كيمياء لغات - الباب الثالث| CHEMICAL COMBINATION
02 Theories Of Covalent Bondig
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كيمياء لغات - الباب الثالث| CHEMICAL COMBINATION
CHEMICAL COMBINATION
Introduction: for an element to be stable, its outermost energy level should be completely filled, so that all elements (except noble gases) undergo chemical reactions to acquire an identical electronic configuration. as that of the nearest noble gas, that’s by losing , gaining or sharing, through forming chemical bonds.
*If a chemical bond is not formed or broken amongst the atoms , there is no
chemical reaction
For example :- *If Iron fillings mixed with sulphur powder , the product
is not a chemical compound but it is a mixture
*If this mixture is heated to high enough temp. it forms new
compound and this is called chemical reaction
Chemical reaction:-
Def :- Breaking bonds between atoms in reactants and forming new bonds
between atoms in products
These bonds are: - 1) Ionic bond 2) Covalent bond 3) Coordinate bond
4) Hydrogen bond 5) Metallic
1) IONIC BOND
Def : - * It’s a bond formed by transfer one or more electrons
from metal to non-metal
* Electrostatic attraction force takes place between cation and anion. So ionic bond has no materialistic existence.
Steps 1) Metal loses one or more electrons to form positive ion[ cation]
2) Non-metal gains that electron to form negative ion (anion)
3) Cations and anions are attracted to each other forming ionic
compound.
EX.
Na 1S2 , 2S2, 2P6 , 3S1 17Cl 1S2 , 2S2, 2P6 , 3S2,3P5
Na+1 1S2 , 2S2 ,2P6 Cl-1 1S2 , 2S2 , 2P6 , 3S2 , 3P6
Attraction force
Na+1 ــــــــــــ Cl-1
G.R.F
1- Ionic bond is formed between the two extremes of the periodic table.
2-Elements of group 1A form Ionic bond with elements of group 7A
*Because elements at the left hand side of the periodic table
characterized by a large volume and a small ionization energy so electrons
easily loosed from the outer level forming cation.
but elements at the right hand side of the periodic table characterized by
small volume and high electron affinity so electrons easily gained [anion].
* Effect of electronegatirity on the strength of the ionic bond
* There is a relation between the difference in electronegativity between atoms in ionic compound and the strength of ionic bond.
* When the difference in electronegativity increases the strength of ionic bond increases. as shown in the table
* Ionic compounds are formed if the difference in electronegativity between the two bonded atoms is more than (1.7).
Group I II III
Element Sodium Magnesium Aluminium
Electro negativity 0.9 1.2 1.5
Chloride compound NaCl MgCl2 AlCl3
Diff in E.n 3- 0.9 = 2.1 3 –1.2 = 1.8 3 – 1.5 = 1.5
M.p 810C0 714 C0 190 C0
B.p 1465 C0 1412 C0 Sublimes'
Conductivity Very good conductor Good conductor Does not conduct
2) COVALENT BOND
Def :- It’s a bond formed between two non-metals by sharing electrons in order to reach the stable electronic configuration as the nearest noble gas.
* Types of covalent bonds
Pure covalent bond Polar covalent bond
It is formed between two similar atoms have the same electronegativity
EX H2 , O2 , N2 It is formed between two different atoms have the difference in electronegativity
HCl , NH3 , H2O
Diff. in electronegativity between atoms equals zero Diff in electronegativity between atoms is less than 1.7
Every atom in the molecule has the same ability to attract electrons of the chemical bond The atom of more electronegativity has greater ability to attract electrons of the chemical bond
The electron pair spends the same time in the vicinity of each atom The electron pair spends more time in the vicinity of each atom
*When difference in electronegativity equals zero bond will be pure covalent
*When difference in electronegativity is less than 1.7 bond will be Polar covalent
*When difference in electronegativity more than 1.7 bond will be ionic
G.R.F
1- The bond formed between two Fluorine ,Chlorine or Hydrogen atoms in
a diatomic molecule is a pure covalent bond.
*Because each atom in the molecule has the same ability to attract the two shared electrons of bond so the electron pair spends the same time in the vicinity of each atom and the net charge = zero.
4- The bond formed between Hydrogen and Chlorine atoms in Hydrogen
Chloride molecule is a polar covalent bond.
*Because Chlorine has a higher electronegativity than Hydrogen so it carries a partial negative charge and Hydrogen carries a partial positive charge due to unequal sharing of the electrons pair .
* Comparison between ionic bond and covalent bond
Ionic-bond Covalent-bond
1 Formed between metal and non-metal Formed between non-metals only
2 one or more electrons transfer from metal to non-metal No transferring electrons but sharing takes place between two non-metals.
3 Ions are formed ions are not formed
4 Electrostatic attraction takes place between cations and anions. Sharing of electrons takes place between the bonded atoms.
5 Has no materialistic existence (just attraction) G.R.F Has a materialistic existence by the shared electrons.
6 Difference in electronegativity is more than 1.7 Difference in electronegativity is less than 1.7
7 Strong bond G.R.F Weak bond
THEORIES OF COVALENT BONDING
1) Octet-rule “Electronic Theory of Valence” E.T.V
“With the exception of Hydrogen , Lithium and Beryllium ,atoms of all elements tend to reach the octet structure ”
By scientists named (Kossel and Lewis 1916 )
EX
H2 1S1 H H EX H2O 8O 1S2 , 2S2 , 2P4
H O H
EX 17Cl 1S2,2S2,2P6,3S2,3P5
Cl Cl EX NH3 7N 1S2,2S2,2P3
H N H
H
* Inadequacies of octet rule :- [Exam 2007]
1) It can’t explain the bonding in some molecules such as
boron trifluoride BF3 and phosphorus pentachloride. PCl5
2) It is not sufficient to explain many of covalent properties
such as the stereo structure and angle between bonds
G.R.F Boron triflouride and phosphorus penta chloride disobey octet rule.
Which of the following compounds obey or disobey the octet rule
Sulphur trifluoride SF3
Sulphur hexafluoride SF6
Sulphur dioxide SO2
Phosphorus pentaoxide P2O5
Phosphorus trichloride PCl3
Carbon dioxide CO2
2) Valence Bond Theory V.B.T
*This theory was based on the conclusions of quantum mechanics that consider electron not only as negative particle that moves in a definite orbit , but as a material with wave property which can exist in any position in the space around the nucleus
Def.:- It explains the formation of the covalent bond as a result of overlap of an atomic orbital of one atom contains single electron with orbital of another atom contains single electron.
* HYBRIDIZATION
*It’s the mixing of orbitals of the same atom closed in energy forming number of equivalent hybridized orbitals which take part in a chemical bonding.
* Types of Hybridization
Point of comp. Methane CH4 Ethylene C2 H4 Acetylene C2 H2
Type of hyb. Sp3 Sp2 Sp
Number of hyb. orbitals S + 3(P) = 4SP3 S + 2(P) = 3SP2 S + P = 2SP
Shape tetrahedral pyramid
[tetrahedron] planner triangle linear planner
Angle between the hybridized orb. 109o.28 120o 180o
Structure
H
H C H
H H H
C C
H H H ــ C C ــ H
Bonds between 2C - have one pi and one sigma in c-c two pi bonds and one sigma bond
number of bonds four sigma bonds five sigma bonds and one pi bond three sigma bonds and two pi bonds
Note:-
*The hybridized molecular orbitals must protrude to the outside
to be more capable of overlapping than the pure atomic orbital.
HYBERIDIZATION OF METHANE CH4
Ground state 2P2
2S2
6C 1S2
Excited state 2P3
2S1
1S2
Hyb. state
1S2
HYBERIDIZATION OF ETHYLENE C2 H4
2P2 2P2
2S2 2S2
6C 1S2 1S2 6C
2P3 2P3
2S1 2S1
6C 1S2 1S2
pz Pz
1S2 1S2
H H
C C
H H
HYBERIDIZATION OF ACETYLLENE
2P2 2P2
2S2 2S2
6C 1S2 1S2 6C
2P3 2P3
2S1 2S1
6C 1S2 1S2 6C
1S2
Shape:-
Linear planner
Angle:-
180o
Type:- SP
EX. Acetylene or Ethyne
*Conditions of hyberidization
1-Hybridization takes place between orbitals of the same atom.
2-Hybridization takes place between orbitals which closed in energy.
3-Number of hybridized orbitals equals orbitals which share in
hybridization and takes its symbols.
G.R.F
1- Angle between hybridized orbitals in Methane equals to 109.28
2- Angle between hybridized orbitals in Ethylene equals to 1200
3- Angle between hybridized orbitals in Acetylene equals to 1800
* Because the orbitals must go a part as far as possible from the
other orbitals to decreases the repulsion force between orbitals and
at this angle the molecule will be more stable. [ 2006 ]
SP3 hyb. :- it is the common hybridization takes place on mixing one
S orbital and 3p orbitals
3) MOLECULAR ORBITAL THEORY
M.O.T
Def:- All the atomic orbitals of the combined atoms are mixed to form molecular orbitals. which have the symbols (sigma , pi , delta )
V.B.T M.O.T
The molecule is formed of two atoms or more. The molecule is a one unit as a big atom with multi nuclei.
Hybridization occurs between some atomic orbitals. Hybridization occurs between all atomic orbitals.
* Compare between ( sigma ) and ( pi ) bonds.
Sigma bond Pi bond
1 Usually formed due to overlap of hybridized orbitals (head to head) Usually formed due to overlap of hybridized orbitals (side by side)
2 Strong bond G.R.F Weak bond
3 Due to greater orbital overlap Due to less orbital overlap
4 The overlapped orbital are on
one line (collinear-overlap)
along one axis Overlap between two parallel orbitals (collateral overlap)
3) COORDINATION BOND
Def :- It’s a special type of covalent bond in which the two electrons required for the bond are donated by one of the two atoms. to the other atom.
Or It’s a special type of covalent bond formed between two atoms one of them is donor and other is acceptor.
Or It’s a special type of covalent bond formed between two atoms one of them has lone pair of electron and other has empty orbital.
The atom, which gives that pair of electrons is called donor atom.
The atom, which gains that pair of electrons is called acceptor atom.
EX. (1) – Hydroxonium[Hydronium] ion [ H3O]+1
H O +H+ [ H O H+]
H H 8O 1S2 , 2S2 , 2P4
h4
* Oxygen is a donor atom
* Hydrogen ion is an acceptor
EX. (2) Ammonium ion [ NH4]+
H H
H N +H+ [ H N H+
H H 7N 1S2 , 2S2 , 2P3
* Nitrogen is a donor atom
* Hydrogen ion is an acceptor atom
Compare between Covalent bond and Coordination bond
Covalent bond Coordination bond
DEFINITION
SOURCE OF ELECTRONS OF CHEMICAL BOND
Types of chemical bonds in ammonium ion
1- Polar covalent bond 2-Coordination bond
Types of chemical bonds in ammonia solution
1- Polar covalent bond between Nitrogen and Hydrogen atom
2- Coordination bond between Nitrogen atom and Hydrogen ion .
3- Ionic bond between positive ion and negative ion.
4) THE HYDROGEN BOND
Def :- It’s a bond formed between hydrogen in a polar molecule and atom in other molecule has a high electronegativity such as Nitrogen , Oxygen and Fluorine.
*So Hydrogen is a bridge between two bonds one of them is
a polar covalent and other is hydrogen bond
Shapes of hydrogen bond
( linear – closed – open net shaped )
EX : [ water , liquid hydrogen fluoride , ammonia. ]
1) H2O Hـــ O…… Hــــ O ……… Hـــــ O……… HـــO
H H H H
F
2) HF F F F F F H H
H H H H H F F
H Closed ring
H H H
3) NH3 H N H N H N
H H H
G.R.F H2O (18) M.w H2S (34) M.w
Boiling point of water is higher than boiling point of H2S (hydrogen sulphids) OR [Anomalous of boiling point of water]. G.R.F
*Oxygen atom has a small atomic radius and high electronegativity[3.5] than
hydrogen [2.1] So water is a polar molecule So hydrogen bonds between
water molecules are formed which need amount of energy for broken
*Compare between covalent bond and hydrogen bond .
Point of comparison Covalent bond Hydrogen bond
Length 1A0 [ Shorter ] 3A0 [ Longer ]
Energy 418 Kj [ stronger ] 21 Kj [ weaker ]
Def.
5) METALLIC BOND
*The positive ions of the metal are hold together by the moving electron cloud.
Def :- It’s produced from the electron clouds of valence electrons which decreases the repulsion forces between the positive metal ions in the crystal lattice.
11Na 1S2 , 2S2 , 2P6 , 3S1
* It contains (1) electron in delay level
12Mg 1S2 , 2S2 , 2P6 , 3S2
* It contains (2) electrons in delay level
13Al 1S2 , 2S2 , 2P6 , 3S2 , 3P1
* It contains (3) electrons in delay level
G.R.F * Aluminum is hard metal * Manganese is mild * Sodium is soft
If No. of valance electrons increases “strength of metallic bond increases” and melting point increases. [Sodium has one electron in the outer level but Magnesium has two electrons but Aluminum has three electrons]
G.R.F M.P of Al greater than Magnesium greater than Sodium
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