2.6 shapes of molecules and intermolecular forces

what does a single bond count as?

1 bonding pair

what does a double or triple bond count as?

1 region of electron density which counts as the same as bonding pair

do bonding pairs repel or attract each other?

repel

what do lone pairs count as? And do they repel more or less than bonding pairs? What does this do?

a region of electron density

repel more than bonding pairs, decreasing the bond angle around the central atom

what does a straight line on shapes of molecules show?

show bonds in the same plane as the paper

what does dashes on shapes of molecule show?

bond going into plane of paper

what does wedges on shapes of molecule show?

a bond  coming out of the plane of paper

what is the name of molecule with 4 bonded pairs, 0 lone pairs? what angle?

tetrahedral

109.5 degrees

what is the name of molecule with 3 bonded pairs, 1 lone pairs? what angle?

trigonal pyramid

107 degrees

what is the name of molecule with 2 bonded pairs, 2 lone pairs? what angle?

non-linear

104.5 degrees

what is the name of molecule with 2 bonded pairs, 0 lone pairs? what angle?

linear

180 degrees

what is the name of molecule with 3 bonded pairs, 0 lone pairs? what angle?

trigonal planar

120 degrees

what is name of molecule with 5 bonded pairs, 0 lone pairs? what angle?

trigonal biopyramid

90 and 120 degrees

what is name of molecule with 6 bonded pairs, 0 lone pairs? what angle?

octahedral

90 degrees

what does electronegativity mean?

the ability of an atom to attract the pair of electrons in a covalent bond

what scale is electronegativity based on?

Pauling electronegativity scale

what are the three factors that electronegativity depends on?

• size of positive charge on nucleus. (periodic table left to right, more protons, more positive charge, increased attraction between nucleus and a pair of electrons in covalent bond)

• atomic radius (smaller atomic radius, the closer the bonding electrons will be to the nucleus of the atom, periodic table left to right, atomic radius decreases so gets more electronegative)

• shielding of nucleus by electrons in inner shell (electrons in inner shell screen electrons in the outer shell from positive charge of the nucleus, greater number of inner shells, lower the electronegativity)

what are the two ways to show a molecule is polar?

• delta positive s+ and delta negative s- to show charges with delta negative being more electronegative element

• arrows point towards the more electronegative element which is the delta negative

what are the three types of intermolecular forces?

• induced dipole-dipole interactions

• permanent dipole-dipole interactions

• hydrogen bonds

explain how a induced dipole-dipole interactions form?

• 2 xenon atoms, their electrons are moving constantly

• at any one instant the electron cloud may be distributed unequally

• this causes partial charges s+ and s- to develop

An instantaneous dipole forms

• Instantaneous dipole on one atom induces a dipole on a neighbouring atom

An induced dipole forms

• Electrostatic forces of attraction between s- of one atom and s+ on another atom

Induced dipole-dipole attraction

• present in all molecules but are weak

If a molecule has polar bonds and is not symmetrical in every plane, what dipole is it and is it polar?

permanent dipole

yes, its polar

If a molecule has polar bonds and is symmetrical is it polar or not?

no it’s not polar because the opposite charges cancel out

which are stronger permanent dipole-dipole attractions or induced dipole-dipole interactions?

permanent dipole-dipole attractions

Explain how a permanent dipole-dipole interactions form? Eg hydrogen chloride

• chlorine atom has a greater electronegativity than hydrogen atom

• so electrons pair in the covalent bond is more attracted to the chlorine atom

• this makes the chlorine atom slightly negative compared to the hydrogen atom

• when two molecules of hydrogen chloride go close, the s- on the chlorine is attracted to the s+ of the hydrogen on another atom

This is called a permanent dipole-dipole interaction

sulphur hexafluoride SF6, exists as a non-polar covalent molecules with an octahedral shape.

Explain why a molecule of SF^ has an octahedral shape (2 marks)

• Sulphur has six bonded pairs and no lone pairs

• Electron pairs repel one another equally

Fluorine has a higher electronegativity than sulphur , yet SF6 molecules are non-polar

Explain what is meant by the term electronegativity and suggest why SF6 molecules are non-polar. (3 marks)

• The ability of an atom to attract electrons 

• in a covalent bond

• The octahedral shape is symmetrical

At room temperature, chlorine is a gas and iodine is a solid. When heated together chlorine reacts with iodine to form iodine mono chloride, ICl.

ICl has a higher boiling point than Cl2.

Explain, in terms of intermolecular forces present, why Icl has a higher boiling point than Cl2. (2 marks)

• Icl has permanent dipole dipole interaction AND Cl2 has only induced dipole dipole interactions

• More energy is needed to overcome forces in ICl

Nitrogen can also form a fluoride, NF3, which has a permanent dipole.

Explain why NF3 has a permanent dipole (2 marks)

• F is more electronegative than N

• Dipoles do not cancel out or NF3 is asymmetrical