Transcript Covalent Bonding - Effingham County Schools

Covalent Bonding
• A molecule is a neutral group of atoms held
together by covalent bonds.
• A chemical compound whose simplest units are
molecules is called a molecular compound.
Water is a molecular compound.
molecule
Molecular Compounds
• The models for water,
oxygen, and sucrose,
represent a few
examples of the many
molecular compounds
in and around us.
Atoms within molecules
may form one or more
covalent bonds.
•A chemical formula indicates the relative
numbers of atoms of each kind in a chemical
compound by using atomic symbols and
numerical subscripts.
Water:
2 hydrogen atoms
1 oxygen atom
Sucrose:
12 carbon atoms
22 hydrogen atoms
11 oxygen atoms
•A molecular formula shows the types and
numbers of atoms combined in a single
molecule of a molecular compound.
Both chemical formula and
molecular formula.
•A diatomic molecule is a molecule containing
only two atoms.
Formation of a Covalent Bond
• As atoms near each
other the charged
particles begin to
interact.
• The nuclei and
electrons are attracted
to one another, but at
the same time the two
nuclei repel each other
and the electrons repel
each other.
• The attraction forces
are stronger than the
repulsion forces.
Formation of a Covalent Bond
Remember
the attraction
forces are
stronger than
the repulsion
forces.
•As independent particles, most atoms are at relatively high
potential energy.
•Nature, however, favors arrangements in which potential
energy is minimized.
•This means that most atoms are less stable existing by
themselves than when they are combined.
•By bonding with each other, atoms decrease in potential
energy, thereby creating more stable arrangements of matter.
Potential energy changes during the formation of a hydrogenhydrogen bond.
(a) The separated hydrogen atoms do not affect each other.
(b) Potential energy decreases as the atoms are drawn together
by attractive forces.
(c) Potential energy is at a minimum when attractive forces are
balanced by repulsive forces.
(d) Potential energy
increases when
repulsion between
like charges
outweighs
attraction between
opposite charges.
Characteristics of the Covalent Bond
•The distance between two bonded atoms at
their minimum potential energy, that is, the
average distance between two bonded atoms, is
the bond length.
•Bond energy is the energy required to break a
chemical bond and form neutral isolated atoms.
•All atoms release energy as they change from
isolated individual atoms to parts of a molecule.
•The same amount of energy must be added to
separate the bonded atoms.
The Octet Rule
•The noble gases are stable because their
atoms’ outer s and p orbitals are completely
filled by 8 electrons.
•Other main group elements can fill their
outermost s and p orbitals with electrons by
sharing electrons through covalent bonding.
•Octet Rule – chemical compounds tend to
form so that each atom, by gaining, losing, or
sharing electrons, has an octet of electrons in
its highest occupied energy level.
Exceptions to the Octet Rule
• Hydrogen forms bonds in which it is
surrounded by only two electrons.
• Boron has just three valence electrons, so it
tends to form bonds in which it is surrounded
by six electrons.
• Main group elements in Periods 3 and up can
form bonds with expanded valence, involving
more than eight electrons.
Electron-Dot Notation
•Electron-dot notation is an electronconfiguration notation in which only the valence
electrons of an atom or a particular element are
shown, indicated by dots placed around the
element’s symbol.
Hydrogen has 1 valence electron
so one dot is placed around the
symbol.
How many valence electrons
does carbon have?
Notice one dot is placed on
each side of the symbol.
How many valence electrons
does fluorine have?
Notice one dot is placed on each
side of the symbol before placing
a second.
Electron-Dot Notation for Groups 1, 2,
13-18
Lewis Structures
•Electron-dot notation can also be used to
represent molecules.
•A hydrogen molecule, H2, is represented by
combining the notations of two individual
hydrogen atoms.
Remember that
hydrogen is “happy”
with two electrons
instead of eight.
Remember each
hydrogen has one
valence electron that
it brings to the bond.
Shared electron pair
(called a bonding pair).
Will two fluorine atoms form a bond?
Each fluorine atom has 7 valence electrons so each fluorine
atom wants to gain one electron to achieve an octet.
Neither atom will give up an electron – remember fluorine is
highly electronegative.
What’s the solution - what can they
do to achieve an octet?
The octet is achieved by each atom sharing the electron
pair in the middle.
A lone pair is a pair of
electrons that is not
involved in bonding and
that belong exclusively
to one atom.
octet
octet
This is the shared pair called the
bonding pair.
It is a single bonding pair and is called
a single bond.
Often the shared pair of electrons in a covalent
bond is replaced by a long dash.
Shared pair
Replace with long dash.
•These representations are all Lewis Structures,
formulas in which atomic symbols represent
nuclei and inner-shell electrons, dot-pairs or
dashes between two atomic symbols represent
electron pairs in covalent bonds, and dots
adjacent to only one atomic symbol represent
unshared electrons.
Unshared electrons.
Electron pair in
covalent bond.
Atomic symbol –
represents nuclei
and inner-shell
electrons.
•A structural formula indicates the kind, number,
arrangement, and bonds but not the unshared
pairs of the atoms in a molecule.
Structural formula
Multiple Covalent Bonds
•A double bond is a covalent bond produced by
the sharing of two pairs of electrons between
two atoms.
•A triple bond is a covalent bond produced by
the sharing of three pairs of electrons between
two atoms.
O
O
How will two oxygen atoms bond?
Each atom has two unpaired electrons.
Each atom wants to gain two electrons to
achieve an octet.
7 electrons
O O
7 electrons
If we share one pair of electrons will each
oxygen atom have an octet?
O O
6 valence electrons
plus 2 shared electrons
= full octet
6 valence electrons
plus 2 shared electrons
= full octet
What if we share two pairs of electrons?
Both electron pairs are shared.
O O
two bonding pairs
making a double bond