bonding & structure Flashcards
metallic bonding found?
in pure metals (sodium, iron) and in alloys (steel)
-metal atoms lose electrons to form + ions
these ions become surrounded by a sea of delocalised electrons- which binds the structure together.
typical metal properties
1) most -solids at room temp with high melting points
2) have high electrical/thermal conductivity
3) are malleable and ductile
ionic bonding found?
- in compounds containing metal and non metals
- metals lose electrons to form positive ions and non metals gain electrons to form negative ions
properties of ionic compounds?
solids at very high melting points
-soluble- in polar solvents (water)
only conduct electricity when dissolved or molten
covalent bonding found?
between non metal atoms
the non metals share outer shell electrons to form a stable structure
properties of covalent substances?
usually gases, liquids, or solids with low MP
do not dissolve in water- but do dissolve in non polar solvents
do not conduct electricity
metallic bond definition
is the electrostatic force of attraction formed between metal ions in a lattice and the sea of delocalised electrons between them
why are metals malleable and ductile?
because of the layers of metal ions that can slide over one another without breaking the metallic bonds.
comparing Na, Mg, Al variation in metallic bonding
1) MELTING POINTS increase as u go along:
a) the charges on ions increase from Na+, to Mg2+ etc
b) number of delocalised electrons increases
c) the ions get smaller- therefore closely pack together
2) ELECTRICAL CONDUCTIVITY increases:
because the number of delocalised electrons increases- so more electrons are able to move when PD is applied
ionic compounds are stable because?
energy is released when they form.
-metals have low ionisation energies and lose electrons easily
-non metals have exothermic electron affinities and gain E easily
- this means that positive and negative ions can form when metals and non-metals react
- as these ions attract, they become arranged into a giant ionic lattice structure contain billions of IB
THE STRENGTH OF IONIC BONDING
the force of attraction between ions in a lattice depends on
a) the charges on the ions:
high charge results in strong ionic bonds
b) the sizes of the ions:
small ions pack together more efficiently and form strong ionic bonds
covalent bonding usually leads to the formation of
molecules in non metal atoms
the shared pair of electron is attracted to the nucleus of both atoms and therefore holds them together
two types of covalent bonds?
normal and dative covalent bonds
Dative covalent bonds?
a dative covalent bond is a shared pair of electrons formed when one atom donates both electrons to the bond
dative covalent bonds are also formed when
metals dissolve in water
THE STRENGTH OF COVALENT BONDS
why do small atoms produce strong covalent bonds?
because the shared pair of electrons lies close to both nuclei.
-the strength of the covalent bond is measured by how much energy is needed to break it.
BONDING IN MOLECULAR IONS
molecular ions are basically molecules which have lost or gained electrons to become stable
-eg. Carbonate ions (CO3,2-)
In dot and cross diagrams, the triangle represents the extra electron gained from sodium.
3 types of covalent structures:
1) simple molecules eg. CH4, H20,CO2
2) giant covalent lattices eg. diamond, graphite
3) Polymers eg. poly (ethene), poly (propene)
SIMPLE MOLECULAR STRUCTURES;
- have low melting points (liquids, gases at RTP)
-because although covalent bonds inside the molecules are strong, there are only weak forces between the molecules
very little heat energy is needed to break these intermolecular forces.
- DONT CONDUCT AT ALL= BC NO CHARGE
GIANT COVALENT LATTICES
all atoms across the whole structure are joined by strong covalent bonds.
therefore are mainly solids with extremely high melting points bc huge amount of energy needed to break the strong covalent bonds
GIANT COVALENT LATTICE 2
What is the structure of diamond?
1) each and every C atom is covalently bonded to 4 others
2) does not conduct electricity (no free electrons)
3) pure silicon and silicon dioxide have similar structures to diamond
what is graphene?
a single layer of graphite, composed of hexagonally arranged carbon atoms linked together by strong covalent bonds.
uses of graphene:
1) used in touch screen, to make them less brittle
2) is light, flexible and transparent and conducts (electronics)
what is the structure of graphite?
each carbon atom has 3 covalent bonds to other carbon atoms in layers
- there are delocalised electrons between the layers which can move and conduct electricity
- feels greasy as weak forces between layers allow them to slide
what is a polymer?
1) a long chain molecule made up of many smaller molecules (monomers) joined together.
- the chains themselves are strong as they are made with covalent bonds.
- however there are only weak intermolecular forces between the neighbouring chains and so many polymeric substances have LMP and are stretchy.
what is the valence shell electron pair theory ?
what are its rules?
is used to explain why molecules have different shapes.
3 RULES:
1) basic shape arises from the number of electron pairs in the outer (valence) shell of the carbon atom in the molecule.
2) electron pairs repel eachother and therefore move as far apart as possible.
3) lone pairs have a greater repulsive effect than covalent bonding pairs, reducing the angle between any bonds
Types of bonding pairs
1) 2 bonding pairs= linear molecule eg. BeCl2
have a 180* angle in between
2) 3 bonding pairs= trigonal planar molecule eg. BCl3
have a 120* angle in between
3) 4 bonding pairs= tetrahedral molecule eg. CH4 and silicon tetrachlroride CCl4
4) 5 bonding pairs= trigonal bipyramidal molecule
eg. PCl5
5) 6 bonding pairs= octahedral molecule eg. SF6
6) 3 bonding pairs + 1 lone pair= trigonal pyramid molecule eg. NH3, PCL3
7) 2 bonding pairs + 2 lone pairs= V-shaped molecule
eg. H20 and SCl2
BONDING PAIRS
11) comparing methane, ammonia and water
all these molecules are tetrahedral shape as all have 4 pairs of electrons in the outer shell of the central atom
- however nitrogen in ammonia has one lone pair, oxygen in water has two lone pairs.
- this affects the bond angles because lone pairs carry a greater repulsive effect than bonding pairs.
BONDING PAIRS
2) molecules containing double bonds
in VSEPR theory, a double bond is counted as a single bonding pair.
INTERMOLECULAR FORCES
these forces are found between neighbouring molecules.
weak nature gives rise to low Melting and BP of covalent liquids and gases.
INTERMOLECULAR FORCES
What are the three types of intermolecular forces?
1) london forces (induced dipole-dipole interactions)
2) permanent dipole-dipole forces
3) hydrogen bonds
what are london forces?
- forces found between all atoms and molecules
- are the weakest of the intermolecular forces- arise bc electrons are always on the move.
for tiny fractions of a sec, there may be more electrons on one side of an atom, therefore the atom gains a TEMPORARY DIPOLE.
LONDON FORCES
Whats happens when an atom has a temporary dipole?
it will attract the neighbouring atoms around it by inducing them with a dipole.
the more electrons present in an atom or molecule, the stronger the london forces.
LONDON FORCES
boiling points of the noble gases?
increase down the group because the atoms have more electrons and stronger london forces and so more heat energy is needed to break them
LONDON FORCES
boiling points of the alkanes?
increases as the length of the carbon chain increases because bigger molecules have more electrons and stronger london forces and more energy is needed to break them
Permanent dipole-dipole forces
- these are found between molecules which have permanent dipoles.
- polar molecules arise from electronegativity differences in the atoms
- the + charge of one molecule is attracted to the - of another to form a dipole-dipole force.
