[عاطف خليفة]

* COVALENT BOND

The atoms of the same electronegativity or of less Electronegativity (the difference between them less than 1.7) bind together by sharing of electron pairs.

(1) Pure Covalent bond

The two bonded atoms have equal electronegativity each atom keeps the bonded electrons the same time and the net charge of each atom is zero. (hydroexgen molecule).
(2) Polar covalent bond
The two bonded atoms are different in electronegativity. the electron pairs spend long time of the higher electronegativity atom so. it acquires a partial negative charge while the other atom acquires a partial positive charge
* Explanation

* 1) The Octet rule
(Electronic theory of valence)

The theory was proposed by Lewis and Kosel (1916)

Except of H2 , Li , and Be - All atoms tend to reach the octet structure

The covalent bond is formed when outer electrons of the two bonded atoms are shared (electron pairs) So the outer shell of each atom contains 8 electrons.




The inadequacies of octet rule

1- The bonding in many molecules can’t be explained on the basis of the octet rule
Ex.1
- In Phosphorus penta-chloride PCl5 phosphorus atom is surrounded by ten electrons.



Ex.2
-In boron trifluoride molecule the boron atoms is surrounded by six electrons

2- The theory is not sufficient to explain many properties of molecule such as

a) The stereo structure
b) The angles between bonds


* 2) The Valence Bond Theory (V-B-T)

It is based on quantum mechanics and dual nature of electron .

The bond occurs due to overlap of an atomic orbital contains a single electron with similar orbital of another atom

Ex.1
Hydrogen molecule: H2

It is formed due to the overlap of the electron of the 1S orbital of each atom to form hydrogen molecule H2




Ex.2
Methane molecule: CH4

Carbon combines with four atoms of hydrogen by four bonds equal in length and strength
It is tetrahedron with angles of 109ْ 28\ between the four bonds.


* Explanation

The carbon atom in ground state has two orbital with single electrons . 1S2 , 2S2 , 2Px1 , 2Py1

The atom acquires a given amount of energy , so electron from 2S travels to the empty orbital 2Pz (excited atom).



* Hybridization
It is combination between atomic orbitals in the same atom to form new *****alent hybrid orbitals
N.B.
a) Hybridization occurs between the orbitals of the same atom
b) Hybridization occurs between orbits of close energy.
c) Number of hybrid orbital equals the number of pure orbitals undergoing hybridization and takes their symbol.
d) Hybrid orbitals are more protrude to the outside than pure orbitals and more capable of overlapping.



In excited carbon atom

Hybridization takes place between orbital 2S and the three orbitals 2P to form four *****alent orbitals SP3
1(2S) + 3(2P) -> 4(SP3 )

Each hybrid orbital contains a negative electron so these orbitals go apart from each other to decrease the repulsive force between orbitals at the angle of 109º 28`.
The hybridized orbital sp3 of carbon overlap with the lower 1S electrons of the four hydrogen atoms to form methane


Ex.3
Ethane: C2H4

1- The carbon atom, as excited, the structure lS2
2S1 , 2Px1 ,2Py1 , 2Pz1.

2- Hybridization occurs between 2S orbital and 2Px , 2Py only (2Pz stays without overlapping ).
- This hybridization is called sp2

3- To overcome the repulsive force the orbitals go apart at angle of 120º

4- The overlapping occurs as follows:

a) 2 hybrid orbitals of kind SP2 overlap with two orbitals lS of two atoms of hydrogen to form two bonds of kind sigma()(C-H).

b) Two hybrid orbitals of carbon overlap together to form a bond of sigma() type (C-C)

c) The two orbitals 2 Pz of the two carbon atoms overlap to form a bond of pi() → (C-C). Sigma the orbitals overlap head by head Pi the orbitals overlap side by side



* 3) molecular orbital Theory

It considered:
The molecule as one unit (big multinuclei atom) all atomic orbitals mixed or hybridized forming molecular orbitals.
The symbols of molecular orbital are sigma()- Pi()-delta()

The sigma bond

- the atomic orbitals overlap in Collinear overlap (head by head).
Ex.1
Overlap of hybrid sp2 orbital of carbon with 1S orbital of hydrogen .
Ex.2
Overlap of hybrid sp3 orbital of carbon with 1S orbital of hydrogen to form C-H bond in molecule CH4

The Pi bond ( II )

- The atomic orbital overlap in collateral overlap (side by side).

As in fig c2H4

Ex.1
overlap of 2P2 of carbon atom with similar orbital in C2 H4 .
- In Acetylene molecule C2 H2
1- The Carbon atom is exited

2- Hybridization occurs between 2Px orbital and 2S orbital to form


3- to overcome the repulsive forces between two hybrid orbitals the angle between them becomes 180º

4- Orbitals 2Py , 2pz remain without hybridization.

5- Overlapping occurs between sp of each carbon and 1S of hydrogen -and between sp of each carbon atom together (sigma bond )

6- Overlapping occurs between 2Py of each atom and between 2Pz or each atom to form Pi bondS .


* General properties of covalent compounds

1- Solubility

They don’t dissolve in - polar solvent such as water because they are not formed of ions.

They can dissolve in non-polar organic solvent because they have weak attraction force between their molecules that help covalent compounds to dissolve in it.
2- Electric conductivity

- They don’t conduct electricity because they are not ionizable..

3- Melting and boiling points

They have weak melting and boiling points due to the weak of attraction force between their molecules.