Chapter 19*CHEMICAL BONDS

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Section 1—Stability in Bonding
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Some of the matter around you is in the form
of uncombined element, such as copper,
sulfur, and oxygen. But, like many other sets
of elements, these 3 elements unite
chemically to form a compound when the
conditions are right.
The green coating on the Statue of Liberty
and some old pennies is a result of this
chemical change.
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One compound in this coating, is a new
compound called copper sulfate.
Copper sulfate isn’t shiny and copper colored
like elemental copper. Nor is it a pale-yellow
solid like sulfur or a colorless, odorless gas
like oxygen.
Copper sulfate has its own unique properties.
SODIUM (SHINY, SOFT, SILVERY
METAL)
CHLORINE (POISONOUS
GREENISH-YELLOW GAS)
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A chemical formula tells what elements a
compound contains and the exact number of
atoms of each element in a unit of that
compound.
EXAMPLE—H2O =CHEMICAL FORMULA
H=hydrogen; O=oxygen; and a subscript of 2
= 2 atoms of hydrogen and if no subscript = 1
atom of oxygen
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H2SO4 =
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2 hydrogen atoms
1 sulfur atom
4 oxygen atoms
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Why do atoms form compounds?
Atoms combine when the compound formed
is more STABLE than the separate atoms.
115 elements—MOST combine with other
elements
GROUP 18—the 6 NOBLE GASES seldom
form compounds. WHY?
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These elements all have 8 electrons in their
outer energy level, giving them a COMPLETE
outer energy level. EXCEPT He—only 2
electrons, but a COMPLETE outer energy level.
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An atom is chemically stable (will not form compounds)
when its outer energy level is COMPLETE.
He
= 2 outer electrons
Ne
= 8 outer electrons
Ar = 8 outer electrons
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How does that relate
to their ability to make
compounds?
• HYDROGEN contains 1
electron in its 1 energy
level. (INcomplete,
UNstable)
• This is why so many
hydrogen containing
compounds, including
WATER exist on Earth.
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In contrast, helium’s
outer energy level
contains 2 electrons.
Helium already has a full
outer energy level by
itself and is COMPLETE
and STABLE.
Helium rarely forms
compounds, but, by
itself, the element is a
commonly used gas.
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Atoms with INcomplete outer energy levels
can lose, gain, or share electrons to obtain a
STABLE outer energy level.
They do this by combining with other atoms
that also have partially complete outer
energy levels.
RESULT: Each achieve stability.
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Sodium--GROUP 1
1 electron in its outer
energy level
(INCOMPLETE,
UNSTABLE)
Chlorine—GROUP 17
7 electrons in its outer
energy level
(INCOMPLETE,
UNSTABLE)
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When these atoms
combine, sodium loses
1 electron and chlorine
gains 1 electron.
•
RESULT: Sodium and
chlorine both receive a
COMPLETE and
STABLE outer energy
level.
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In the compound water,
each hydrogen atom
needs 1 electron to fill its
outer energy level.
The oxygen atom needs
2 electrons for its outer
energy level to be stable
with 8 electrons.
HYDROGEN and
OXYGEN share electrons
instead of gaining or
losing electrons.
WATER—
H2O
•When atoms
gain, lose, or
share electrons,
an attraction
forms between
the atoms, pulling
them together to
form a compound.
•This attraction or
force that holds
atoms together is
a CHEMICAL
BOND.
Section 2—TYPES OF BONDS
An atom that has LOST or GAINED electrons is
called an ion.
 An ion is a charged particle, because it now has
either MORE or LESS ELECTRONS than protons.
 The POSITIVE and NEGATIVE charges are NOT
BALANCED.
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BEFORE (INDIVIDUAL ELEMENTS): PROTONS =
ELECTRONS
NOW (GETTING READY TO FORM COMPOUNDS):
PROTONS ≠ ELECTRONS
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Some of the most common compounds are
made by the LOSS and GAIN of just 1
ELECTRON. (Group 1 element + Group 17
element) Ex. NaCl
Examples…Sodium chloride (table salt);
sodium fluoride (anticavity ingredient in
toothpaste); and potassium iodide
(ingredient in iodized salt)
Na
 LOSES AN ELECTRON
Na+
(1 LESS ELECTRON = POSITIVE CHARGE)
The 1+ charge is shown as a superscript written
AFTER the element’s symbol, Na+, to indicate its
charge.
 F
GAINS AN ELECTRONF (1 MORE ELECTRON = NEGATIVE CHARGE)
 RESULT: The compound has a neutral charge, because
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positive and negative charges of the ions cancel each other.
Ex. NaF (Another Ex. Na+ and Cl
NaCl )
An ionic bond is a force of attraction between
the opposite charges of the ions in an ionic
compound. (Metal + Nonmetal)
 Ionic bond with more than 1 electron
involved…
MgCl2
MagnesiumGroup 2 and ChlorineGroup 17
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At the same time, 2 chlorine atoms GAIN 1
electron each and become NEGATIVE
charged ions, Cl-.
A MAGNESIUM atom has 2 electrons to
LEND, but a single CHLORINE atom needs to
BORROW only 1 electron.
Therefore, it takes 2 chlorine atoms to take
the 2 electrons from the 1 magnesium ion.
Magnesium LOSES 2
electrons = Mg2+
 2 Chlorine atoms GAIN
2 electrons = ClCl(Each Cl atom GAINS 1
electron)
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The POSITIVE CHARGE of the 1 magnesium
ion is exactly EQUAL to the NEGATIVE
CHARGE of the 2 chloride ions.
***Therefore, the compound is NEUTRAL.
***Ionic bonds usually are formed by
bonding between METALS and NONMETALS
(losing/gaining electrons).
(Examples: NaCl and MgCl2)
Some atoms of NONMETALS are unlikely to LOSE
or GAIN electrons.
 For example, the elements in GROUP 14 have 4
electrons in their outer energy level. They would
have to lose or gain 4 electrons in order to have a
stable outer energy level.
 The loss of this many electrons takes a great deal of
energy. Each time an electron is removed, the
nucleus holds the remaining electrons even more
tightly.
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The attraction that forms
between atoms when they
SHARE electrons is known
as a covalent bond.
A neutral particle that
forms as a result of
electron sharing is called a
molecule.
O = 6 electrons
H = 1 electron
 H = 1 electron
 2 SINGLE COVALENT BONDS
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N = 5 electrons
N = 5 electrons
***3 pairs of electrons
represent 3 bonds
 O = 6 electrons
 O = 6 electrons
***2 pairs of electrons
represent 2 bonds
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Electrons are not always shared equally
between atoms in a covalent bond.
The strength of the attraction of each atom
ot its electrons is related to…
SIZE of the atom
CHARGE of the nucleus
TOTAL NUMBER OF ELECTRONS the atom
contains
C = 4 electrons
Cl = 7 electrons
***4 Chlorine atoms needed
to equally share electrons,
so that carbon and chlorine
all have 8 electrons in their
outer energy levels.
 CCl4
 This type of molecule does
not have oppositely
charged ends.
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Section 3—WRITING FORMULAS AND NAMING COMPOUNDS
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The first formulas of compounds you will
write are for binary ionic compounds.
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A binary compound is a compound that is
composed of 2 elements.
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EXAMPLE—Potassium iodide, the salt
additive, is a binary ionic compound.
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1.
2.
What do you need to know before you can
write the formula?
Which ELEMENTS are involved?
What NUMBER OF ELECTRONS are lost,
gained, or shared? (IN ORDER TO BECOME
STABLE)
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An OXIDATION NUMBER
tells you how many
electrons an atom has
GAINED, LOST, or
SHARED to become
stable. (p. 588)
For ionic compounds, the
OXIDATION NUMBER is
the same as the CHARGE
OF THE ION.
Na ion
Cl ion
=
=
CHARGE
1+
1-
OX. #
1+
1-
• Na = G-1, 1 outer electron (LOSES 1
ELECTRON OR LOSES 1 NEG—CHARGE IS
POS)
• Cl = G-17, 7 outer electrons (GAINS 1
ELECTRON OR GAINS 1 NEG—CHARGE IS
NEG)
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Although the individual IONS in a compound carry
CHARGES, the COMPOUND itself is NEUTRAL.
***A formula must have the right number of
POSITIVE IONS and the right number of NEGATIVE
IONS so the charges BALANCE.
EXAMPLE…Sodium chloride (NaCl)
Na+
1+
Cl10
(Compound is neutral/0 charge)
Ca2+
=
2+
F1=
1In this case, you need to have 2 fluoride ions for every
calcium ion in order for the charges to cancel and the
compound to be NEUTRAL.
 Ca
F
 2+
+
1 2+
+
2(1-)
 2+
+
2=0 (COMPOUND IS NEUTRAL)
 1 ATOM OF Ca and 2 ATOMS OF F;
CaF2
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Al3+
=
3+
O2=
2What is the least
common multiple of 3
and 2?
 6
(2 x 3)
 2 Al ions and 3 O ions
are needed in order to
have a 6+ charge and a
6- charge
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IONS CHARGES
Al= 2 x 3+
= 6+
O= 3 x 2= 60
• Therefore, the nuetral
compound =
• Al2O3
1.
2.
Write the symbol of the element or polyatomic ion (ions
with more than 1 atom)that has a positive oxidation number
or charge. Ex. Na
***H, ammonium ion (NH4), and all metals have positive
oxidation numbers.
Ex. Na = metal = +
Write the symbol of the element or polyatomic ion with the
negative oxidation number. Ex. Cl
***Nonmetals (other than H) and polyatomic ions (other
than NH4) have negative oxidation numbers.
Ex. Cl = nonmetal = -
3.
Use subscripts next to each ion so that the
sum of the charges is ZERO.
Na ion
Cl ion =
CHARGE
1+
=
1-
Ex. Therefore, 1 atom of sodium and 1
atom of chlorine is needed…NaCl
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Not all compounds are binary (contain 2
elements). Baking soda, used in cooking, has
the formula:
NaHCO3
Which 4 elements does it contain?
Na = sodium H = hydrogen
C = carbon O = oxygen
poly = many
polyatomic = many
atoms
 A polyatomic ion is a
positively or negatively
charged covalently
(nonmetals) bonded
group of atoms.
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1.
You can name a binary
ionic compound (2
elements—1 metal and 1
nonmetal)
(Using TABLE 2-p.588,
check to see if the
positive ion is capable of
forming more than 1
oxidation number.) Write
the name of the positive
ion (METAL). Ex. Barium
2.
3.
Write the root name of the negative ion
(NONMETAL). The root is the first part of
the element’s name. (chlorine = chlor-;
oxygen = ox)
Ex. Fluorine = fluor
Add the ending –ide to the root.
Ex. BaF2 = Barium fluoride
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1.
2.
The polyatomic ion in baking
soda is the hydrogen carbonate
ion. (HCO3-)
To name a compound that
contains one of these ions…
Write the name of the positive
ion (METAL). Ex. Sodium
Write the name of the negative
ion (NONMETALS)
TABLE 4—P.591-Hydrogen
carbonate (Sodium hydrogen
carbonate)
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Potassium sulfate
WRITING FORMULAS FOR COMPOUNDS
CONTAINING POLYATOMIC IONS, FOLLOW
THE RULES FOR BINARY COMPOUNDS,
WITH 1 ADDITION.
***When more than 1 polyatomic ion is
needed, write parentheses around them
before adding the subscript…
Ba2+
 ClO31 Ba
ClO3
_____________________
2+
11 (2+) +
2(1-)
(2+)
+
(2-) = 0
Ba(ClO3)2


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