Lesson 2 Molecules & Extended Structures
Essential Question: How are atoms combined to form different molecules and extended
structures?
Introduction
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Scientists use models to explore how atoms of elements group together to form the many different
materials and objects in the world around you. Even though atoms molecules occur on the atomic
scale, by using proportional models, you can visualize how atoms combine.
1. Molecules and Chemical Bonds
a. There are millions of materials that we use and encounter every day, yet there are only 92
naturally occurring elements.
b. Atoms of elements can come together to form more complex particles to form the millions of
materials
c. Chemical Bonds: The connections between atoms in more complex particles
d. Molecules: a group of two or more atoms that are held together by chemical bonds; a unique
molecule has a unique composition and structure
e. Some types of molecules have stronger bonds than other types of molecules
f. Just as atoms are too small to be seen with your eyes, molecules are also too small to be seen,
so scientists use models.
i. Model: A representation of an object that makes important aspects easier to see.
ii. By using models, you can quickly depict molecules in a visual way that everyone can see.
iii. 2D drawings called line-angle formulas to represent molecules where atoms are
depicted with their atomic symbol and the bonds are shown with lines
iv. 3D models called ball-and-stick models atoms are depicted as round balls and the the
bonds between atoms are shown as narrow sticks
1. Scientists use different colors and sizes to represent the different elements
a. Oxygen = Red
b. Hydrogen = White
c. Carbon = Black
d. Nitrogen = Blue
2. Composition and Structure of Molecules
 Molecules come in all sorts of shapes and sizes
 A unique molecule always has a unique composition and structure
a. Composition
i. Atomic Composition: The types of elements that make up a substance, as well as the
specific ratios of each kind of element that make it up
1. The number and type of elements of the molecule affect the characteristics of
the molecule and what it looks like
2. Example: Na3C6H5O7
a. Na3 = 3 Sodiums
b. C6 = 6 Carbons
c. H5 = 5 Hydrogens
d. O7 = 7 Oxygens
b. Structure
i. Not only do molecules vary in their composition that affect their characteristics but so
does the chemical structure of the molecules
ii. Chemical Structure: A specific way in which the bonds and atoms are arranged within
the molecules of a substance
iii. Butane and Isobutane have the same atomic composition, but they have different
atomic structures which makes these two molecules have very different characteristics.
3. Crystals and Polymers
a. Crystals
i. Crystal: A solid structure whose atoms or molecules are arranged in repeating, threedimensional subunits
ii. Can be formed with only one element or with multiple elements
iii. Can go on for forever in all three dimensions
iv. Just as individual atoms can combine to form crystals, so can molecules that are held
together in repeating patterns by strong attractions
b. Polymers
i. Polymer: A structure that is made of smaller repeating subunits in a chain
ii. Unlike crystals, in which the subunit could be atoms or molecules, in polymers, the
subunit is always a molecule
c. Extended Structures
i. Crystals and polymers fall into the larger category of extended structures
ii. Extended Structures: A structure whose subunits occur in a constant ratio and are
arranged in a repeating pattern; the subunit can be individual atoms or a molecule
iii. Does not have a specific composition; the composition of an extended structure is the
ratio of elements that make it up.
4. Patterns You Can See
a. Although you cannot see the atoms of different molecules and extended structures, their
atomic-level structure result in the patterns that you can see
Lesson Summary
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Molecules and Chemical Bonds: Atoms can group and form chemical bonds with each other. Though
we cannot see the molecules that these atoms have formed, we can use models to help us visualize
and understand them.
Composition and Structure of Molecules: Molecules are groups of atoms held together by chemical
bonds. A molecule has a unique composition and chemical structure. Its composition is the number of
each type of atom, and its structure is the way that those atoms are bonded together. Both a
molecule's composition and structure determine its characteristics.
Crystals and Polymers: Sometimes atoms form extended structures, a group of atoms or molecules in
a distinct ratio that are arranged in a repeating pattern. Crystals can be made up of atoms of one
element grouped together, atoms of more than one element grouped together, or molecules grouped
together. Polymers are a chain of smaller molecules bonded together.
Patterns You Can See: The atomic level structure of extended structures results in patterns that you
can see.