Transcript Chapter 6 Chemical Bond

Chapter 6
Chemical Bonds
Why Bond?
• An atom’s goal is to be stable
• This means that the highest occupied
energy level is filled with electrons
• For most elements the highest
energy level is filled when it contains
8 electrons.
• (i.e. Noble Gases)
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Examples of electron dot
configurations
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1)Ionic Bonds- chemical bond
between a metal and a
nonmetal.
 Electrons are transferred from
metal to nonmetal
 Results in positively charged
metal cation and negatively
charged nonmetal anion
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What does this look like?
When sodium reacts with chlorine, an
electron is transferred from a sodium
atom to a chlorine atom. Each atom ends
up with a more stable electron
arrangement than it had before the
transfer.
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Formation of Ions
When an atom gains or loses an electron,
the number of protons is no longer equal to
the number of electrons.
• The charge on the atom is not balanced, and
the atom is not neutral.
• An atom that has a net positive or negative
electric charge is called an ion.
• The charge on an ion is represented by a plus
or a minus sign.
• If an atom gains an electron the charge is 1• If an atom loses an electron the charge is 1+
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Examples of common ions…
Anions: O2S2N3-
FIP3-
Cations: Cr2+
Fe3+
Cu +
Mn2+
Co3+
Cu2+
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Ionization Energy
• The amount of energy used to remove an
electron is called ionization energy
• It varies from element to element
• It increases from left to right in the
periodic table.
• It takes more energy to remove an
electron from a nonmetal than a metal
• It generally decreases from top of a group
to bottom.
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2) Covalent Bonds- chemical bond
between two nonmetals.
 Electrons are shared, but not always
equally
 Non-metals bond covalently to
achieve
stability (8 valence electrons)
 Not as strong as ionic bond
This illustration shows four ways to
represent a covalent bond between two
hydrogen atoms.
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• Many nonmetal
elements exist as
diatomic
molecules.
Diatomic means
“two atoms.”
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Covalent Bonding:
sharing electrons
 When two atoms share
one pair of electrons, it is
called a single bond.
Example Cl:Cl or Cl-Cl
 Two atoms sharing two
pairs of electrons =
double bond.
 Two atoms sharing
three pairs of electrons =
triple bond.
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Polar Covalent Bonds
• A covalent bond in which electrons
are not shared equally.
• When atoms form a polar covalent
bond, the atom with the greater
attraction for electrons has a
partial negative charge. The other
atom has a partial positive charge
• The type of atoms in a molecule
and its shape are factors that
determine whether a molecule is
polar or nonpolar.
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Attraction Between
Molecules
• In a molecular compound
there are forces of attraction
between molecules.
• Attraction between polar
molecules are stronger than
between nonpolar molecules.
• When a molecule has only
two atoms, it will be
polar.
• When molecules have
more than two atoms, the
answer is not obvious.
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Naming Compounds &
Writing Formulas
• The name of an ionic compound
must distinguish the compound from
other ionic compounds containing the
same elements. The formula of an
ionic compound describes the ratio of
the ions in the compound.
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Binary Ionic Compounds
•A compound made from only two
elements is a binary compound.
•The names have a predictable
pattern: the name of the cation
followed by the name of the
anion.
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• This table lists
eight common
anions. The
name of an
anion is
formed by
adding the
suffix –ide to
the stem of
the name of
the nonmetal.
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Metals With Multiple Ions
Many transition metals form more than
one type of ion. Many paint pigments
contain compounds of transition
metals.
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Polyatomic Ions
• A covalently bonded
group of atoms that
has a positive or
negative charge and
acts as a
unit is a polyatomic
ion. Most simple
polyatomic ions are
anions.
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Time to try one…
• What is the formula for the ionic
compound calcium chloride?
– Start off by listing the symbols and
charges for the cation and anion.
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List the symbols and charges for the
cation and anion.
Ca with a charge of 2+ and Cl with
a charge of 1–
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Determine the ratio of ions in the
compound.
It takes two 1– charges to balance
the 2+ charge. There will be two
chloride ions for each calcium ion.
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Answer…
•CaCl2
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Naming Molecular
Compounds
• The general rule is that the most
metallic element appears first in the
name. These elements are farther to
the left in the periodic table.
• If both elements are in the same
group, the more metallic element is
closer to the bottom of the group.
• The name of the second element is
changed to end in the suffix -ide, as
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in carbon dioxide.
There may be more than
one molecular compound
that can exist with the
same two elements.
The Greek prefixes in the
table are used to name
molecular compounds. The
prefix octa-means “eight,”
as in the eight tentacles of
an octopus.
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Two compounds that contain nitrogen and
oxygen have the formulas N2O4 and NO2.
• The name of the compound with the formula
N2O4 is dinitrogen tetraoxide.
• The name for the compound with the formula
NO2 is mononitrogen dioxide. The prefix monooften is not used for the first element in the
name, so a more common name is nitrogen
dioxide.
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Let’s try one…
What is the formula for diphosphorus
tetrafluoride?
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• Because the compound is molecular, look for
elements on the right side of the periodic
table.
• Phosphorus has the symbol P. Fluorine has the
symbol F.
• Di- indicates two phosphorus atoms, and tetraindicates four fluorine atoms.
• The formula for the compound is P2F4.
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Last but not least…
Metallic Bonds
• A metallic bond is the attraction between metals.
• Valence electrons move freely among atoms of
metals in a sea of shared electrons.
• The mobility of electrons explains some of metals
properties such malleability and being good
conductors of heat and electricity
• Although the electrons are moving among the
atoms, the total number of electrons does not
change. So, overall, the metal is neutral.
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• Scientists can design alloys with specific
properties by varying the types and
amounts of elements in an alloy.
• An alloy is a mixture of two or more
elements, at least one of which is a metal.
Alloys have the characteristic properties of
metals.
• Examples: Brass & bronze are alloys of
Copper. Steel is an alloy of iron
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