1.3 Bonding Part 2 Flashcards
Electronegativity
The power of an atom to attract the pair of electrons in a covalent bond
Top 5 electronegative elements
Fluorine, oxygen, nitrogen, chlorine and bromine
what is the trend in electronegativity across a period
increases
why does electronegativity increase across a period
nuclear attraction on outer electrons decreases
what is the trend in electronegativity down a group
decreases
why does electronegativity decrease down a group
nuclear attraction on outer electron decreases
do noble gases have electronegativity values?
no because they don’t normally form covalent bonds
what is electronegativity like in a non polar bond
same atom so same electronegativity
electrons are evenly distributed
what is electronegativity like in a polar covalent bond
significant difference in electronegativity
covalent bond where electrons are unevenly distributed
what is electronegativity like in an ionic bond
difference so large that electrons permanently go to one of atoms forming ions
what does delta mean
difference
how do we show a negative charge on an atom involved in a polar bond
delta negative
how do we show a positive charge on an atom involved in a polar bond
delta positive
what do charges depend on
how electronegative the atom is- more electronegative- more power to attract electrons- negative
what are hydrocarbons non-polar
carbon and hydrogen have similar electronegativity
what do polar bonds mean in a simple molecule
the whole molecule has an uneven distribution of electrons
what happens in more complex molecules
the dipoles of the polar bonds may cancel out
what decides whether dipoles cancel out or not
whether the molecule is symmetrical or not
what are intermolecular forces
forces of attraction between molecules
what are the three types of intermolecular force
- van der Waals’ force
- permanent dipole-dipole forces
- hydrogen bonds
which substances have van der Waals’ forces within them
all molecules and atoms
which molecules contain permanent dipole-dipole forces (and VDW)
between polar molecules
what is a hydrogen bond
special case of permanent dipole-dipole force
where do hydrogen bonds occur (and VDW and permanent dipole-dipole)
where theres a delta plus hydrogen atom and either a nitrogen, oxygen, fluorine with a lone pair
example of a molecule with hydrogen bonds
water
what is special about intermolecular forces
molecules with hydrogen bonds also contain permanent dipole-dipole forces and VDW forces
molecules with permanent dipole-dipole forces also contain VDW forces
What are VDW forces caused by
The movement of electrons which unbalances the charge distribution within the molecule. This creates an instantaneous dipole across the molecule
The instantaneous dipole is constantly forming and disappearing
What does the dipole constantly forming and disappearing induct
A dipole in neighbouring molecules, resulting in weak forces of attraction between molecules
VDW forces are present between all molecules but…
Not ions or metals
What properties do non-polar molecules have
Relatively low boiling points
Generally gases/volatile liquids at room temperature
Why do bigger molecules have larger induced dipoles
They have more electrons
What do larger induced dipoles in bigger molecules result in
Stronger VDW forces between molecules
Where do permanent dipole-dipole forces occur
Between molecules which have a permanent dipole
They occur in addition to VDW forces
The delta positive end of one molecule is attracted to the delta negative end of a neighbouring molecules
Stronger than VDW forces
Where does hydrogen bonding occur
Between molecules which contain a hydrogen atom bonded to either F, O or N
Between a delta positive hydrogen atom in one molecule and a lone pair of electrons in an N, O or F in a neighbouring molecule
Occur in addition to VDW
Strongest intermolecular force
Why do substances which contain hydrogen bonds have higher boiling points than expected
Due to the strength of the hydrogen bonds (strongest intermolecular force)
Why do substances which contain hydrogen bonds dissolve in water
They form hydrogen bonds with water
Why does ice float on water
The density of ice is less than that of water.
Why does water expand when it solidifies (ice)
As the temperature gets to 0 degrees the water molecules are held further apart by hydrogen bonds in an open lattice
What is sublimation
When a material goes straight from a solid to a gas eg. Iodine and carbon dioxide
Energy is needed to change a substance from a solid to a gas to…
Overcome forces of attraction
When is a bond polar
When there is an uneven distribution of electrons
When is a bond polar
When there is an uneven distribution of electrons
When is a bond polar
When there is an uneven distribution of electrons
when can an electric current flow
if there are charged particles which are free to move ie. delocalised electrons or mobile ions
when can substances dissolve
if solute and solvent molecules attract one another
what can ionic and polar substances dissolve in
polar solvents such as water
what can non polar substances dissolve in
non polar solvents such as hexane
what is the structure in ionic compounds
giant ionic lattice
how are the ions arranged in the lattice
negative and positive ions alternate
. each ion surrounded by oppositely charged ions in all directions
in soldium chloride, what is each sodium ion surrounded by
6 chloride ions
in sodium chloride, what is each chloride ion surrounded by
6 sodium ions
what is the giant ionic lattice held together by
strong ionic bonds
why do ionic compounds have high melting points
lots of strong ionic bonds need to be broken
do ionic compounds conduct electricity
when solid no
when dissolved/molten yes
why do ionic compounds not conduct electricity when solid
ions held fixed in the lattice
why can ionic compounds conduct electricity when molten/dissolved
ions free to move
why are ionic substances brittle
if enough force is applied the layers slide over eachother because like charges move next to eachother, causing repulsion and the lattice structure breaks down
what is the structure of a metal
positive metal ions surrounded by a sea of delocalised electons
positive ions fixed
electrons free to move
why do metals have high melting points
giant structure and metallic bonds are strong
why can metals conduct electricity when solid/liquid
delocalised electons can flow through structure and carry the current
why are metals strong
metallic bonds are strong and extend through the giant metallic lattice
what does metallic bond strength depend on
size and charge of metal ion (smaller and higher charged ions are stronger)
what is a malleable substance
can be hammered or pressed into shape without breaking or cracking
what is a ductile substance able to do
be drawn into a wire
why are metals malleable/ductile
layers of ions in giant metallic lattice can slide over each other into new positions without disrupting metallic bond
what are the two main types of covalent substance
simple and giant/simple and macro-molecule
describe the structure of a molecular crystal of iodine
covalent bonds between iodine atoms- weak VDW forces between i2 molecules
why do simple covalent molecules have low melting points
all that Is needed is to overcome weak intermolecular forces (VDW,dipole-dipole, H-bonds)
are simple covalent molecules soluble
usually insoluble in water unless they can form hydrogen bonds/react with water
why don’t simple covalent molecules conduct electricity
no charged partices which are free to move, don’t contain ions/delocalised electrons
examples of macromolecular crystals
diamond, graphite and graphene
why do macromolecular crystals have a high melting point
strong covalent bonds between all atoms. Lots of energy needed to break these
why does diamond not conduct electricity
electrons are localised in the covalent bonds so are not free to move- no ions present
why does graphite conduct electricity
one electron per carbon not involved in bonding and is delocalised along the layer
why does graphene conduct electricity
better conductor than silver! delocalised electrons carry current
why are the macromolecular crystals insoluble in water
covalent bonds are very strong and the lattice does not break up when any solvent is added
