Chemical Bonds

One way to achieve a stable outermost shell is to pair up with another atom. Such joining up is called a chemical bond.

Bonds can be formed between two or more atoms of the same element or between different elements.

Bond formation

Covalent bond

In a covalent bond an electron pair is shared between two atoms to achieve a stable, outer shell.

Non-polar covalent bond

In a non-polar covalent bond, the electron pair is equally shared, the bond is not polarized e.g. H2 and O2.

Hydrogen gas

Oxygen gas

Non-polar bonds can also be formed between different elements e.g. C : H

Polar covalent bonds

Electrons not always shared equally.

In a bond between two different atoms, one might attract the shared electron pair more strongly pulling it away from the other atom.

The shared pair is shifted from the center between the two participating atoms making one end of the molecule positive and the other end negative. The bond is polarized.

Which atom attracts the pair of electrons is determined by the atom's electronegativity.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons.

The more one atom attracts electrons, the more electronegative it is e.g. O and N are very electronegative and their bonds with hydrogen will be polarized.

In a polar covalent bond participating atoms acquire partial charge.

Ionic bond

In the extreme case of polarization, the "shared" electron is stolen by the more electronegative atom e.g. in NaCl chlorine steals a lone electron from sodium.

The atoms acquire now a full electronic charge: the ones with extra electrons are called anions, those with missing electrons are cations.

Hydrogen bonds

Hydrogen bonds arise from the weak electrostatic attraction of the partial charges in adjacent polar covalent bonds.

The partial positive charge on hydrogen and the partial negative charge of the electronegative atom (in a another polar covalent bonds) form a hydrogen bond.

Though weak, the large number of these bonds make them energetically significant.

van der Waals interactions

At very close distances synchronous motion of electrons in electrically neutral atoms results in a weak attraction called van der Waals interaction.

Bond strength

Bond	Strength (kcal/mole)
covalent	70-100
ionic in solid	90-140
ionic in water	10
hydrogen	4-5
Van der Waals	1-2

The strong bonds are important for structural molecules which form the building blocks of the cell.

The weak bonds (hydrogen and van der Waals) are more important in biological reactions (metabolism), since the available thermal energy at ambient temperatures is about 10 kcal/mol) and weak bonds are easier to break or form.

P. Fajer Chemical Bonds 4/3 Chem-2 12/30/95 11:59 PM 4