shapes and intermolecular forces Flashcards
what is the electron pair repulsion theory?
-electron pairs repel as far as possible
-lone pairs repel more than bonding pairs (for each lone pair the bonding angle decreases by 2.5 degrees)
define electronegativity
a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
what happens if one element is more electronegative than the other?
the shared electrons will sit closer to the element that is more electronegative, giving it an S- charge (polarity)
what is a polar covalent bond?
a bond with a permanent dipole, having S- and S+ partial charges on the bonded atoms
describe a London force
-the weakest intermolecular forces
-also known as ‘instantaneous-induced dipole dipole’ forces
-e.g. H—H (same element so equal electronegativity- no permanent dipole)
how is a London force formed?
electrons move around randomly at high speeds between the two elements, so at some points they will be closer to one than the other. this gives the closest atom a small negative dipole (and the other a positive one) for an instant before the electrons move again
once a London force is formed in a molecule, what happens to the other molecules around it?
the positive instantaneous dipole will attract the pair of electrons in a neighbouring molecule, causing them to move closer to one side. this therefore induces a negative dipole on the atom on that side, and acts as a ripple affect across other molecules
what affects the strength of London forces?
the number of bonded electrons present (more bonded electrons = more strength = higher melting point)
describe a permanent dipole-dipole force
- medium strength
- e.g. H—Cl (unequal electronegativity)
how is a permanent dipole formed?
e.g. H—Cl
Cl is more electronegative than H, so it attracts the bonded pair of electrons, gaining a partially negative charge (H loses some negativity so gains a partially positive charge)
-because this will always happen, it is called a permanent dipole
how is a permanent dipole dipole force formed?
the positive dipole attracts the negative dipole from its neighbouring molecule (e.g. H S+ attracts a Cl S- from another molecule)
describe a hydrogen bond
-the strongest intermolecular forces
-e.g. H—F, H—O or H—N
F, O and N are the only elements which can give hydrogen a strong enough S+ dipole to attract a lone pair of electrons from another O, N or F.
how should you draw a hydrogen bond?
the bond must be drawn between the lone pairs
describe the anomalous properties of water
the hydrogen bonds are extra forces above the London forces and require more energy to overcome, so the MP/BP or water is higher, so it is a liquid at room temperature despite being a simple covalent molecule
describe a tetrahedral shape
bond angle: 109.5 degrees
bonding pairs: 4
lone pairs: 0
