Post on 13-Dec-2015
2
Molecular Geometry and Polarity
Part A: Chemical Bonding Review
Dr. Chin Chu
River Dell Regional High School
3
Nature of Chemical Bond• Atoms are held together by electrostatic attraction
between positively charged nuclei and negatively charged electron clouds.
• Chemical Bond: a link between atoms that result from mutual attraction of their nuclei for electrons.
• Bond energy: the energy required to break a bond.
• Forces in substances:– Attractive: between electron clouds and respective bonding
nuclei (of the two atoms that bond).– Repulsive: between all the electron clouds in the bonding
atoms; between positively charged nucleus.
4
Formation of Bonds• Bonding involves only the valence electrons (those in the
highest energy level).
• Use the periodic chart to guide determination of valence electrons
• WHEN BONDING OCCURS:
– Atoms attain an OCTET: a stable Noble Gas configuration.
– the resulting system is at the lowest possible potential energy level.
– The process of bonding is, therefore, exothermic: energy is being released. If the energy released is Large we get a strong bond; small ΔE bond is weak
5
Types of Bonds
• Ionic bond: formed by transfer of electrons from the valence energy level of one atom to another’s
• Covalent bond: formed when atoms share electrons.
• Metallic bond: ions of metals are surrounded by sea of electrons that bind all ions together.
6
Lewis Theory: An Overview• Valence e- play a
fundamental role in chemical bonding.
• e- transfer leads to ionic bonds.
• Sharing of e- leads to covalent bonds.
• e- are transferred or shared to give each atom a noble gas configuration – the octet.
7
Lewis Symbols (Structures)
• A chemical symbol represents the nucleus and the core e-.
• Dots around the symbol represent valence e-.
Si
N••
••
• P••
••
• As••
••
• Sb••
••
• Bi••
••
•
••Al••
Se••
•••
Ar••
••
••I •••
••
•••
8
Metallic Bond
• Metals consist of crystalline lattice in which positive ions (kernels) are arranged in fixed patterns.
• The valence electrons are free to move and they belong to the entire crystal.
• “Electron Sea” model
10
Ionic and Molecular Bonds
• Formation of sodium chloride (ionic):
• Formation of hydrogen chloride (covalent):
A metal and a nonmetal transfer electrons to form an ionic bond. Two nonmetals share electrons to form a covalent bond.
Na + Na+ [ ]Cl
Cl
H + Cl
Cl
H
H ClThe closer they get to the chlorine atom,
the more negative it gets.The farther they get from the hydrogen,
the more positive it gets.
H Cl+
_
But the charge is only partial.Hydrogen has not lost the electronsas in the formation of an ion.
The degree of sharing (equal to unequal)is determined by the electronegativitydifference between the two atoms.
H ClTwo atoms with a small difference in electronegativity will share unequally,
resulting in partial charge.
3.02.1
H ClTwo atoms with a small difference in electronegativity will share unequally,
resulting in partial charge.
–+
FKTwo atoms with a large difference in electronegativity will result in a loss
of an electron,resulting in a full charge.
4.00.8
44
Electronegativity
• A measure of how strongly the atoms attract electrons in a bond.
• The bigger the electronegativity difference the more polar the bond.
• 0.0 - 0.4 Covalent nonpolar• 0.4 - 1.0 Covalent moderately polar• 1.0 -1.7 Covalent polar• >1.7 Ionic