The Nature of the Chemical Covalent Bond Section 4.2

Valence Bond Theory A half-filled orbital in one atom can overlap with another half-filled orbital of a second atom to form a new bonding orbital The new bonding orbital form the overlap of atomic orbitals contains a pair of electrons of opposite sign The total number of electrons in the bonding orbital must be two When atoms bond, they arrange themselves in space to achieve the maximum overlap of their half-filled orbitals. Maximum overlap produces a bonding orbital of lowest energy.

Hybrid Orbitals Hybridization: A theoretical process involving the combination of atomic orbitals to create a new set of orbitals that take part in bonding Hybrid orbitals only take place to form a new molecule, they do not exist on their own. Two problems from Lewis theory: inability to explain the four equal bonds represented by the four pairs of electrons in a carbon cmpd & the existence of multiple bonds

Forms of Hybridization Depending on the types of orbitals that overlap you will get different shapes of molecules: sp hybridization occurs when an s and a p orbital overlap –two sp combine result = linear sp2 hybridization occurs when an s and 2 p orbitals overlap, – three sp2 combine result = trigonal planar sp3 hybridization occurs when one s and three p orbitals overlap – four sp3 combine result = tetrahedral

Single Covalent Bonds Single bonds form from the overlap of end to end orbitals The original bond formed between two atoms is called a sigma (denoted by the Greek lowercase s, ) bond

Double and Triple Covalent Bonds Double and Triple bonds form from the side to side overlap of p orbitals over the sigma bond. They are called pi (denoted by the Greek lowercase p, ) bonds, a double bond has one  bond and a triple bond has 2  bonds

Double bonds: Triple Bonds:

Polar Covalent Bonds Polar bonds happen when the differences in electronegativities between two non-metals is greater than 0.5

Polar Molecules Occur when there are polar bonds AND there is an unequal sharing of the positive and negative sides. If there are unpaired electrons, treat them as negative charges.

Polar or Not CBr4 HBr NF3 BF3 H2S