Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

If the electronegativity of an atom is high, then it attracts and holds on to electrons.

If the electronegativity of an atom is low, then

it tends to give electrons away. Electronegativity differences thus

determine bond type and explains what is meant by polar bonds and polar molecules.

• If the difference in electronegativities is between:

– 1.7 to 4.0: Ionic – 0.3 to 1.7: Polar Covalent – 0.0 to 0.3: Non-Polar Covalent Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2 So this is an ionic bond .

What is a bond?A bond is a link or force that binds two or

a group of atoms together to form a compound.

The type of bonds found in biomolecules

can be divided into two types ;1-Covalent bonds.2-Non-covalent bonds.(including , H

bonds, ionic bonds, hydrophobic bonds, Van derWaals forces ,).

Weak bonds are bonds that form between different molecules or within different parts of a large molecule. While these bonds are not strong enough to hold a molecule together they are extremely important because of their large number. There are three basic types of weak bonds (Hydrogen bonds, Hydrophobic Interactions and Van der Waals Forces).

Hydrogen bonds

What are they?

• A special case of permanent dipole-dipole interactions

They are stronger than van der Waals forces.Molecules with hydrogen bonds have higher boiling points than molecules that don’t.

What do you need?

• A hydrogen atom covalently bonded to an electronegative atom … N, O or F.

If only one of these conditions is met, you don’t get hydrogen bonding.

• A lone pair of electrons on the electronegative atom.

Give me an example!

This does not have any hydrogen bonds. Carbon is not very electronegative, and it has no lone pairs of electrons in methane.

• methane, CH4 …

Give me a real example!

This does have hydrogen bonds.

Nitrogen is very electronegative, and it has one lone pair of electrons in ammonia.

• ammonia, NH3 …

Give me another example!

This has not one, but two hydrogen bonds.

Oxygen is very electronegative, and it has two lone pairs of electrons in water.

• water, H2O …

Remember, you need:

• A hydrogen atom covalently bonded to an electronegative atom … N, O or F.

If only one of these conditions is met, you don’t get hydrogen bonding.

• A lone pair of electrons on the electronegative atom.

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Generally chemists consider this to be the strongest of all bonds, as long as no water is present. In biological systems, ionic bonds are weak due to this fact.

•Helium, neon and argon are atoms which do not react with other atoms.

•We call them the Inert Gases (or Noble Gases) because of this.

•Each of these gases has a full outer electron shell (orbit).

•2p2n •10p10n •18p22n

•He•4•2

•20•10

•Ne•Ar•18

•40

•2,8•2,8,8

•Sodium

•Na•11

•23

•e.c. 2,8,1

•11 protons•12

neutrons•11 electrons

•11p

•12n

•17 protons

•Chlorine

•Cl•17

•35

•17p

•18n •18 Neutrons•17 electrons

•e.c. 2,8,7

•Na•e.c. 2,8,1

•(Na +)

•Ion

•Atom

•e.c. (2,8)+

•The Sodium loses 1 electron to leave a

complete outer shell.

•It is now a Sodium ion with a charge of 1 +

•The Sodium atom has 1 Electron in it’s outer

shell.

•+

•Cle.c.

2,8,7

atom

•(Cl - )

•Ion

•e.c. (2,8,8)-

•The Chlorine gains 1 electron to gain a

complete outer shell.

•It is now a Chlorine ion with a charge of 1 -

•The Chlorine atom has 7 electrons in it’s outer

shell.

•-

•Sodium atom

•Na

•Sodium ion

•(Na +)

•Chlorine atom

•Cl

•Chlorine ion

•(Cl -)

•The Ionic Bond

•The sodium atom loses one electron to attain a complete outer shell and become a positive ion (Na +).

The Chlorine atom gains one electron to attain a complete outer shell and become a negative ion (Cl –).

Strong electrostatic forces attract the sodium and chlorine ions.

•+ •-

•Ionic Compounds

• Held together by strong electrostatic forces

• Opposite charges attract

• High melting points and boiling points

• Regular structures – giant ionic lattices

• When melted or dissolved ionic compounds conduct electricity

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•Chlorine atom 2,8,7

•2 Chlorine atoms

•Outer shells only

•Chlorine molecule Cl2

•Molecules have no overall electric charge

•Molecular or covalent compounds are usually gases or liquids – they have low melting points and low boiling points

•Forces between molecules very weak

•Forces (bonds) between atoms in the molecule very strong

•Each outer shell has 8 electrons

•Electrons shared

•Covalent bonds can be represented in 3 ways:

•Cl

•Cl

•Chlorine Cl2

•Cl

•Cl

•Cl

•Cl

•Covalent Bonds:

•Water H2O

•Methane CH4

•Ammonia NH3

•Hydrogen H2

•Hydrogen Chloride HCl

•Oxygen O2

•N.B. Oxygen has a double bond

•Oxygen

•O•16

•8

•Oxygen atom 2,6

•2 Oxygen atoms (outer shells only)

•Double covalent bond

•O •O

Covalent bonds; a covalent bonds forms when two atoms share a pair of electrons together ( thus each atom will be donating an electron to form the bond) .If each atom donates 2electrons a double bond is formed which is stronger and more rigid, and a triple bond is formed when 3electrons is donated by each atom.

-It is the strongest chemical bond.-It can be a non- ploar covalent bond , which arises

when the two atoms involved are of the same element thus having the same electronegativity thus share the pair of electrons equally.

Non-polar covalent bond.

Or a polar covalent bond when it is formed between two different atoms of different electronegativity ,thus one atom has a stronger pull on the pair of electrons resulting in a shift of electron density toward the more electronegative atom.

Such a covalent bond is polar, and will have a dipole (one end is positive and the

other end negative). The degree of polarity and the magnitude of the bond dipole will be proportional to the difference in electronegativity of the bonded atoms.

. Hydrophobic Interactions;When non-polar substances such as fats or oils are placed in

water they tend to clump together. The attraction of the hydrophobic (or nonpolar) parts of molecules to each other in the presence of water (or another polar fluid). Molecules containing substantial non-polar regions will attract one another as a result of these hydrophobic interactions.

Van der Waals Forces;van der Waals' forces are forces that exist between

MOLECULES of (they are intermolecular bonds or forces) the same substance . Which can be resembled by the attraction that occurs between polar molecules (between their opposite partial charges) ,when they are in close proximity to one another. It also occurs between non-polar molecules due to the small attractions resulting from the constant movement of the electrons around the atoms of the molecules. These very weak attractions are known as Van der Waals Forces