Unit 1: Section 3 - Bonding CDS * Flashcards
ionic bonding covalent bonding shapes of molecules polarisation and intermolecular forces
what is formed when different elements bond together?
a compound
what are the 2 main types of bonding in compounds?
ionic
covalent
how are ions formed?
when 1 or more electrons are transferred from 1 atom to another.
the 2 ions are totally distinct from 1 another
what are the simplest ions?
single atoms which lose or gain electrons to get a full outer shell
examples of simple ions:
sodium atom loses 1 electron, Na+
magnesium loses 2, Mg2+ (Mg –> Mg2+ + 2e-)
chlorine gains 1, Cl-
oxygen gains 2, O2- (O + 2e- –> O2-)
how do you know what ion is formed from an atom?
every element in a group has the same number of outer electrons. so they lose or gain the same number of electrons, so have the same charge
what charge ions does each group have?
Group 1 = 1+ ions
Group 2 = 2+ ions
Group 6 = 2- ions
Group 7 = 1- ions
what are compound ions?
ions that are made up of groups of atoms with an overall charge
what are the formulas of some compound ions?
sulfate = SO4 2- hydroxide = OH- nitrate = NO3 - carbonate = CO3 2- ammonium = NH4 +
how can you work out the formula of an ionic compound from the charges of individual ions?
the charges must balance out so if one ion is 1- and the other 2+ you need to of the 1- ion to cancel out the other
in general swapping the charges of the ions to become the number of the opposite ions works
e.g. X2- and Y3+
X3Y2
what are ionic crystals?
giant lattices of ions
what is a giant ionic lattice?
lattice - a regular structure
giant - it’s made up of the same basic unit repeated over and over
what is the structure of sodium chloride?
the Na+ and Cl- ions are packed together. the sodium chloride lattice is cube shaped - different ionic compounds have different shaped structures, but they’re all still giant lattices
what properties do ionic compounds have from their ionic structure?
conduct electricity when they’re molten or dissolved - but not when they’re solid
ionic compounds have high melting points
ionic compounds tend to dissolve in water
why do ionic compounds conduct electricity when fluid but not solid?
the ions in a liquid are free to move and carry a charge
in a solid the ions are fixed in position by strong ionic bonds
why do ionic compounds have high melting points?
giant ionic lattices are held together by strong electrostatic forces. it takes loads of energy to overcome these forces, so melting points are very high
what is the melting point of sodium chloride?
801 *C
why do ionic compounds tend to dissolve in water?
water molecules are polar - part of the molecule has a small negative charge and other bits have small positive charges. these charged parts pull ions away from the lattice, causing it to dissolve
how do molecules form?
when 2 or more atoms bond together - they could be the same or different
they’re held together by strong covalent bonds
what does a single covalent bond contain?
a shared pair of electrons
what happens in covalent bonding?
2 atoms share electrons, so they’ve both got full outer shells of electrons. both the positive nuclei are attracted electrostatically to the shared electrons
what do giant covalent structures have?
a huge network of covalently bonded non-metal atoms, sometimes called macromolecular structures
why can carbon atoms form giant covalent structures?
because they can each form 4 strong, covalent bonds.
what are 3 examples of giant covalent structures?
graphite
diamond
SiO2
what is the structure of graphite?
the carbon atoms are arranged in sheets of flat hexagons covalently bonded with 3 bonds each. the fourth outer electron of each carbon atom is delocalised
what properties does graphite have?
slippery electrical conductor low density very high melting point insoluble
how does graphite’s structure make it slippery?
the weak bonds between the layers in graphite are easily broken, so the sheets can slide over each other.
its used as a dry lubricant and in pencils
how does graphite’s structure make it an electrical conductor?
the delocalised electrons in graphite aren’t attached to any particular carbon atoms and are free to move along the sheets carrying a charge
how does graphite’s structure make it low density?
the layers are quite far apart compared to the length of the covalent bonds, so graphite has a low density and is used to make strong, lightweight sports equipment
how does graphite’s structure give it a high melting point?
the strong covalent bonds in the hexagon sheets give it its high melting point, sublimes at over 3900 K
how does graphite’s structure make it insoluble?
its insoluble in any solvent. the covalent bonds in the sheets are too strong to break
what is the structure of diamond?
made up of carbon atoms, each carbon atom is covalently bonded to four other carbon atoms, the atoms arrange themselves in a tetrahedral shape
what properties does diamond have due to its strong covalent bonds?
very high melting point - sublimes at over 3900 K extremely hard good thermal conductor can't conduct electricity insoluble diamond can be cut to form gemstones it sparkles
what uses does diamond have by being extremely hard?
its used in diamond-tipped drills and saws
how is diamond a good thermal conductor?
vibrations travel easily through the stiff lattice, so its a good thermal conductor
why can’t diamond conduct electricity?
all the outer electrons are held in localised bonds
why does diamond sparkle?
its structure make it refract light a lot
what it dative covalent bonding?
it’s where both electrons come from 1 atom
also called co-ordinate bonding
what’s an example of dative covalent bond?
the ammonium ion NH4 +
its formed when the nitrogen atom in an ammonia molecule donates a pair of electrons to a proton H+
how is dative covalent bonding shown in diagrams?
its shown in diagrams by an arrow pointing away from the ‘donor’ atom
what determines the shape of a molecule?
the number of pairs of electrons in the outer shell of the central atom.
electron pairs will try to sit as far apart from each other as possible
what are charge clouds?
areas where you have a really big chance of finding an electron pair, the electrons are constantly moving, bonding and lone pairs exist as charge clouds
what kind of electron pairs repel each other the most?
lone pair-lone pair - largest angle
lone pair-bonding pair
bonding pair-bonding pair - smallest angle
what is the system for naming the shapes of angles called?
valence-shell electron-pair repulsion theory
how to work out the shape of a molecule?
find number of electrons in the outer shell of central atom
add the number of atoms bonded to central atom
divide by 2 for number of electron pairs
subtract by number of bonds to find number of lone pairs
shape and angle of molecule with 2 electron pairs?
linear - 180*
shape and angle of molecules with 3 electron pairs?
no lone pairs: trigonal planar - 120*
1 lone pair: angular - 117.5*
shape and angle of molecules with 4 electron pairs?
no lone pairs: tetrahedral - 109.5*
1 lone pair: trigonal pyramid - 107*
2 lone pairs: angular - 104.5*
shape and angle of molecules with 5 electron pairs?
no lone pairs: trigonal bipyramid - 120, 90
1 lone pair: seesaw - 102, 87
2 lone pairs: T-shaped - 88*
3 lone pairs: linear - 180*
shape and angle of molecules with 6 electron pairs?
no lone pairs: octahedral - 90*
2 lone pairs: square planar - 90*
what is electronegativity?
a measure of the tendency of an atom to attract a bonding pair of electrons in a covalent bond
what is the most electronegative element?
fluorine
what makes a bond polar?
in a covalent bond between 2 atoms of different electronegativities the bonding electrons are pulled towards the more electronegative atom.
the greater the difference in electronegativity the more polar the bond
what’s a permanent dipole?
the difference in charge between the 2 atoms caused by a shift in electron density due to difference in electronegativity of atoms
what makes a molecule polar?
when the polar bonds in a molecule create an uneven distribution of charge across the whole molecule
why are not all molecules with polar bonds polar molecules?
if the polar bonds are arranged symmetrically in the molecule, then the charges cancel out and there is no permanent dipole
what intermolecular forces do polar molecules have?
permanent dipole-dipole forces between the 𝛿+ and 𝛿- charges on neighbouring molecules
what are intermolecular forces?
forces between molecules
what are the 3 main types of intermolecular forces in order of weakest to strongest?
van der waals
dipole-dipole
hydrogen bonding
how are van der waals forces formed?
instantaneous dipoles, create an induced dipole in another molecule and forces can form between the temporary 𝛿+ and 𝛿- charges
how are instantaneous dipoles formed?
electrons in charge clouds are constantly moving.
at any 1 moment there could be more electrons on 1 side of an atom than another, making it slightly 𝛿-
why are van der waals so weak?
because electrons are constantly moving, dipoles are created and destroyed all the time
how can van der waals be stronger?
the larger a molecule, the more electrons, the stronger the van der waals
unbranched molecules have stronger van der waals as they can lie closer to each other
what happens when intermolecular forces are broken?
the substance changes state, the stronger the intermolecular forces, the more energy that is needed to break them and melt/boil the substance
when does hydrogen bonding occur?
when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
why are hydrogen bonds so strong?
F, N and O are very electronegative, so draw electrons away from the hydrogen atom. the bond is so polarised, and H has such a a high charge density that the hydrogen atoms form weak bonds with lone pairs of electrons on F, N or O atoms of other molecules
what affect does hydrogen bonding have on the properties of all substances?
increase their melting and boiling points because of the extra energy needed to break the hydrogen bonds
why is ice less dense then water?
ice has more hydrogen bonds then liquid water that arrange themselves into a hexagonal regular lattice structure
this makes H2O molecules further apart on average than in liquid water
what structures do metals have?
giant metallic lattice structures
what is metallic bonding?
the outermost shell of electrons of each metal atom is delocalised.
these electrons are free to move about the metal
strong forces of attraction form between the positive metal ions and the sea of delocalised electrons
what properties do metals have?
high melting points good conductors insoluble malleable high density
what gives metals a high melting point?
strong electrostatic FoA between positive ions and delocalised electrons
the more delocalised electrons, the higher the melting point
why are metals good thermal conductors?
delocalised electrons can pass kinetic energy to each other, making them good thermal conductors
why are metals good electrical conductors?
the delocalised electrons can move and carry a charge
why are solid metals insoluble?
because of the strength of the metallic bonds
what are the particles of a solid like?
particles are very close together
this gives it a high density and makes it incompressible
the particles vibrate about a fixed point and can’t move freely
what are the particles of a liquid like?
similar density to solids, virtually incompressible
particles move about freely and randomly within a liquid, allowing it to flow
what are the particles of a gas like?
particles have loads of energy, and are much further apart
density is generally low, very compressible
particles move about freely, with little attraction between them so it’ll quickly diffuse to fill a container
what must be done to melt or boil a simple covalent compound?
you only have to overcome weak intermolecular forces that hold molecules together, not break the strong covalent bonds between atoms
which type of bonding has the highest melting and boiling points?
lowest to highest
simple covalent
ionic
metallic
giant covalent
which types of bonding can conduct electricity as solids?
giant covalent - only graphite
metallic - delocalised electrons
which types of bonding can conduct electricity as liquids?
ionic - ions are free to move
metallic - delocalised electrons
which types of boding are soluble in water?
simple covalent - depending on how polarised the molecule is
how are metals malleable?
they have layers that can slide
why are metals high density?
closely packed
how does the melting points of metals change down a group?
melting points decrease:
bigger ions
weaker giant metallic lattice
weaker forces of attraction between less tightly packed ions
how does the melting points of metals change across a period?
melting point increases: smaller ions more delocalised electrons e.g. Na+ has 1 delocalised electron per ion, Mg2+ has 2 stronger FoA and giant metallic lattice closer packed
what does isoelectric mean?
same electronic structure
which covalent bonds have the highest bond strengths?
the more bonds, the higher the bond strength
more shared electrons
so stronger FoA
so atoms are closer together
therefore single bonds are longer than multiple
what are pure covalent substances?
they have an even sharing of electrons, where electrons in the covalent bond are equally distributed between the 2 nuclei
how does electronegativity change across a period and down a group?
increases across a period
decreases down a group
what determines how electronegative a substance is?
size of nuclear charge
distance between nucleus and outer electron shell
shielding
how does nuclear charge effect electronegativity?
an atom with lots of protons in its nucleus will attract electrons strongly
as the nuclear charge increases so does the attraction between the nucleus and the electrons in the covalent bond
so electronegativity increases
how does the size of an atom effect electronegativity?
as the size of the atom increases, the pair of electrons in the covalent bond will be further from the nucleus
this means there is a decreased attraction from the nucleus
therefore electronegativity decreases
how does shielding effect electronegativity?
the more shielding, the further the pair of electrons are from the nucleus
this means there is a decreased attraction from the nucleus
therefore electronegativity decreases
why does electronegativity increase across a period?
all electrons are the same distance from the nucleus
protons are being added as you move across
shielding is the same
atoms are getting smaller
why does electronegativity decrease down a group?
the increased shielding means there is a reduced attraction from the nucleus on the electrons
the effect of increase in atomic radius means electrons are further away so increased nuclear charge doesn’t matter
why are hydrogen bonds between water molecules so strong?
oxygen has 2 lone pairs, so the forces of attraction are even stronger with the positive hydrogen on another water molecule
what important uses does hydrogen bonding have?
ice is less dense than water water has much higher melting and boiling point than expected protein folding DNA base airings enzyme reactions
