Rock Cycle - Chemistry (3)
Pre Lab 

   
OBJECTIVES:
  • Exploring elements and compounds
  • Comparing elements on the periodic table.
VOCABULARY:
  • element
  • compound
  • matter
  • periodic table
MATERIALS:
  • Periodic Table Placemats

Students compare elements with compounds.

BACKGROUND:

All known substances can be classified as solids, liquids, gases, or plasma. In addition, a fifth state of matter, the Bose-Einstein condensate has been discovered recently. However, it is not stable at normal earth conditions. Likewise, although plasma is the most abundant state of matter in the Universe, it is not common on the Earth under normal conditions, except for lightning. Most matter that students are familiar with will therefore be in a solid, liquid, or gaseous state.

An element is a pure substance that cannot be decomposed into simpler substances by normal chemical means. There are 109 different elements. Ninety of these are naturally occurring; the rest have been created in laboratories. Elements 110 and 118 are still being researched on. There will be more elements as technology can identify them. A symbol is used to represent the full name of an element. For example, H represents hydrogen; O represents oxygen, and Al represents aluminum. Sometimes the Latin name for an element is used as the basis for its symbol, for instance K represents potassium (kalium in Latin).

Three subatomic particles compose elements: protons, neutrons, and electrons. Protons, which have an electrical charge of +1, and neutrons, which have a neutral charge, make up the nucleus of an element. This nucleus is surrounded by a "cloud" of electrons, each of which as a charge of -1. The electrons spin around the nucleus in what are called orbits or shells. Each of the orbits can contain a set number of electrons. For instance, the first orbital from the nucleus has 2 electrons, the second has 8, the third has 8, the 4th has 16 and the fifth has 32, and so on. Each shell may not be full, depending on the number of electrons in the element, and the inner shells fill before the outer shells fill. Sodium, for example, has 11 electrons, which are located in the first, second, and third shells (2+8+1.)

An element has a uniform composition. Different elements may join together; these combinations are called compounds. A compound can be separated into its component elements by chemical means. For example, common table salt is a compound made of two elements: sodium and chlorine. Table salt can be broken down into sodium and chlorine by mixing it with water. However, sodium and chlorine cannot be easily broken down into any simpler forms.

PROCEDURE:

  1. Discuss the properties of elements with the students. Review the structure of the periodic table. Ask students questions about the different elements and see if they can locate them on the periodic table.

  1. Review the difference between an element and a compound. The students should realize that an element cannot be broken down, whereas a compound can be subdivided into elements. You may wish to explain that in many instances, forming or breaking down a compound requires energy. For example, if you place a mixture of iron and sulfur in a bowl, they will not react. No compound will form. However, if iron and sulfur are mixed and then heated, they will combine and form a compound.
      

  2. Write the following examples of compounds and their constituent elements on the board. At this point, do not be concerned with explaining the "endings" to the chemical words, such as chlorine versus chloride. These endings reflect the molecular structure of the compound.

    ELEMENTS

    COMPOUNDS

    Na-sodium

    NaCl (sodium chloride)

    Cl-chlorine

    AgCl (silver chloride)

    K-potassium

    KCl (potassium chloride)

    Ag-silver

    KClO (potassium perchlorate)

    O-oxygen

    H2O (water)

      

  3. Use the Periodic Table placemats to explore elements with the students. When they examine the chart, the students may ask the meaning of the numbers surrounding the element symbols. The number in the upper left corner is the atomic number, i.e., the number of protons inside the nucleus of the element. The number in the lower left is the atomic mass or atomic weight, which is essentially a measurement of how heavy the element is.
      
  4. Explain the basic subatomic structure of elements. Tell the students that protons and neutrons reside inside the nucleus. The electrons spin around the nucleus in what are called orbits or shells. Each of the orbits represents a set number of electrons. For instance, the first orbit from the nucleus has 2 electrons, the second has 8, the third has 8, the 4th has 16 and the fifth has 32, and so on. Sodium for instance, has 11 electrons located in the first, second, and third shells (2+8+1.)

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