III. Chemical Bonds

fulfillment through sharing

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

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

Bond formation

Ø Covalent bond

electronic marriage

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

Non-polar covalent bond

equal partners

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

Ø Hydrogen gas

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

Polar covalent bonds

some are more equal than others

Ø Electrons not always shared equally.

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

bond polarization

Ø 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.

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

Electronegativity

Ø Electronegativity is the tendency of an atom to attract electrons.

Ø 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.

partial charge

Ø In a polar covalent bond participating atoms acquire partial charge.

Animation 2.3.1 Covalent Bon.MOV

Animation 2.3.4 Polar and No.MOV

Ionic bond

the grand theft

Ø 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.

Animation 2.4 Ionic Bonds.MOV

anions and cations

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

Hydrogen bonds

love but no commitment

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

Ø 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.

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

van der Waals interactions

office elevator

Ø 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

building blocks

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

power through weakness

Ø 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.