Chapter 6: Shapes of molecules and intermolecular forces

An electron has a negative charge, so electron pairs repel one another. * The electron pairs surrounding a central atom determine the shape of the molecule or ion * The electron pairs repel one another so that they are arranged as far apart as possible. * The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape. * Different numbers of electron pairs result in different shapes.

The shape adopted by a simple molecule or ion is that which keeps repulsive forces to a minimum.

A solid line represents a bond in the plane of paper. A solid wedge comes out of the plane of the paper. A dotted wedge goes into the plane of the paper.

• All electron pairs repel • Lone electron pairs repel more strongly than bonded pair of electrons (because a lone pair of electrons is slightly closer to the central atom)

bonded-pair/bonded-pair < bonded pair/lone pair < lone pair/lone pair

no = 2 Linear 180 degrees no = 3 Trigonal planar 120 degrees no = 4 Tetrahedral 109.5 degrees no = 5 Triangular bipyramid 90 and 120 degrees no = 6 octahedral 90 degrees

Each lone pair of electrons decreases the bond angle by ~ 2.5 degrees

4 bonded pairs, 0 lone pairs: tetrahedral shape 109.5 degrees 3 bonded pairs, 1 lone pair: pyramidal 107 degrees 2 bonded pairs, 2 lone pairs non-linear 104.5 degrees

Square planar Bond angle is 90 degrees

• Draw outer shell electrons of central atom • for every +ve charge on ion, remove an electron from outer shell / for every -ve charge on ion, add an electron to the outer shell • Pair up electrons in usual way, then work out shape and bond angle(s).

The measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.

In molecules of elements (e.g. H2) where both elements in the covalent bond are the same, the bonded electron pair is shared equally, In a covalent bond, between two different elements, the electron density is not shared equally. - nuclear charges are different - atoms may be different sizes - shared pair of electrons may be closer to one nucleus than the other

IN a non-polar bond, the bonded electron pair is shared equally between the bonded atoms. (resulting in a 100% - pure - covalent bond) This occurs when: - the bonded atoms are the same - the bonded atoms have the same or similar electronegativity. Examples include H2, C-H bond, Cl2 etc.

In a polar bond, the bonded electron pair is shared unequally between the bonded atoms. A bond will be polar when the bonded atoms are different and have different electronegativity values, resulting in a polar covalent bond. It has a PERMANENT DIPOLE.

If the bonding atoms are different, one of the atoms is likely to attract the bonding electrons more. The atom with a greater share of the electrons will be slightly negative, and the other slightly positive. A permanent dipole exists across the bond. The bond is polar.

The separation of opposite charges.

Small charge difference across a bond that results from the difference in the electronegativities of the bonded atoms. delta sign means small. d+ and d- charges are partial charges, and are much smaller than a full + and - charge.

The greater the difference in electronegativity between the bonded atoms, the greater the permanent dipole. The more electronegative atom will take the d- charge.

• Atomic radius: - As this increases, the bonding pair of electrons become further away from the nucleus. - They are therefore less attracted to the positive charge of the nucleus, resulting in LOWER electronegativity • Number of unshielded protons: - The greater the number of protons in a nucleus, the greater the attraction to the electrons in the covalent bond, resulting in higher electronegativity 2b/3: Number of electron shells (electron shielding) - But full energy levels of electrons shield electrons in the bond from increased attraction of greater nuclear charge, reducing electronegativity

• Atomic radius decreases • Charge on nucleus increases without significant extra shielding • Electronegativity INCREASES

• Atomic radius increases • Charge on nucleus increases BUT shielding increases significantly. This is because electrons added down a group fill new energy levels. • Electronegativity decreases.

Fluorine is the most electronegative atoms. So are oxygen, then nitrogen and chlorine. Group 1 metals, including lithium, sodium, and potassium have the least electronegative atoms.

If the electronegativity of both atoms in a covalent bond is identical, the electrons in the bond will be equally attracted to both of them This results in a symmetrical distribution of electron density around the 2 atoms.

NO - molecules containing polar bonds are NOT always polar. Non polar molecules: If the polar bond are arranged symmetrically, the partial charges cancel out and the molecule is non-polar. Polar molecules: If the polar bond are arranged non-symmetrically, the partial charges do not cancel out and the molecule is polar.