UNIT 7 SECTION 1
NOTES
Compounds and Molecules
ELEMENTS: A
REVIEW
• Made of individual atoms.
• Cannot be broken down into simpler
substances.
• Listed on the periodic table.
• The building blocks of all matter.
MIXTURES: A REVIEW
• Different substances just placed together
• Substances keep their individual
properties
• Example: Mixing yellow and blue paint
to make green paint: color has changed
but not the properties
• Can be separated (Example: sand and
water)
+
→
COMPOUNDS
• Compound: 2 or more elements chemically
joined together in definite ratios
• The characteristics of each element are lost
when a compound is formed
• Think of Table salt: Sodium, a metal, combines
with chlorine, a gas, to make table salt, a small
white crystal.
+
→
MOLECULES
• Molecules are the smallest units of a
compound that retains the
characteristics of the compound.
This is one
molecule of
caffeine:
COMPOUND
CHARACTERISTICS
• Formed through CHEMICAL BONDING
• Compounds have a chemical formula
• Compounds are not easily separated
• New properties: different than the elements
that it is made from
CHEMICAL FORMULAS
• A compound ALWAYS has the same chemical
formula.
• A chemical formula shows the types and
numbers of atoms or ions making up the
simplest unit of the compound.
EXAMPLES OF
CHEMICAL FORMULAS
• Note: When there is no subscript, it is
understood to be “1”.
• The larger number in front is called a
coefficient
• H2O: Water: 2 Hydrogen atoms; 1 Oxygen
atom
• C12H22O11: Sugar: 12 Carbon atoms; 22
Hydrogen atoms; 11 Oxygen atoms
• NaCl: Table Salt: 1 Sodium atom; 1 Chlorine
atom
EXAMPLES OF CHEMICAL
FORMULAS
Use the chemical formula to list each element by
symbol and tell how many atoms there are in the
compound.
•
•
•
•
•
•
•
•
CO2
2H2O
Mg(OH)2
3NaHCO3
2H2SO4
2CaCO3
3C6H8O7
4Be(OH)2
EXAMPLES OF CHEMICAL
FORMULAS
Use the chemical formula to list each element by
symbol and tell how many atoms there are in the
compound.
•
•
•
•
•
•
•
•
CO2
2H2O
Mg(OH)2
3NaHCO3
2H2SO4
2CaCO3
3C6H8O7
4Be(OH)2
C=
H=
Mg =
Na =
H=
Ca =
C=
Be =
O=
O=
O=
H=
H=
C=
S= O=
C=
O=
H=
O=
O=
H=
O=
EXAMPLES OF CHEMICAL
FORMULAS
Use the chemical formula to list each element by
symbol and tell how many atoms there are in the
compound.
•
•
•
•
•
•
•
•
CO2
2H2O
Mg(OH)2
3NaHCO3
2H2SO4
2CaCO3
3C6H8O7
4Be(OH)2
C=1 O=2
H=4 O=4
Mg = 1 O = 2
Na = 3 H = 3
H=4 S=2
Ca = 2 C = 2
C = 18 H = 24
Be = 4 O = 8
H=2
C=3
O=8
O=6
O = 21
H=8
O=9
LAW OF DEFINITE
PROPORTIONS
• Law of Definite
Proportions:
Compounds are
ALWAYS made of
the same
elements in the
same proportion
by mass.
CHEMICAL
STRUCTURE
• Chemical Structure is the way the
compound’s atoms are bonded to make
the compound.
• Just as the structure of buildings can be
represented by blueprints, the structure of
chemical compounds can be shown by
various models. Different models show
different aspects of compounds.
MODELS OF
COMPOUNDS
• Ball and Stick Model
• Space Filling Model
• Structural Formula
PROPERTIES OF
COMPOUNDS
What determines many of a compound’s
properties?
• The atoms present in a compound
• The way the atoms are arranged
(chemical structure)
This shows how a crystal
lattice is formed from
sodium and chlorine
atoms to form table salt.
STRUCTURE AND
PROPERTIES
• Network Crystals: typically based on
nonmetals like carbon, silicon, and boron
• Held together by COVALENT bonds
• VERY strong network or lattice
• VERY high melting point and great
physical strength; takes a lot of energy
to weaken the bonds.
• Poor conductors of electricity
• Often very hard and strong; typically will
not dissolve in water.
STRUCTURE AND
PROPERTIES
• Examples of network
crystals:
• Quartz: made of silicon dioxide,
SiO2 Every Si atom is bonded to 4
oxygen atoms; all bond angles are the
same at 109.5˚, which holds the atoms
together in a strong, rigid structure
• Diamond
STRUCTURE AND
PROPERTIES
• Molecular Crystals: Some compounds are made
of molecules
• Compounds made of molecules DO NOT have
a strong attraction holding atoms together, so
they have a lower melting and boiling point than
compounds with ionic bonds.
• Example: A molecule of sugar is made between
bonds of carbon, hydrogen, and oxygen and
the attraction between the molecules is much
weaker than an ionic bond.
STRUCTURE AND
PROPERTIES
• Ionic Crystals: Some networks are made of
bonded ions, which have very strong
attractions between the positively and
negatively charged ions
• The strong attraction requires a lot of energy
to break the bonds, which is why these
network structures have high melting and
boiling points.
• Examples: table salt, NaCl; magnesium
oxide, MgO
STRENGTH OF ATTRACTION
BETWEEN MOLECULES
• The weaker the force of attraction
between molecules, the easier the
substance will be to break apart
• Molecules of gases have weak attractions
to one another, so they spread out and
take up a lot of space.
DIATOMIC
MOLECULES
• Diatomic Molecule: 2 atoms of the same
element that have covalently bonded together.
• 7 Diatomic molecules: Oxygen, Iodine,
Hydrogen, Nitrogen, Chlorine, Fluorine, and
Bromine
• Pneumonic device:
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