Molecular Geometry and Polarity Part A: Chemical Bonding Review Dr. Chin Chu River Dell Regional High School

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.

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

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.

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.

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

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

Metallic Bond

Ionic and Molecular Bonds Formation of sodium chloride (ionic): Cl · · · ® Na+ [ ] Cl · · Na · + Formation of hydrogen chloride (covalent): Cl · · · ® Cl · · H H · + A metal and a nonmetal transfer electrons to form an ionic bond. Two nonmetals share electrons to form a covalent bond.

We know a COVALENT bond comes from sharing the bonding pair of electrons.

F F Shared pair (bonding pair)

F F The nucleus of each atom pulls on the bonding pair.

F F Both atoms have equal pull, so the bonding pair is shared equally.

H Cl If two different atoms share a bond, one will pull more strongly on the bonding electrons.

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

H Cl

The bonding electrons carry negative charge. Cl The bonding electrons carry negative charge.

H Cl The 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 electrons as in the formation of an ion.

H Cl There is an unequal sharing of electrons.

H Cl d + d – The partial charge is denoted by a + or – and the Greek letter delta, d

d + d – The partial charge is denoted by a + or – and the Greek letter delta, d

The degree of sharing (equal to unequal) is determined by the electronegativity difference between the two atoms.

F Two atoms of equal electronegativity will share the bond equally

resulting in partial charge. Cl 3.0 2.1 Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

resulting in partial charge. d – d + H Cl Two atoms with a small difference in electronegativity will share unequally, resulting in partial charge.

The bonding pair is, on average, Cl This is a polar bond: The bonding pair is, on average, closer to one atom.

d – d + H Cl

resulting in a full charge. 4.0 0.8 F K Two atoms with a large difference in electronegativity will result in a loss of an electron, resulting in a full charge.

F K

F K

F K

K F

K F

K F

positive ion negative ion + _ K F positive ion negative ion

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 - 0.3 Covalent nonpolar 0.3 - 1.0 Covalent moderately polar 1.0 -1.7 Covalent polar >1.7 Ionic 4/17

Covalent: DC = 0 Polar: 0.3 < DC < 1.7 Ionic DC > 1.7