Untitled - Kelly Walsh High School

Chemical Bonding 133
Quick Tip
Never put more than three covalent bonds between a pair of atoms.
In the hydrogen molecule, H 2, the atoms share the electrons equally. Each
hydrogen nucleus has one proton equally attracting the bonding pair of electrons.
A bond like this is a nonpolar covalent bond, or simply a covalent bond.
In cases where the two atoms involved in the covalent bond are not the same,
then the attraction is not equal. The bonding electrons are pulled more toward
the atom with the greater attraction (more electronegative atom). This bond is
a polar covalent bond. The atom that has the greater attraction takes on a partial
negative charge and the other atom a partial positive charge.
Relative electronegativity values are important here. It is important to know
which element has the greater attraction for the electrons. This is the atom with
the greater electronegativity. Consider for example, hydrogen fluoride, HF. The
fluorine has a greater attraction for the bonding pair of electrons. For this reason,
it takes on a partial negative charge. This will leave the hydrogen with a
partial positive charge. The presence of partial charges gives a polar covalent
bond. Many times, we use an arrow, in place of a dash, to represent this type of
bond. The head of the arrow points toward the atom that has the greater attraction
for the electron pair:
d H S F d
The charges formed are not full charges as found in ions. These are only partial
charges, indicated by a delta, d. Many times these polar bonds are responsible
for the entire molecule being polar. The molecule is polar if it has a negative
end and a positive end. Polar molecules attract other polar molecules, and this
attraction may greatly influence the properties of that substance. (We will see
the consequences of this in Chapter 11.)
A Lewis structure can show the bonding pattern in a covalent compound. In
Lewis formulas, we show the valence electrons that are not involved in bonding
as dots surrounding the element symbols. The valence electrons involved in
bonding are present as dashes. There are several ways of deriving the Lewis structure,
but here is one that works well for most compounds that obey the octet rule.
Draw the Lewis structural formula for CH 4O.