topic four/fourteen Flashcards
what is an ionic bond
an electrostatic attraction between oppositely-charged ions. Ionic bonds are formed by elements with an electronegativity difference of approximately 1.8 units and greater.
howa re ions making up an ionic compound arranged
a lattice structure
why is ionic bonding described as non directional
the force of attraction occurs in all directions around the individual ions.
hwo to find the coordination number
number of ions in contact with other ions
what are polyatomic ions
ions that consist of more than one atom
what is the type of bonding between a polyatomi ion and another
ionic
why are ionic compounds charges cancel out
they are electrically neutral
4.1.2
charges etc
why do ionic compounds have high mp
relatively large amounts of energy are required to break these forces of attraction between the ions.
what does the mp of an ionic compound depend on
the ionic charge and the ionic radius of its component ions
what does voltaility mean
a term used to describe how easily a substance evaporates
insoluble ionic compounds
Hydroxides (OH–) and sulfides (S2–) are insoluble except when combined with group 1 ions, the ammonium ion and Ca2+, Sr2+, and Ba2+
Carbonates, CO32–, and phosphates, (PO43–) are insoluble, except when combined with group 1 ions and the ammonium ion
how are ionic compounds dissovled
firstly, the breakdown of the lattice structure, then the ions being surrounded by water molecules (hydration)
If the forces of attraction between the water molecules and the ions in the lattice structure are strong enough to overcome the attractions between the ions themselves, then the ionic compound is what
soluble
Non-polar solvents cannot disrupt the lattice structure in the same way as polar solvents, therefore, the solubility of ionic substances in solvents such as hexane and propanone is what
limited
why do ionic ompounds conduct electricity when molten
ions are free to m ove and carry current
why are ionic compounds brittle
This occurs because the movement of the ions within the lattice when the force is applied places ions of the same charge next to each other. The forces of repulsion between ions of the same charge cause the lattice structure to split and fracture.
if something has a small ionic radius does it have a high or low mp
high
what is a covalent bond
the electrostatic attraction between positively charged nuclei and shared pairs of bonding electrons.
whcih exist as diatomic molevules
halogens
hydrogen
oxygen
nitrogen
what is differnt bout diatomic oxygen and nirtogen
atoms are bonded via double and triple covalent bonds
how is a coordiante covlen tbond different
both bonding electrons come from one atom
atoms that are electron deficient can or can’t form coordinate covalent bonds
can
dimer def
a larger molecule composed of two identical smaller molecules and can be linked by coordinate covalent bonds or by hydrogen bonds.
what is bond order
the number of bonds between a pair of atoms
over 1.8 units difference in electronegativity
ionic
0.5-1.7 units difference in electronegativity
polar covalent
0.1-0.4 units difference in electronegativity
non polar or weakly polar covalent
0
pure covalent
The greater the electronegativity difference between the atoms, the greater the polarity of the bond and the greater its
ionic character
when do pure covalent bonds occur
between atoms that have no differene in electronegativity, such as those in molecular oxygen chlorine nitgoen
when do polar covalent bonds occur
between atoms that have a difference in electronegativity of between 0.5 and 1.7 units
what causes a bond dipole
unequal sharing of electrons in a covalent bond
what does the octet rule state
the most stable arrangement for an atom is to hae eight electrons
exceptions to the octet rule
Hydrogen is stable with only two electrons in its outer shell.
Atoms such as boron, beryllium and aluminium (in compounds) are stable with fewer than eight electrons in their outer shell.
Atoms in period three and higher, such as sulfur, can form expanded octets with up to twelve electrons in their valence shell.
what are incomplete octets
less than eight electrons
in a resonance hybrid structure
intermediate and identical in strength and length between a single and double
benefit of delocalised electrons
give greater stability to a molecule or polyatomic ion
electron domains
bonding nad non bonding electrons
which cause more repulsion nb electrons or b electorns
nb, lone pairs
2 electron domains
liner molecular and electron
Electron domain and molecular geometries for up to four electron domains
Number of electron domains Number of bonding domains Number of non-bonding domains Electron domain geometry Molecular geometry
4 4 0 tetrahedral tetrahedral
4 3 1 tetrahedral trigonal pyramidal
4 2 2 tetrahedral bent / V-shaped
3 3 0 trigonal planar trigonal planar
3 2 1 trigonal planar bent / V-shaped
2 2 0 linear linear
do polar or non polar molecules have net dipole movement
polar
what is the net dipole moment
a measure of its overall polarity. It is the sum of all the bond dipoles in a molecule.
giant covalent structure / network covalent structure or macromolecular structure
silicon / diamond
diamond properties
hard
high mp
poor electrical conductor
no delocalised electrons within its structure
silicon and silicon dioxide
giant covalent structure
tetrahedral
109.5 degree bond angle
strong covalent bonds
poor conductor of electricity
how to improve the conductivit of silicon
The electrical conductivity can be improved by a process known as doping. This involves the addition of small amounts of elements such as phosphorus or boron to pure silicon.
waht are the allotropes of carbon
diamond
graphite
fullerenes
graphite
layered structure
planar sheets of hexagonally arranged carbon atoms
The layers are held together by relatively weak London dispersion forces.
Each carbon atom has an electron which becomes delocalised across the plane. The presence of delocalised electrons explains the ability of graphite to conduct electricity along the plane of the crystal when a voltage is applied.
fullerene
bond angle is 120
poor conductor of electricity
rings of 5 and 6 carbon atoms
fullerene C60 is by definition a simple molecular substance even though it contains so many bonded carbon atoms
The structure is made up of carbon atoms bonded together in 20 hexagons (six-carbon rings) and 12 pentagons (five-carbon rings), known as a truncated icosahedron.
graphene
Its tensile strength is 1000 times greater than steel.
It behaves as a semi-metal, so it is very suitable for electronic devices.
Adding only 1% content of graphene to plastics could allow those plastics to conduct electricity.
graphene is the most chemically reactive. This is because of the reactive edges of the structure, where there are carbon atoms with unoccupied (‘dangling’) bonds. This reactivity could be used in important ways. For example, membranes of graphene oxide have been shown to be preferentially permeable to water, which could be useful in desalination and water purification.
The three main allotropes of carbon show differences in their structure. How many atoms is each carbon directly bonded to in these allotropes?
diamond graphite fullerene, C60
ii 4 3 3
intramolecular bonds
bonds within the molecule
london dispersion forces
weakest type of intermolecular force
what is a temporary dipole
A temporary dipole is caused by changes in electron density within an atom or molecule. At a certain moment in time, the electrons may be concentrated on one side of an atom or molecule, giving this side a slight negative charge, and the opposite side a slight positive charge. These opposite charges are given the symbols δ+, (for a partial positive charge) and δ- (for a partial negative charge). A molecule with a temporary dipole can induce a dipole in a neighbouring molecule; this is known as an induced dipole
waht does the strength of london forces depend on
The ease with which the electrons in an atom or molecule form a temporary or induced dipole (their polarisability).
The surface area of the molecule.
when does polarisabikity increase
alongside molar mass of a molecule. so does melting point and strength of London dispersion forces
dipole dipole forces
only exist between polar molecules that have a permanent dipole
hydrogen bonding
occurs between molecules that have an electronegative nitrogen, oxygen or fluorine atom directly bonded to a hydrogen atom.
a stronger type of dipole–dipole attraction
like dissolves like means
it means that polar substances are soluble in polar solvents and non-polar substances are soluble in non-polar solvents. Ethanol, for example, is a polar molecule and is soluble in water (a polar solvent) because of the hydrogen bonds that form between ethanol molecules and water molecules
metals are
good conductors of heat and electricity,
ductile (can be made into wires),
malleable (can be bent into shape) and
shiny when polished.
metallic structure
delocalised electrons in ionised metal atoms.
non directional (force of attraction in all directions between the positive ions and delocalised electrons within the lattice structure)
why are mtals malleable
one layer of metal ions can slide over another without disrupting the metallic bond; the bond remains intact
When a potential difference (voltage) is applied across a metal,
a direction is imposed on the movement of the delocalised electrons. They are repelled from the negative electrode and move towards the positive electrode. This orderly flow of delocalised electrons in a given direction constitutes the flow of an electric current.
how do metals conduct heat
the electrons move through the metal, carrying kinetic energy (in the form of vibrations) from the hotter part of the metal to the colder part of the metal. These delocalised electrons are also responsible for the shininess of metals, because they reflect wavelengths of visible light.
what does the strength of the metallic bnd depend on
charge ont he metal ion and ionic radius of the metal ion. they also affect density
what is an alloy
homogeneous mixtures composed of two or more metals or a metal and a non-metal
why are alloys harder and more resitant to corrosion
layers cannot sldie over eachother as easily
what is added to metals to make alloys
carbon or phosphorus perhaps but mainly other metals
how is a sigma bond formed
direct head-on (axial) overlap of atomic orbitals
The two 1s atomic orbitals overlap head-on, forming a sigma bond.
pi bond formation
formed by the sideways overlap of two unhybridised p orbitals
electron density ina sigma bond
The electron density in a σ bond is concentrated in the region directly between the nuclei of the bonding atoms; this can also be described as cylindrical symmetry along the bond axis (Figure 2). This allows free rotation around a σ bond.
what makes up a double bond
one sigma one pi
which bond is stronger pi or sigma
sigma
where does the extra strength of the sigma bond come from
The extra strength of the sigma bond comes from the greater overlap of the atomic orbitals in the bond. In a pi bond, the atomic orbitals cannot overlap as much which results in a weaker bond.
purpose fo formal charge
used ot determin ewhich lewis structure is the preferred one when there is more than one possibility
formal charge equation
(number of valence electrons) − ½ (number of bonding electrons) − (number of non-bonding electrons)
the sum of formal charges in a neutral molecule must equal
zero
For a polyatomic ion, the sum of the formal charges must equal the
overall charge of the ion
delocalised pi electrons
electrons that are shared between more than two nuclei; they are present in molecules and polyatomic ions for which there is more than one possible Lewis structure (known as resonance structures). They originate from the overlap of pi bonds in a molecule or ion.
benzene carbon atoms are what hybridisation
sp2
which is more stable resonance structure or resonance hybrid structure
resonance hybrid
resonance structures of ozone
CFC
Chlorofluorocarbons (CFCs) are highly stable compounds. This stability was initially seen as an advantage but it meant that CFC molecules released into the lower atmosphere could remain intact and reach the upper atmosphere. Here, when exposed to UV radiation, compounds such as trichlorofluoromethane (CCl3F) decompose to produce chlorine free radicals (Cl*), as shown in the equation below.
which elements can accomodate more than eight electrons in their valence shells
Elements in period 3 and beyond. This is due to the availability of d orbitals which can be used for bonding
