OBJECTIVES:

• Examining the periodic table.
• Determining why elements combine.

VOCABULARY:

• element
• negative
• periodic table
• positive

MATERIALS:

• Periodic Table placemats

Students explore why elements join together to form compounds.

Hematite

BACKGROUND:

The more exposure students have to the Periodic Table the more familiar they will become with the elements. The Periodic Table is arranged so that nonmetals are on the right side of the chart and metals on the center and to the left. The color coding on the chart indicates this classification. Note the right-descending "staircase line" separating nonmetal and metals. The elements to the immediate left of the "staircase line" are called "semi-metals" or "metalloids" because they can behave chemically as both metals or nonmetals. The metallic elements are characterized by a metallic luster and high electrical and thermal conductivity. The nonmetals may be gases, liquids or crystalline solids.

An element that has a charge is called an "ion." Generally, the metals have a positive charge (a positive ion) and the nonmetals a negative charge (a negative ion). There are several exceptions to this rule but there is no need to point this out to the students at this time. A chemical reaction can take place when a positively charged element meets a negatively charged element.

Minerals are compounds made of elements that have both negative and positive charges, which combine so that the overall charge equals zero, making the compound electrically neutral. For example table salt (the mineral halite) is composed of sodium, which has a +1 charge, and chlorine, which has a charge of -1. Since there are equal numbers of sodium and chlorine atoms in halite, the overall charge is zero. We will not be concerned with balancing charges in this activity, because the main point is to illustrate that positive and negative elements come together.

PROCEDURE:

1. Pass out the Periodic Table placemats. Explain the metals vs. nonmetals arrangement of the elements on the table to the students. Point out the staircase line. On the right of the line are the nonmetals. These can be liquids, gasses or crystalline elements. On the left of the line are the metals. The elements right beside the staircase line are "strange" elements that are called 'metalloids' or 'semi-metals'. They can act as both metals and nonmetals.
    
2. Tell the students that those elements to the left of the staircase line are attracted to those elements on the right. The metals have a positive (+) charge and the nonmetals have a negative (-) charge. This causes them to want to stick together so they can become neutral. A positive plus a negative produces a neutral compound.
     
3. Tell the students that when elements join together they form a new material called a compound. Compounds can look and act very differently from the elements that made them.
     
4. Write the following on the board:

   table salt = NaCl	quartz = SiO2	hematite = Fe2O3
   pyrite = FeS	galena = PbS	fluorite = CaF

5. With the class determine which is the positive and negative part of the minerals (as in chart below). Use NaCl (halite) as your example, by asking students to find Na and Cl on the periodic table. Have them identify whether the element is positive or negative by looking at its location on the periodic table.

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