*The Covalent Bond: Sharing Electrons

*A covalent bond consists of a pair of electrons shared between two atoms.In the millions of chemical compounds that exist, the covalent bond is the predominant chemical bond.

*Substances which covalently bond exist as molecules.Carbon dioxide bonds covalently. It exists as individually bonded covalent molecules containing one carbon and two oxygen atoms.11.7

*The term molecule is not used when referring to ionic substances.Sodium chloride bonds ionically. It consists of a large aggregate of positive and negative ions. No molecules of NaCl exist.11.7

*Covalent bonding in the hydrogen molecule11.8

*11.9Covalent bonding in the chlorine moleculeEach chlorine now has 8 electrons in its outermost energy level.

*Covalent bonding with equal sharing of electrons occurs in diatomic molecules formed from one element.A dash may replace a pair of dots.

*Electronegativity

*electronegativity The relative attraction that an atom has for a pair of shared electrons in a covalent bond.

*If the two atoms that constitute a covalent bond are identical then there is equal sharing of electrons.This is called nonpolar covalent bonding.Ionic bonding and nonpolar covalent bonding represent two extremes.

*If the two atoms that constitute a covalent bond are not identical then there is unequal sharing of electrons.This is called polar covalent bonding.One atom assumes a partial positive charge and the other atom assumes a partial negative charge.This charge difference is a result of the unequal attractions the atoms have for their shared electron pair.

*:Shared electron pair.:The shared electron pair is closer to chlorine than to hydrogen.Partial positive charge on hydrogen.Partial negative charge on chlorine.Polar Covalent Bonding in HClThe attractive force that an atom of an element has for shared electrons in a molecule or a polyatomic ion is known as its electronegativity.

*A scale of relative electronegativities was developed by Linus Pauling.

*Electronegativity decreases down a group for representative elements.Electronegativity generally increases left to right across a period.

*The electronegativities of the metals are low.The electronegativities of the nonmetals are high.11.1

*The polarity of a bond is determined by the difference in electronegativity values of the atoms forming the bond.

*If the electronegativity difference between two bonded atoms is greater than 1.7-1.9 the bond will be more ionic than covalent.If the electronegativity difference is greater than 2, the bond is strongly ionic.If the electronegativity difference is less than 1.5, the bond is strongly covalent.

*If the electronegativities are the same, the bond is nonpolar covalent and the electrons are shared equally.The molecule is nonpolar covalent.11.10

*If the electronegativities are the same, the bond is nonpolar covalent and the electrons are shared equally.The molecule is nonpolar covalent.Electronegativity Difference = 0.011.10

*If the electronegativities are not the same, the bond is polar covalent and the electrons are shared unequally.Hydrogen Chloride MoleculeThe molecule is polar covalent.Electronegativity Difference = 0.911.10

*If the electronegativities are very different, the bond is ionic and the electrons are transferred to the more electronegative atom.The bond is ionic.No molecule exists.Electronegativity Difference = 2.111.10

*A dipole is a molecule that is electrically asymmetrical, causing it to be oppositely charged at two points.

*An arrow can be used to indicate a dipole.The arrow points to the negative end of the dipole.Molecules of HCl, HBr and H2O are polar .

*A molecule containing different kinds of atoms may or may not be polar depending on its shape.The carbon dioxide molecule is nonpolar because its carbon-oxygen dipoles cancel each other by acting in opposite directions.

*11.11Relating Bond Type to Electronegativity Difference.