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.

Chemical bonding and molecular shapes

How atoms connect to one another?

• • •Cl• •

• •

•C•

•

•Unpaired electrons

Unpaired electronPaired

electrons

Chlorine Carbon

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 protons11 electrons

Na+ 11 protons10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 electrons

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

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.

How to judge the composition of an ionic compound? Balance of electric charge

Ruby

Sapphire

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-

More than one electron can be shared in an atom: double bond and triple bond.

O C O

double bonds

N N

triple bond

F F

single covalent bond

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.

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