1. Chemical bonding and molecular shapes
How atoms connect to one another?
An atomic model is needed to understand how atoms bond
Electrons in the outermost occupied shell of any atom are responsible for the
atom’s chemical properties.
The electrons that participate in chemical bonding are called valence
electrons,the shell they occupy is called the valence shell of an atom.
Valence electrons can be conveniently represented as a series of dots
surrounding an atomic symbol. (Electron-dot structure or Lewis dot
symbol).
Valance electrons can be either paired or unpaired. Paired electrons usually
do not form chemical bonds with other elements.
3. Atoms can lose or gain electrons to become ions
When the number of protons in the
nucleus of an atom equals to the
number of electrons in the atom,
the charges balance and the atom
is electrically neutral. If one or
more electrons are lost or gained,
the atom takes on a net electric
charge. Any atom having a net
electric charge is called an ion. If
the electrons are lost, the ion’s net
charge is positive. If the electrons
are gained, the ion’s net charge is
negative.
Na 11 protons
11 electrons
Na+ 11 protons
10 electrons
Cl 17 protons
17 electrons
Cl-
17 protons
18 electrons
4. Shell model can be used to deduce the type of ion
an atom tends to form: Atoms tend to lose or
gain electrons so that they end up with an
outermost occupied shell that is filled to capacity.
1+ 2+ 3+ 4- 3- 2- 1- 0
5. Ionic bonds result from a transfer of electrons
When an atom that tends to lose electrons is
placed in contact with an atom that tends
to gain them, the result is that an electron
transfer and the formation of two
oppositely charged ions. The two
oppositely charged ions are thus attracted
to each other by the electric force, which
holds them together. This electric force of
attraction between two oppositely charged
ions is called an ionic bonds). All
compounds containing ions are referred to
as ionic compounds.
6. How to judge the composition of an ionic
compound? Balance of electric charge
Ruby
Sapphire
7. Covalent bonds result from a sharing of
electrons
The electrical attraction in
which atoms are hold
together by their mutual
attraction for shared
electrons is called an
covalent bond. A
compound composed of
atoms held together by
covalent bonds is a
covalent compound.
F + F
7e- 7e-
F F
8e- 8e-
8. More than one electron can be shared in an atom:
double bond and triple bond.
O C O
double bonds
N N
triple bond
single covalent bond
F F
9. Valence electrons determine
molecular shape
How to determine the three
dimensional structure of a
covalent compound?
Valence shell electron-pair
repulsion (VESPR): any
given pair of valence-shell
electrons strives to get as
far away as possible from
all other electron pairs in
the shell.
10. Polar covalent bonds result from an uneven
sharing of electrons
What is the distribution of a
shared pair of electrons in
a covalent bond?
In HF the shared electrons
are drawn more close to F
atom, so the fluorine side
of bond is electrically
negative while the
hydrogen side of bond is
electrically positive. This
kind of charge separation
is called a dipole.
In H2, evenly shared
in HF, unevenly shared
