Grade Level: High School
Time: 45 minutes
Content Standard: NSES Physical Science, structure & properties of matter: Bonds are createdwhen electrons are transferred or shared among elements. Atoms or ions may bind together to form molecules or crystalline solids (e.g. salts).
Ocean Literacy Principle 1e: Most of Earth's water (97%) is in the ocean. Seawater has unique properties: it is saline, its freezing point is slightly lower than fresh water, its density is slightly higher, its electrical conductivity is much higher, and it is slightly basic. |
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Big Idea |
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Sodium chloride or common salt is the chemical compound NaCl, composed of the elements sodium and chlorine. Salt occurs naturally in many parts of the world as the mineral halite. Seawater has lots of salt; it contains an average of 2.6% (by weight) NaCl, or 78 million metric tons per cubic kilometer. |
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Key Concepts |
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- Molecular compounds are made of individual molecules that are bound together by shared electrons (i.e. covalent bonds).
- Ionic compounds consist of two or more ions that are held together by electrical attraction. On of the ions has a positive charge (called a cation) and the other has a negative charge (termed anion).
- Salt is an ionic compound, consisting of a crystal, lattice structure of the two ions Na+ and Cl-.
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Essential Questions |
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- Where does salt come from?
- Why is the ocean salty?
- What is salt used for?
- Will salt dissolve in all liquids?
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Knowledge and Skills |
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- Explain the general relationship between an element's Periodic Table Group Number and its tendency to gain or lose electron(s).
- Explain the difference between molecular compounds and ionic compounds.
- Use a model to demonstrate sodium chloride's cubic form which results from its microscopic crystal lattice.
- Describe the nature of the electrostatic attraction of the oppositely charged ions that holds the structure of salt together.
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Prior Knowledge |
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- Atoms may stick together in well-defined molecules, or may be packed together in large arrays. Different arrangements of atoms into groups compose all substances.
- When substances interact to form new substances, the elements composing them combine in new ways. In such recombinations, the properties of the new combinations may be very different from those of the old.
- Atoms are made of a positive nucleus surrounded by negative electrons.
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Common Preconceptions |
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Salt water is full of sodium chloride molecules.
Salt is NOT made of NaCl molecules. Salt is made of a three dimensional checkerboard of
oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNa. When salt dissolves, it turns
into independent atoms.
Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine. The atoms are not poisonous and
reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron.
Because of this, the remaining electrons behave as a filled electronic shell, so they cannot easily react and form chemical bonds with other atoms except by
electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so like sodium it too cannot bond with other atoms.
These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from
each other as the water molecules surround them, and they float through the water separately.
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Concept Map |
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Primarily for the teacher's use, the map provided here relates to the branch "Properties of Water" from the comprehensive Aquarius Concept Map - Water & its patterns on Earth's surface. |
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Background |
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Chemically, table salt consists of two elements, sodium (Na) and chloride (Cl). Neither element occurs separately and free in nature, but are found
bound together as the compound sodium chloride. It occurs naturally in many parts of the world as the mineral halite and as mixed evaporites in salt lakes. Seawater contains an average
of 2.6% (by weight) sodium chloride, or 78 million metric tons per cubic kilometer, an inexhaustible supply. Table salt, when viewed with a magnifying glass, can be seen
to consist of tiny, cube-shaped flakes.
The microscope image seen at the right, shows table salt crystals magnified by 100. - From The Salt Institute.
Did you ever wonder why the oceans are filled with salt water instead of fresh? Just where did salt come from? And is it the same salt you find on a dining room table?
Most of the salt in the oceans came from land. Over millions of years, rain, rivers, and streams have washed over rocks containing the compound sodium chloride
(NaCl), and carried it into the sea. Some of the salt in the oceans comes from undersea volcanoes and hydrothermal vents. When water evaporates from the surface of the
ocean, the salt is left behind. - From the Office of Naval Research |
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Materials |
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Periodic Table; Chemistry reference book or web access (optional); Spheres of styrofoam (some spheres should be about twice the size of others), clay, etc.; Toothpicks, straws, etc. |
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Preparation |
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This is a good precursor to the “ Electrolysis of Salt Water” activity. |
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Activity |
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- Have the students read the "Background" section (above) and then find sodium and chlorine on a Periodic Table. Ask them to record the following information on their Student Worksheet: Atomic Weight, Atomic Number, and Periodic Table Group Number. If appropriate resources are available (e.g., reference books, web access), also ask the students to research the "State" (i.e., solid, liquid, or gas) and "Color" of these elements at room temperature.
- If students are not already familiar with the general relationship between an element's Periodic Table Group Number and its tendency to gain or lose electron(s), have them research this topic. The relevant Groups for the elements (i.e., Na and Cl) under consideration in this activity:
- Group 1 (or I) Elements – Have one electron in their outer shell. Each element in this group has a tendency to lose a single electron to form a singly charged positive ion. Elements in this group include Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium. These are known as "Alkali Metals" and are so reactive with water that they must be stored under oil.
- Group 17 (or VIIA) Elements – Have seven electrons in their outer shell. Each element in this group has a tendency to gain a single electron to form a singly charged negative ion. Elements in this group include Fluorine, Chlorine, Bromine, and Iodine. These are known as "Halogens" or "salt formers." They form ionic compounds with metals such as sodium.
- This information should help students classify whether Na and Cl are cations (i.e., positively charged) or anions (i.e., negatively charged) on the Student Worksheet. Likewise, the students should also be able to supply the chemical formula for sodium chloride on the Worksheet (NaCl).
- In chemistry, a compound is a pure substance that consists of atoms or ions of two or more different elements that cannot be readily separated by physical means.
- Molecular compounds are made of individual molecules – not ions. Atoms in the compound are bound together by shared electrons (i.e., covalent bonds). Water (H2O) and carbon dioxide (CO2) are examples.
- Ionic compounds are made of ions – not moleclues. Cations and anions are held together by electrostatic forces (i.e, ionic bonds). Potassium bromide (KBr) and magnesium chloride (MgCl2) are examples.
- Ask the students: "Are salts molecular compounds or ionic compounds?" (Ionic compounds.)
- In ionic compounds, the strong electrostatic force of attraction reaches out in all directions. Thus each ion is surrounded by other ions of opposite charge. The term "crystal lattice" is often used to describe the structure of the cations and anions in ionic compounds. Look at the microscope image (above) of table salt. What type of structure would you expect it to have at the atomic level? (Cubic lattice.)
- On the Student Worksheet, Atomic Radius data have been provided for Na and Cl. These measurements are in nanometers (nm), one billionth of one meter (i.e., 10,000 times smaller than the width of a human hair). These data can be used to answer the question posed in the Student Worksheet: "What is the size ratio between sodium and chlorine ions?" (0.186 : 0.099 or about 2 : 1)
- Given all the information they've acquired, challenge the students to construct a model of a typical sodium chloride compound. They may choose to use toothpicks, straws, styrofoam balls, clay spheres, etc. The most accurate models will have Na+ ions about twice the size of the Cl- ions.
- Students can check their results by viewing the Naval Research Laboratory's 3-D interactive salt crystal lattice structure (click on "visualize the structure").
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Assessment / Questions |
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- Using data from the Student Worksheet, answer the following questions:
- What is the molecular weight of sodium chloride? (58.4428)
- What is the relative weight percentage of Na in sodium chloride? (39.34%)
- What is the relative weight percentage of Cl in sodium chloride? (60.66%)
- Table salt is usually clear or white at room temperature. How does this compare to the state / color of Na and Cl at room temperature? Investigate salts' constituent elements to understand how the properties of Alkali Metals and Halogens change with increasing Atomic Number (i.e., from top to bottom of their column in the Periodic Table). What combinations of Alkali Metals and Halogens are found in nature? What are these salts used for?
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Adapted with permission from the Salt Institute's activity "Salt: The Essence of Life"; Glossary definitions from " EverythingBio.com" |
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