Chapter 6 - Shapes Of Molecules & Intermolecular Forces Flashcards
Electron pair repulsion theory (EPR)
- electrons have negative charges
- around a central atom they repel each other so they are arranged as far apart as possible
4 bonding regions
tetrahedral
109.5
2 bonding regions
linear
180
3 bonding regions
trigonal planar
120
6 bonding regions
octahedral
90
what repels more - bonded or lone pairs?
lone pairs repel more
- for every lone pair the bond angle is reduced by 2.5
3 bonded, 1 lone
pyramidal
107
2 bonded, 2 lone
non-linear
104.5
electronegativity
the ability for an atom to attract the electron pair in a covalent bond
how is electronegativity measured and trend?
- Pauling electronegativity values
- increases as period does and going up a group as nuclear charge increases and atomic radius decreases
- fluorine is the most electronegative element with a value of 4.0
effects of electronegativity difference
- larger the difference, the more an atom has control of electrons - becomes more ionic
- covalent - 0
- polar covalent - 0-1.8
- ionic - >1.8
non-polar bonds
- electron pair shared equally
- when bonded atoms are the same e.g oxygen
- or when they have same/similar electronegativity
- small difference in C-H bond so considered electronegative
polar bonds
- bonded electron pair is shared unequally due to different electronegativity values
- it’s polarised forming a small partial +ve & -ve charge
dipole
separation of opposite charges
polar molecules
when polar charges don’t cancel out due to molecule shapes
intermolecular forces
weak interactions between dipoles of different molecules - 3 types:
- induced dipole-dipole interactions (London forces)
- permanent dipole-dipole interactions
- hydrogen bonding
Induced dipole-dipole interactions (London forces)
- exists between all molecules
- electrons move around creating instantaneous dipoles
- induces one on a neighbouring molecule
- continues inducing on more molecules
strength of London forces
- more electrons the larger the instantaneous dipoles
- greater induced dipole-dipole interactions
- stronger attractive forces between molecules
- more energy needed to overcome
- higher m.p/b.p
permanent dipole-dipole interactions
- act between permanent dipoles in polar molecules
- stronger then London forces
- act in addition to London forces
- more energy needed to overcome
- higher m.p/b.p
simple molecular substances
- made up of simple molecules
- form simple molecular lattice - molecules held together by weak intermolecular forces, atoms within molecules have strong covalent bonds
m.p/b.p of simple molecular substances
- can exist as any state at room temp
- weak intermolecular forces need little energy to be broken so low melting and boiling points needed
solubility of simple molecular substances
- non-polar dissolves in non-polar
- i.f. form between solute and solvent, weakens i.f. in simple molecular substance, breaks them then dissolves
- non-polar insoluble in polar
- polar soluble in polar sometimes
- depends on strength of dipole, if similar strengths then will dissolve
electrical conductivity of simple molecular substances
- no mobile charged particles
- doesn’t conduct electricity
hydrogen bond
- special type of permanent dipole-dipole between:
- electronegative atom - F, O, N with a lone pair
- H atom attached to an electronegative atom
- strongest intermolecular interaction
anomalous properties of water
- solid (ice) is less dense then water (liquid)
- H bonds hold molecules apart in open lattice structure, further away then when liquid
- makes it less dense and floats on water
- relatively high m.p/b.p - forces added to London & permanent dipole-dipole forces
- more energy needed to break forces so high m.p/b.p
