Chemical Bonding

*A Chemical Bond is a mutual electrical attraction between the nuclei of one atom and the valence electrons of another atom.

*This attraction binds the atoms together.

Bonding between atoms occurs because:

*it creates a more stable arrangement for atoms

*all atoms want to be like the noble gases and have a completely filled outer shell

3 Types of Bondinga. Metallic bonding – “metal to metal”

• Results from the attraction between metal atoms and the surrounding sea of electrons

b. Ionic bonding – “nonmetal to metal”

• Results from the electrical attraction between large number of cations and anions. It involves the transfer of electrons from one atom to another

c. Covalent bonding – “nonmetal to nonmetal”

• Results from the sharing of electron pairs between two atoms

*Ionic Bonding* Many atoms transfer electrons and other atoms accept them. This creates cations (positive ions) and anions (negative ions).

sodium atom chlorine atom

Na Cl

sodium ion - Na+1 chlorine ion – Cl-1

Na Cl

Ionic Bonding

Ionic bonding is the electrostatic attraction between positive cations and negative anions. This is the strongest bond type.

Covalent Bondinga “tug-of-war”

*In many cases electrons do not completely transfer from one atom to another. The electrons between atoms are shared -like a “tug of war”

*Consider two hydrogen atoms:

Hydrogen Hydrogen

H H

The two hydrogen atoms SHARE the 2 electrons equally, giving each atom 2 electrons

H:H

NON-POLAR Covalent Bonding:

an EQUAL “tug-of-war”

the electron orbitals overlap or “share” electrons

When electrons are shared equally the bond is called a nonpolar or pure covalent bond

*POLAR Covalent Bonding:

an UNEQUAL “tug-of-war”*A polar covalent bond is when the electrons between two atoms are NOT shared equally. It is an unequal tug-of-war.

*Each Atom donates one electron to be shared to give each a stable, filled outer energy level

*POLAR Covalent Bondingan UNEQUAL “tug-of-war”

But sometimes, the two electrons are shared UNEQUALLY…one atom “tugs” harder on the shared electron pair (in this case the Cl is stronger). This creates an uneven distribution of charge. The H will have a partial positive charge ( ) and the Cl will have a partial negative charge ( ).

*POLAR Covalent Bonding

an UNEQUAL “tug-of-war”

Since this bond has two “poles” – a positive pole and a negative pole – it is

called a Polar Covalent Bond

H Cl

Ionic, Polar Covalent or Non-

Polar Covalent ??????*We use electronegativities to

determine whether we have an ionic, polar or non-polar bond

*An electronegativity value is a number ranging from 0.0 to 4.0 that shows the tendency of an atom to attract electrons towards itself.

Electronegativity Chart

The electronegativity values for the elements are found on an “electronegativity chart”, which looks like the periodic table. The numbers on the chart are the electronegativity values for that element. The HIGHER the number, the STRONGER the attraction for the electrons.

*Ionic, Polar or Non-Polar ??????

*How do we determine what type of bond will occur between two atoms?

*The DIFFERENCE between their electronegativity values (EN) will tell us:

electronegativity bond

difference ( EN) type

1.7 and higher ionic

0.3 – 1.7 polar

0.0 – 0.3 non-polar

Ionic, Polar or Non-Polar ??????*What type of bond will occur

between Iodine (EN= 2.7) and the following elements: cesium (EN=0.8), iron (EN=1.8) and sulfur (EN=2.5)Bond between

Iodineand

Electronegativity difference Bond Type

More negative atom

cesium 2.7 – 0.8 = 1.9

iron 2.7 – 1.8 = 0.9

sulfur 2.7 – 2.5 = 0.2

*Determine the type of bond between the following pairs:

Bonding between:

EN difference Bond Type More negative

atom

Na and Cl

S and O

Ca and Br

P and H

Si and Cl

S and Br

1.7 and greater IONIC0.3 – 1.7Polar0.0 – 0.3 Non-Polar

*What if the electronegativity difference is 0.3 or 1.7? What type of bond is it?

*The cut-off numbers are guidelines, not definite delineation points. It is more of a gradual change from one bond type to another.

Draw pictures of the bonds in the previous table.

Show the distribution of charge.

Sodium and chlorine

ionic bond

Sulfur and oxygen

polar covalent

S O

Draw these bonds. Show the distribution of charge if

polar*Calcium and Bromine

*Phosphorus and hydrogen

*Silicon and chlorine

*Sulfur and bromine