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Grade
Level: High
School | Time:
45 min. 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.
Big
Idea
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.
Key
Concepts
- 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-.
Essential
Questions
- Where
does salt come from?
- Why
is the ocean salty?
- What
is salt used for?
- Will
salt dissolve in all liquids?
Knowledge
and Skills
- 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.
Prior
Knowledge
- 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.
Common
Preconceptions
- "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.
Concept
Map
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.
Background
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
Materials:
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.
Preparation:
This
is a good precursor to the “Electrolysis
of Salt Water” activity.
Activity
- 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").
Assessment
/ Questions
- 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?
Original
source:
Adapted
with permission from the
Salt
Institute's activity
"Salt: The Essence
of Life";
Glossary definitions from "EverythingBio.com"
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