Transcript Chemical Bonding What are two elements joined together called? • Two elements chemically bonded together are called a compound. • A compound has different.

Chemical Bonding
What are two elements joined
together called?
• Two elements chemically bonded together are
called a compound.
• A compound has different properties than the
elements that make it up
• A compound is represented
by a chemical formula
Chemical Formulas
• A chemical formula tells the elements that
make up a compound and the number of
atoms of each element in one unit of it.
• A compound always has the same
chemical formula
H20
NaCl
Si02
C12H22O11
Why do elements bond?
• Elements bond together in order to
achieve stability
• The noble gases rarely bond because they
are already stable
• An atom is chemically stable when it’s
outer energy level is complete
• Atoms do this by gaining, losing, or sharing
electrons with other atoms
• An atom that has lost or gained an
electron is called an ion
Valence Electrons
• Valence electrons are the electrons in the
highest occupied energy level of an
elements atoms.
• Valence electrons can be found by looking
at the periodic table.
Ion Formation - Cation
• An ion with a positive charge is called a cation
and is formed when an atom loses electrons
• This happens most commonly with metals
• An atom can become more stable by ionizing
• Na: 1s22s22p63s1 -> Na+: 1s22s22p6 octet
• Sodium now has the same electron configuration
as neon by losing one electron
Ion Formation - Anion
• An anion is formed when an atom gains
electrons
• This happens most typically with nonmetals
• Cl: 1s22s22p63s23p5 -> Cl-: 1s22s22p63s23p6 octet
• Ions produced from the atoms of the halogen
group are called halide ions
Types of Bonding - Ionic
• Compounds formed from cations and
anions are called ionic compounds
• The net charge on the compound is zero
• The electrostatic forces that hold the
compound together are called ionic bonds
• An ionic bond is usually
found between metals
and nonmetals
Ionic Compounds
• Ionic compounds exist as a
collection of positivly and
negativly charged ions
arranged in a repeating
pattern.
• The formula for ionic
compounds is refered to as
a formula unit, or the
lowest whole-number ratio
of ions in an ionic
compound
Properties of Ionic Compounds
• Ionic compounds are usually crystal solids at room
temperature
• The components are arranged in repeating three
dimensional patterns
• In solid NaCl, each sodium ion is surrounded by 6
chloride ions and each chloride ion is surrounded
by 6 sodium ions
• Each ion is strongly attracted to it’s neighbor and
repulsion effects are minimized
• The large attractive forces result in a stable
structure with a high melting point
Properties of Ionic Compounds
• The coordination number of an ion is the number of all
ions of opposite charge adjacent to it in a crystal
• NaCl has a coordination number of 6
• If the chemical formula of an ionic compound does not
contain equal parts of each element, each element will
have a different coordination number
• In TiO2, Ti4+ has a coordination number of 6 while O2- has
a coordination number of 3
Properties of Ionic Compounds
• Ionic compounds can conduct an electric current
when dissolved or melted in water
• When the substance is dissolved in water, ions
are free to move from one electrode to another
which produces a flow of electricity
Metallic Bonding
• A metallic bond is a bond
between two metals
• The valence electrons of
metal atoms can be viewed
as a sea of electrons, they
are mobile and can drift
freely from one part of the
metal to another
Metallic Bonding
• This sea of electrons explains many of the physical
properties of metals
• Metals are good conductors of electricity because
electrons can flow freely in them, as electrons enter one
end, an equal number leave the other
• Metals are ductile and malleable, meaning they can be
stretched into wires and hammered into sheets
• This is because the sea of valence electrons insulates the
metal cations from one another. When subjected to
stress, the metal cations slide past each other like ball
bearings in oil.
• When ionic compounds are subjected to force, like
charges come into contact and the crystal shatters
Crystalline Structure of Metals
• Like ionic compounds, metals also have a
crystalline structure
• Metal atoms are arranged in very compact
and orderly patterns and are the simplest
forms of crystalline structure
• For spheres of equal size there are many
arrangements possible, three of which are
body centered cubic, face centered cubic,
and hexagonal close packed
Body Centered Cubic
• In a body centerd cubic
arrangement, every atom
except those on the
surface has eight
neighboring atoms
• Sodium, potassium, iron,
chromium, and tungsten
crystallize into this form
Face Centerd Cubic
• In a face centerd cubic
arrangement, every atom
except those on the
surface has twelve
neighboring atoms
• Copper, silver, gold,
aluminum, and lead
crystallize in this form
Hexagonal Close Packed
• In a hexagonal close
packed arrangement,
every atom except those
on the surface has twelve
neighboring atoms, but in
a different arrangement
• Magnesium, zinc, and
calcium crystallize into this
form
Alloy
• An alloy is a mixture of a
metal and another element.
• The second element makes
the mixture stronger than
it’s main element.
• The second element
enhances the properties of
the main element
• Two of the most well known
alloys are steel and brass
Alloys
• If the atoms of the second element are of a
similar size to the main element, they can take
the place of the main element in the crystal
structure, and a substitutional alloy is formed
• If the atomsof the second element are small
enough to fit into the interces of the main
element, a intersitial alloy is formed
• Steel is an intersitial alloy
Molecules and Covalent
Compounds
Molecules and Covalent
Compounds
• In nature, only noble gases exist as atoms.
• Atoms can form bonds that are not due to
electrostatic forces.
• They can share electrons in a covalent bond.
• Many elements in nature are found in the
form of molecules.
• A molecule is an electrically neutral group of
atoms joined together by covalent bonds
Molecular Compounds
• A compound composed of molecules is a molecular
compound
• A molecule consisting of two of the same atom is called a
diatomic molecule. Example Oxygen
• Molecular compounds tend to have relatively low
melting and boiling points
• Molecular compounds are usually formed from two or
more nonmetals
Molecular Formulas
• The chemical formula for a molecular compound
is called a molecular formula, and shows the
kinds and numbers of atoms present in a
compound
• It does not tell you about the structure of the
molecule itself
NH3
Types of Bonding - Covalent
• Losing electrons takes energy, and if the
energy required is to much, atoms will
share electrons
• A single covalent bond is made up of two
shared electrons, usually one from each
atom in the bond
• This bond results in a stable outer level for
each atom or noble gas configuration
Single Covalent Bonds
• Two atoms held together by sharing a pair
of electrons are joined by a single covalent
bond
• When writing the electron dot diagram of
molecules, the electrons that are shared
can either be shown as two dots, or a dash
H:H or H-H
Multiple Covalent Bonds
• A covalent bond can contain more than
one pair of electrons
• An example is N2 , which shares 3 pairs of
electrons between the two nitrogen
molecules which is a triple bond
Coordinate Covalent Bonds
• A coordinate covalent bond is a bond in which
one atom contributes both bonding electrons.
• An example is carbon monoxide. Carbon needs 4
electrons to become stable, and oxygen needs
two. For the final two carbon needs, oxygen
donates them. This is represented by drawing an
arrow from oxygen to carbon.
Polyatomic Ions
• A polyatomic ion is a group of atoms tightly bound
together with a positive or negative charge that function
as a unit
• They are held together with covalent bonds and
coordinate covalent bonds
• Polyatomic ions can then form ionic bonds with other
atoms
Bonding
• The total energy required to break
the bond between two covalently
bonded atoms is known as the
bond dissociation energy.
• A large bond dissociation energy
corresponds to a strong covalent
bond
Polar & Nonpolar Molecules
• Electrons are not always equally shared
between atoms.
• More electronegative atoms attract
electrons more
• Because of this, one end gains a slight
positive charge, and the other a slight
negative charge
• This is called a polar covalent bond
• Molecules made of identical elements or
are symmetric are nonpolar molecules
Attractions between molecules
• Molecules can attract each other in a variety of ways.
• These methods are weaker than bonding forces but still
can affect molecules
• The two main ones are called Van der Waals Forces and
hydrogen bonding.
• The two weakest attractions are called Van der Waals
Forces
• Dipole interactions occur when opposite ends of polar
molecules attract eact other
• Dispersion forces occur between nonpolar molecules
and are the weakest of all interactions
• They occur when electrons momentarily move to one
side of an atom causing the the electrons in the
neighboring atom to move momentarily making it
polar
Hydrogen bonding
• A hydrogen bond occurs when hydrogen is bonded to an
extremely electronegative element and is also very weakly
bonded to an unshared pair in another electronegative atom.
The resulting bond has about 5% the strength of a covalent
bond.
Lewis Structures
Lewis Structures
• A Lewis Structure is a way of representing a molecule
and the bonds that it shares.
• A pair of electrons that is shared between two atoms is a
shared pair and is represented by a dash (-)
• An unshared pair of electrons is called a lone pair or
nonbonding pair and is represented by two dots
Resonance Structures
• A resonance structure is a structure that
occurs when it is possible to write two or
more electron dot diagrams for the same
molecule or ion
Lewis Structures
1. First figure out the total number of electrons available for
bonding by writing the electron dot diagram for each element
2. Next figure the total number of atoms to get the noble gas
configuration for each atom.
3. Subtract the first number from the second to get the total
number of bonding electrons.
4. Divide that by 2, that is the number of bonds in the Lewis
structure
5. When writing the bonds, hydrogen and halogens bond once,
oxygen group bonds twice, nitrogen group 3 times, and carbon
group 4.
6. To get the number of unbonded electrons, subtract 3 from 1
7. Pick the central atom, and write it first
8. Write in the bonds to give noble gas configuration
9. Write in any unshared electron pairs.
Bonding Theories
• Molecules are not flat as a Lewis structure would make them
appear, they are actually three dimensional
• VSEPR Theory explains the three dimensional shape of
molecules.
• VSEPR stands for valence shell electron pair repulsion
• It states that the repulsion between electron pairs causes
molecular shapes to adjust so that the valence-electron pairs
stay as far apart as possible
• Unshared pairs repel more strongly than shared pairs because
they are held closer to the nucleus of the atom than bonded
pairs