4.2 Combinations of Atoms

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Earth Chemistry
Section 2: Combinations of Atoms
Preview
• Key Ideas
• Molecules
• Compounds
• Chemical Formulas
• Chemical Equations
• Balancing a Chemical Equation by Inspection
• Chemical Bonds
• Mixtures
• Maps in Action
Section 2
Earth Chemistry
Section 2
Key Ideas
• Define compound and molecule.
• Interpret chemical formulas.
• Describe two ways that electrons form chemical bonds
between atoms.
• Explain the differences between compounds and
mixtures.
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Section 2
Molecules
• Elements rarely occur in pure form in Earth’s crust. They
generally occur in combination with other elements.
• compound a substance made up of atoms of two or
more different elements joined by chemical bonds
• The properties of a compound differ from the properties
of the elements that make up the compound.
• molecule a group of atoms that are held together by
chemical forces
• A molecule is the smallest unit of matter that can exist by
itself and retain all of a substance’s chemical properties.
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Compounds
Click below to watch the Visual Concept.
Section 2
Earth Chemistry
Section 2
Chemical Formulas
• A chemical formula is a combination of letters and
numbers that shows which elements make up a
compound.
• A chemical formula shows the number of atoms of each
element that are required to make a molecule of a
compound.
• In a chemical formula, the subscript that appears after
the symbol for an element shows the number of atoms of
that element that are in a molecule. For example:
H2O = 2 H (hydrogen atoms) + 1 O (oxygen atom)
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Section 2
Chemical Equations
• Elements and compounds often combine through
chemical reactions to form new compounds.
• The reaction of these elements and compounds can be
described in a formula called a chemical equation.
Equation Structure
• In a chemical equation, the reactants (to the left of the
arrow) form the products (to the right of the arrow)
through chemical reactions.
• The arrow means “gives” or “yields.”
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Section 2
Chemical Equations, continued
Equation Structure, continued
• In the following equation, one molecule of methane, CH4,
reacts with two molecules of oxygen, O2, to yield one
molecule of carbon dioxide, CO2, and two molecules of
water, H2O.
CH4
+
methane +
2O2

oxygen yields
CO2
+ 2H2O
carbon
dioxide
+ water
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Chemical Equations, continued
The diagram below shows a chemical equation.
Section 2
Earth Chemistry
Section 2
Chemical Equations, continued
Balanced Equations
• A chemical equation must be balanced to be useful for
showing the types and amounts of the products that
could form from a particular set of reactants.
• An equation is balanced when the number of atoms of
each element on the right side of the equation is equal to
the number of atoms of the same element on the left
side.
• To balance an equation, you must put numbers called
coefficients in front of chemical formulas.
• A coefficient multiplies the subscripts in an equation.
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Balancing a Chemical Equation by
Inspection
Click below to watch the Visual Concept.
Section 2
Earth Chemistry
Section 2
Chemical Bonds
• The forces that hold together the atoms in molecules are
called chemical bonds.
• Chemical bonds form because of the attraction between
positive and negative charges.
• Atoms form chemical bonds by either sharing valence
electrons or transferring them from one atom to another.
• Scientists can study interactions of atoms to predict
which kinds of atoms will form chemical bonds together.
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Section 2
Chemical Bonds, continued
Reading Check
In what two ways do atoms form chemical bonds?
Atoms form chemical bonds by transferring electrons or by
sharing electrons.
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Section 2
Chemical Bonds, continued
Ions
• When an electron is transferred from one atom to
another, both atoms become charged.
• ion an atom or molecule that has gained or lost one or
more electrons and has a negative or positive charge
Ionic Bonds
• ionic bond the attractive force between oppositely
charged ions, which form when electrons are transferred
from one atom or molecule to another
• A compound that forms through the transfer of electrons
is called an ionic compound.
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Section 2
Chemical Bonds, continued
Covalent Bonds
• covalent bond a bond formed when atoms share one or
more pairs of electrons
• A compound that forms through the sharing of electrons
is called a covalent compound.
Polar Covalent Bonds
• A covalent bond in which the bonded atoms have an
unequal attraction for the shared electrons is called a
polar covalent bond.
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Section 2
Chemical Bonds, continued
The diagram below compares ionic bonds and covalent
bonds.
Earth Chemistry
Section 2
Chemical Bonds, continued
Reading Check
Why do water molecules form from polar covalent bonds?
The oxygen atom has a larger and more positively charged
nucleus than the hydrogen atoms do. As a result, the
oxygen nucleus pulls the electrons from the hydrogen
atoms closer to it than the hydrogen nuclei pull the shared
electrons from the oxygen. This unequal attraction forms a
polar-covalent bond.
Earth Chemistry
Section 2
Mixtures
• mixture a combination of two or more substances that
are not chemically combined
• Because the substances that make up a mixture keep
their individual properties, a mixture can be separated
into its parts by physical means.
Heterogeneous Mixtures
• Mixtures in which two or more substances are not
uniformly distributed are called heterogeneous mixtures.
Earth Chemistry
Section 2
Mixtures, continued
Homogeneous Mixtures
• In chemistry, the word homogeneous means “having the
same composition and properties throughout.”
• solution a homogeneous mixture throughout which two or
more substances are uniformly dispersed
• Liquids, gases, and solids can all be solutions.
• An alloy is a solution composed of two or more metals,
such as steel.
Earth Chemistry
Maps in Action
Element Resources in the United States
Section 2
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