1.4 chemistry a level wjec

What is electronegativity?

1. Electronegativity is a measure of how strongly atoms attract electrons in a covalent bond.

2. The higher the electronegativity value, the better the element can attract bonding electrons.

How does a bond become polar?

1. In a covalent bond, the electron pair is not usually shared exactly evenly between the two atoms. They normally have different electronegativities and the bonding electrons are pulled towards the more electronegative atom.

2. This atom will take up a slightly negative charge and the other becomes slightly positive. The bond is said to be polar.

How does a bond become non polar?

If the two atoms are the same (e.g. two Hydrogen atoms), the atoms have equal electronegativities, so the electrons are equally shared and the bond is said to be non polar.

Electronegativity differences and type of bonding

<0.4 - non polar covalent

0.4-1.9 - polar covalent

2.0+ - ionic

What molecules show ionic bonding?

metal + non metal

How does ionic bonding work?

1. The metal atom loses electrons and becomes a cation (+)

2. The non metal gains electrons and becomes an anion (-).

3. There are strong electrostatic forces of attraction between these oppositely charged ions - ionic bonding.

What molecules show covalent bonding?

non metal + non metal

How does covalent bonding work?

Each atom gives one electron to form a bond pair in which the electron spins are opposed.

What is covalent bonding?

The electrostatic attraction between the positive nuclei of the bonded atoms and the shared electrons between them.

What is coordinate bonding?

The same as a covalent bond but both electrons forming the bond pair come from the same atom.

What are permanent dipole-dipole forces?

If the dipoles on polar molecules arrange themselves so that the negative region of one molecule is close to the positive region of another molecule, there will be an attraction between them.

How do temporary dipoles form?

Electrons are in constant motion around the nuclei therefore at any particular moment the distribution of the electron cloud around the nuclei will not be symmetrical. At this moment, the molecule would have a temporary dipole.

how do induced dipole-induced dipole forces work?

The slightly positive end of the molecule can pull the electron cloud of a neighbouring molecule towards it, giving the left side of that molecule a slightly negative charge i.e. it induces a temporary dipole in the neighbouring molecule. The two dipoles are attracted to each other. The second dipole induces a dipole in the third molecule, etc. The dipoles will always be aligned correctly to produce an attraction between the molecules.

How strong are induced dipole-induced dipole forces?

The strength of the induced dipole-induced dipole increases with increasing numbers of electrons in the molecule. The more electrons in a molecule, the greater the fluctuation in the electron cloud around the nuclei and the larger the temporary and induced dipoles created. So, stronger forces between the molecules.

What is hydrogen bonding?

Special intermolecular forces that occur between molecules containing hydrogen atoms bonded to small very electronegative elements which have lone pairs - fluorine, oxygen or nitrogen.

How does hydrogen bonding work?

Because fluorine, oxygen and nitrogen are highly electronegative, and the hydrogen atom is so small, the slightly positive charge in the bonded hydrogen atom is spread over a small volume and so it has a high charge density. The highly polarising slightly positive hydrogen atom then attracts a lone pair of electrons from the highly electronegative atom in the other molecule. The hydrogen atom is sandwiched between two electronegative atoms. It is covalently bonded to one and hydrogen bonded to another.

Effects of hydrogen bonding.

Increases boiling points - When a liquid boils, energy is needed to overcome all the forces of attraction between the molecules of the liquid. Since hydrogen bonds are stronger than van der Waals forces, molecules that form hydrogen bonds have a higher boiling temperature than molecules of a similar size that cannot hydrogen bond.

Increases solubility - The most significant intermolecular forces between water molecules are hydrogen bonds. Covalent compounds that can replace these bonds by forming new hydrogen bonds with water will dissolve.