Physical Unit 1.3: Bonding (inc. Shapes) Flashcards
what is the structure of ionic compounds?
giant 3d lattice
+ve & -ve ions
often metal & non-metal
(metal atoms lose electrons & non-metal atoms gain electrons = transfer of electrons)
strong electrostatic forces of attraction b/w oppositely charged ions
why do ionic compounds have high mp & bp?
a lot of thermal energy is required to overcome strong electrostatic forces of attraction
b/w oppositely charged ions
why do ionic compounds not conduct electricity when solid but do when molten or aqueous?
when solid, ions are held in fixed positions so cannot move
when molten or aqueous, ions are free to move, carrying charge
what factors affect the strength of ionic bonds & how?
the greater the charge on the ions, the stronger the ionic bonding
the smaller the ions, the stronger the ionic bonding
(ions get bigger down a group)
describe the nature of metallic bonding
within metals
metal atoms lose their outer shell electrons
+ve metal ions surrounded by a sea of free-moving delocalised e-s
what is the structure of metals?
giant 3d lattice structure
+ve metal ions surrounded by a sea of free-moving delocalised e-s
+ve metal ions are in layers
explain the properties of metals
conduct electricity when solid & molten
delocalised e-s are free to move through the structure, carrying charge
high mp
a lot of thermal energy is needed to overcome the strong force of attraction b/w +ve metal ions & -ve delocalised e-s
soft & malleable
layers of +ve metal ions can slide over each other
what factors affect the strength of metallic bonds & how?
the smaller the metal ions
the greater the charge on the ions
& the more delocalised outer shell e-s
the stronger the metallic bonding
what is a covalent bond?
shared pair of e-s
attracted to & held b/w the nuclei of 2 non-metal atoms
to form full outer shells
what is a co-ordinate/dative covalent bond?
both e-s come from the same species
i.e. one species ‘donates’ 2 e-s
drawn with an –>
what is the structure of simple molecular substances?
very strong covalent bonds within molecules
relatively weak intermolecular forces b/w molecules
what are the properties of simple molecular substances & explain?
low mp & bp
little thermal energy needed to overcome weak intermolecular forces
often liquids & gases at room temp.
do not conduct electricity
do not have free-moving ions or delocalised e-s
what is the structure of giant covalent substances?
diamond
each C atom form 4 covalent bonds to other C atoms
giant covalent lattice
graphite
each C atom forms 3 covalent bonds to other C atoms
C atoms arranged in layers
delocalised e-s are free to move b/w layers
silicon dioxide
alternating silicon & oxygen atoms
do giant covalent substances conduct electricity?
diamond - no
does not contain free-moving ions or delocalised e-s
graphite - yes
delocalised e-s are free to move b/w layers, carrying charge
silicon dioxide - no
does not contain free-moving ions or delocalised e-s
why do giant covalent substances have high melting & boiling points?
a lot of thermal energy is needed to break the very many very strong covalent bonds b/w atoms
why is graphite soft & malleable?
C atoms are arranged in layers, which can easily slide over each other
what is e- pair repulsion theory & how does it determine the shapes of molecules (&ions) & bond angles?
e-s repel each other as far as possible
lone pairs repel more than bonding pairs
state:
total # e-s in outer shell of central atom
how many e-s from how many atoms bonded to it so total of 8 e-s in outer shell
# of bonding & lone pairs
shape of molecule
bond angles
what are the basic shapes & bond angles for each # of total e- pairs?
2 pairs - linear 180
3 pairs - trigonal planar - 120
4 pairs - tetrahedral 109.5
5 pairs - trigonal bipyramidal 120, 90
6 pairs - octahedral 90
learn table for each # of lone & bonding pairs
.
define electronegativity
the power of an atom to attract the 2 electrons
in a covalent bond
what factors affect electronegativity?
as nuclear charge increases,
more protons
stronger attraction b/w nucleus & bonding pair of e-s
as atomic radius decreases,
e-s are closer to the nucleus
stronger attraction b/w nucleus & bonding pair of e-s
as shielding decreases,
fewer shells of e-s b/w nucleus & bonding pair of e-s
less repulsion
stronger attraction b/w nucleus & bonding pair of e-s
state & explain the trends in en in the periodic table
en decreases down a group
atomic radius increases
shielding increases
so weaker attraction b/w nucleus & bonding pair of e-s
en increases across a period
atomic radius decreases
number of protons increases
same shielding
so stronger attraction b/w nucleus & bonding pair of e-s
why are some covalent bonds non-polar?
the 2 atoms in the covalent bond have the same en
so the 2 e-s in the covalent bond are shared equally/equally attracted to each nucleus
why are some covalent bonds polar?
the 2 atoms in the covalent bond have different en
so the 2e-s are not shared equally
the more en atom attracts the pair of e-s more strongly than the less en atom so gets a greater share of the e-s
e.g H-Cl
why are some molecules non-polar & what intermolecular forces do they have?
VDW only
contains no polar bonds
or
contains polar bonds but dipole moments cancel out bc the molecule has a symmetrical shape
why are some molecules polar & what intermolecular forces do they have?
contains polar bonds & dipole moments do not cancel out bc the molecule does not have a symmetrical shape
VDW & dipole-dipole attractions or H-bonds (depending on the molecule)
state the 3 types of intermolecular forces
Van de Waal’s forces/induced dipole-dipole
permanent dipole-dipole attractions
Hydrogen bonding
how do Van de Waal’s forces arise?
random e- movement causes uneven e- distribution & creates an instantaneous dipole
this induces a temporary dipole in a neighbouring molecule
the partially -ve part of one molecule is attracted to the partially +ve part of another molecule
weakest IMF
what affects the strength of the VDW forces?
the bigger the molecule
the more e-s it has
so the stronger the VDWs
linear molecules (as opposed to branched) can pack closer together
greater surface contact area
so stronger VDWs
how do permanent dipole-dipole attractions arise/b/w what types of molecules are they formed?
exist b/w 2 polar molecules
how does hydrogen bonding arise?
H atom is bonded to an F, O or N atom, which are very en
dipole results from difference in en b/w H & F, O, N
strong attraction from lone pair to exposed H nucleus
strongest IMF
how is H bonding drawn?
dotted line from H to lone pair
180 degrees
show all partial charges
how is mp/bp influenced by IMFs?
non-polar molecules with VDWs only have lowest mp/bp
bc little thermal energy needed to overcome VDWs b/w molecules
the stronger the VDWs, the higher the mp/bp
substances with VDWs & permanent dipole-dipole attractions have higher mp/bp than VDWs only
bc permanent dipole-dipole is stronger than VDWs
so a lot of thermal energy is needed to overcome them
very big molecules with VDWs only can outweigh the effect of dipole-dipole so may have a higher mp/bp
substances with VDWs & H-bonds have highest mp/bp
bc H-bonds are stronger than VDWs & dipole-dipole
so a lot of thermal energy is needed to overcome them
how is mp/bp affected by # of H-bonds formed?
the more H-bonds formed to other molecules, the greater the mp/bp
a substance is soluble if
it forms H-bonds with water molecules
why does bp increase down a group?
molecules are bigger
so have more e-s
so VDWs b/w molecules are stronger
so more thermal energy needed to overcome them
explain the shape of molecules with no lone pairs of e-s
equal repulsion b/w bonding pairs of e-s
what is a permanent dipole?
unequal sharing of electrons
causing a partially +ve side & a partially -ve side
why is an HCl molecule (as an e.g.) polar?
chlorine is more en than H
e-s are attracted towards Cl atom
define polar bond
bonding pair of e-s is shared unequally b/w the 2 atoms
there are partial charges on the atoms of the bond
