultant number would be if the number +20 was added to the number -18.

17. The students should answer +2 which is the charge on elements in group II.

18. Now solicit from the class another element from the right side of the periodic table to be used as the next example. The example used for this lesson is oxygen.

19. Explain to the students that oxygen is in group VIA, the second column from the right, then highlight the example element on the periodic table wall chart using the laser pointer.

20. Explain that oxygen has eight protons and eight electrons and the most common isotope of oxygen has eight neutrons.

21. Explain that oxygen has two “s” electrons in the first period, two “s” electrons and four “p” electrons in the second period.

22. Tell the students that oxygen too is trying to achieve a completed outer shell of eight electrons.

23. Pose this question to the class using your example element: “Would it be easier for oxygen to lose six electrons, two “s” and four “p,” to achieve the completed outer shell of the first period or would it be easier for oxygen to gain two electrons to achieve a completed outer shell of eight electrons in the second period?”

24. The answer from your students should be for oxygen to gain two electrons to achieve a completed outer shell in the second period.

25. Write the number +8 for eight protons having positive charges on the chalkboard or overhead projector.

26. Draw a circle around the number +8. Draw the number -8 for eight electrons having negative charges on the circle. Make a mark through the number -8 and place the number –10 (8 electrons plus 2 electrons) by the number.

27. Ask the students to add the number +8 to the number -10. The resultant should be -2 which is the charge on oxygen in group VIA.

28. Distribute the worksheets from the associated file to the students.

29. Tell the students that there are many rules to follow in all disciplines but there are also exceptions. Explain that the diagram on the worksheet is just a generalization that can be used at this time to learn the charges on elements. Explain that the exceptions to these charges will be learned at a future date when they have amassed more information regarding the charges of elements as ions.

30. Explain to the students that there is a symmetry involved with the diagram and that they should be able to learn the sequence because of the symmetry.

31. Explain the symmetry: Start with +1 and increase by +1 until group IVA is reached at the halfway mark. Explain that group IVA is in the middle so it will be +4 or -4, i.e., it can lose four, +4 , or gain four, -4. Now decrease the numbers by -1 for each new group from IVA to VIIA. Explain that group VIIIA is zero, 0.

32. Explain that all of the “d” block, transition metals, can be considered +2 at sometime as they all have two “s” electrons that can be lost. Tell the students they will be taught the exceptions for these elements at a future date.

33. Tell the students that all of the elements in the “f” block can be +3 as they are almost all capable of that charge. Tell the students they will probably not learn any of the charges on “f,” inner-transition, elements.

34. You may explain at this time that all metals will ONLY form positive ions so that the groups on the far right of the periodic table will have metals toward the bottoms of the groups and so these will not follow the generalities shown by the diagram. Also explain that the nonmetals that are at the tops of these far right groups, are the elements that are most likely to be studied by the students and therefore, the diagram is most helpful in determining the most likely charges for those elements.

Steps #35 - #37, 20 minutes
35. Tell the students that there are terms used for positive and negative ions which are “cation” for positive and “anion” for negative. Tell the students that they can remember the “cation” is positive as it has a “positive” sign in it when the “t” is crossed.

36. Tell the students that they will have twenty minutes to work the problems on the worksheet that you distributed in #27.

37. Walk around the classroom to spot check the students as they work the problems. Help any students that might need further explanation regarding charges on ions.

Steps #38 - #39, 20 minutes
38. When the worksheets are completed by most students, call on individual students to give their answers for the problems.

39. The students should get these problems correct but if there are some students who do not seem to understand the charges, use this last advice for them: “The charges on ions will involve a number and sign. The number of the answer will always be the number of electrons. All you have to do is place a positive sign or negative sign in front of the number. Remember that electrons are negative so if an atom gains electrons, it is gaining negatives so the sign will be negative. The logic is the same with the loss of electrons; if an atom loses electrons, it is losing negatives and therefore the sign will be positive.”

Assessments

1. The teacher makes a formative assessment of the students and their understanding of the information in this lesson by subjective analysis of the students as they follow the lesson, work on the worksheet, and give their answers to the problems and questions on the worksheet.

2. The students are assessed by a summative assessment instrument, specific to this lesson and the information presented, if the teacher feels that mastery of the benchmark has been achieved. The summative assessment instrument, Charges of Ions-Test, is located in the associated file.

3. The summative assessment instrument can be used as a reinforcement tool rather than an assessment instrument if the teacher feels that this will result in a greater understanding of the material by the students.

Extensions

This lesson has been written for Chemistry, however, it can be used in general science classes employing textbooks that include information on charges of ions.