chapter 6

pairs of electrons around a central atom repel each other, so they move as far apart as possible to minimise this repulsion, meaning molecules containing three or more atoms have shapes.

180 linear shape

120 trigonal planar

109.5 tetrahedral

90, 120 trigonal bipyramidal

90 octahedral

Lone pairs are slightly closer to the central atom and occupy more space than a bonded pair, so they repel more, reducing the overall bond angle by 2.5 degrees per pair.

A measure of the ability of an atom in a molecule to attract a pair of electrons in a covalent bond to itself.

Electronegativity increases up the group It increases across a period up to group 7. Grp 1 have the lowest electronegativity Non metals N,O,F,Cl have the most electronegative atoms. F is the most electronegative

In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms. This happens when the bond atoms are the same or have the same/similar electronegativity creating a pure covalent bond Exception–> C-H is always non-polar

1) In the same group, the pull on bonding electrons increases as the nuclear charge increases. 2) In the same group, the pull falls rapidly as electrons become further from the nucleus and the number of shielding inner electrons increases (Factor 2 outweighs Factor 1)

In a polar bond, the bonded electron pair is shared unequally between the bonded atoms. This happens when bonded atoms are different and have different electronegativity values, creating a polar covalent bond.

The bigger the difference in electronegativity between the bonding atoms, the more polar the bond and the greater the ionic character.

The dipoles do not cancel out and the molecule has an overall dipolar.

Symmetrical- dipoles cancel out and molecule is non-polar unsymmetrical- poles will not cancel out, and the molecule is polar.

Polar molecules, particularly water, can form strong enough attractions to ions to permit dissolving in some places. (water can dissolve many but not all ionic substances)

Weak interactions between dipoles of different molecules

• All molecules exert London forces -Electrons in an atom/molecule are continually moving • A temporary uneven distribution of electrons can occur for an instant -This creates an instantaneous dipole -The instantaneous dipole induces a dipole in its neighbour, leading to attraction -Induced dipoles are only temporary and can disappear in an instant

As molecules get larger the number of electrons increases so the size of the induced dipoles also gets larger. As the strength of the London forces increases more energy is required to overcome the forces, so melting and boiling points increase.

-Formed between polar molecules -Do not come and go -Exert forces all the time –> stronger -Opposite dipoles attract eachother

• intermolecular forces are weak, so don’t need much energy to overcome -Most have low mp and bp -many are gases/liquids at room temp

A special type of permanent dipole-dipole reaction containing an electronegative atom with a lone pair of electrons and a hydrogen atom attached to an electronegative atom. (H,N,F)

• partial charges -dashed line between lone pairs -Labelled h bond

Solid ice is less dense than liquid water -H bonds hold water molecules apart in an open lattice structure -Water molecules in ice are further apart than in water Water has a relatively high surface tension -Molecules on the surface experience unbalanced hydrogen bonding forces pulling them in - molecules in the bulk experience balanced forces in all directions