Unit 1.4 Flashcards
Ionic bonding?
Bond formed by the electrical attraction between a cation and an anion
Covalent bond?
pair of electrons with opposite spin shared between 2 atoms with each atom donating 1 electron
Co - ordinate bond?
a covalent bond in which shared electrons come from the same atom
Bonding?
due to electrical attraction and repulsion between protons + electrons
Ionic bonding?
cations and anions
each cation is surrounded by anions and vice versa
maximising electron and minimising repulsion
What does the repulsion mean?
prevents the ions from getting too close
Covalent?
the electrons in the pair repel each other but this is to overcome by their attraction to both nuclei
if the atoms get too close, inner electrons repel so the covalent bond is always a certain length between different elements
electrons spin must be in the opposite direction for a bond to form
What is the Pauling scale?
a measure of electron negativity
Electronegativity?
a measure of the ability of an atom to attract an electron in a covalent bod
Polar bonds?
In a covalent bond, the electron pair is not shared equally due to differences in their electronegativity
How are they classed as non polar covalent bonds?
If 2 atoms are less than 0.4 diference
0.4 - 0.9?
a polar covalent bond
If bigger than 2?
the bond = ionic
Intramolecular?
the bonding within molecules
Intermolecular?
bonding between molecules
strong bonds between the atoms which govern its chemistry
What are intermolecular?
weak bonds holding molecules together + it governs physical properties
How many types of dipole are there?
3
What are dipole?
polar molecules have dipole S+ AND S-
they arrange themselves so the positive of one attract the negative of another
Why can a permanent dipole not always be formed?
Dipoles do not always align to produce an attraction as they move randomly
In methane?
forces = very weak and molecules separate for example liquid boils at -162 degrees but C-H bonds are very strong + need a temp of 600degrees before they will break
What are intermolecular bonding caused by?
caused by electrical attraction between opposite charges
Why will the molecule have a Dipole - Dipole?
molecule may have + and - dipoles and if electronegativities of the atoms are not the same
If dipoles arrange themselves?
the negative region of one molecule is close to the positive region of another molecule, there will be a net attraction between them
Do even molecules with no dipole dipole show intermoleculer bonding?
yes e.g the atoms that come together to form a liquid at 4K
why is this?
the electrons are in constant motion around the nuclei so centres of positive and negative charge do not always
co - incide, giving a fluctuating dipole
come into step with one another as one dipole induces an opposite dipole in a nearby molecule giving an attraction between them
What are the 2 types of IMF?
First dipole - dipole
Second induced dipole induced dipole = VDW
What is the bonding inside molecules?
100 x stronger than between them with VDW strength being around 3KJ -1
Hydrogen bonding?
Special IMF
only occurs between molecules that contain Hydrogen atoms bonded to very electronegative elements having Lone pairs namely
F
N
O
although weak compared with bonding taking place inside molecules, it is much stronger than VDW
H bonds?
30 KJ
300 KJ bonding within molecules
H bonding is much stronger than VDW
since small H+ IS sandwiched between 2 electronegative elements + allows a close approach
When boiled?
energy needs to overcome forces
What has high boiling points?
H2O and NO3 has high boiling points in comparison to Group 4, Group 5 or Group 6 Hydrides
due to Hydrogen Bonding
H bonding in water and ice?
H+ is especially delta + being attracted to the electronegative oxygen atom so that the oxygen on the other molecule is closely attatched to it
When are the bonding the strongest?
when the 3 atoms in a straight line and the internal
O-H bond is shorter than the dotted Hydrogen connected to other molecule
Since Oxygen has 2 lone pairs + 2 hydrogen atoms, a tetrahedral hydrogen bonded structure is formed
What are the effects on Hydrogen bonding on boiling temps + solubility?
Melting + especially boiling temps increase with the strength of IMF
VDW because there is a steady increase when these molecular mass + also as dipoles become larger
What does the diagram show?
that Hydrogen-bond molecules freely Hydrogen bond with neighbours + these bonds must be largely broken before boiling can occur so more energy i.e a higher temp is needed
Solubility in water?
as well as H bonding with other water molecules, they themselves can dissolve other molecules such as the lower alcohols with which they can hydrogen bond
What can non polar organic molecules?
cannot Hydrogen bond with water = insoluble in it + prefer to interact with one another through VDW forces
Shapes of molecules?
shapes of covalent molecules with more than 2 atoms + their ions is governed by the electron pairs around the central atoms such as CH4
2 Points to watch out for?
how many electron pairs is connected to the other atoms
how many electron pairs in total around the central atom
in NH3?
there is 3 connected pairs, however the fourth lone pair pushes molecule in an umbrella shape + when H+ is added to give NH4+ is tetrahedral
Water?
2 lone pairs bend strongly
because since all electron pairs repel one another, molecular shape taken up is that allowing pairs to keep as far away from each other as possible to minimise the repulsion energy
while bonding pairs are spread out between the 2 atoms bonded in the molecule, lone pairs stay close to the central atom and so repel more than bonding pairs giving the repulsion sequence.
What is greater than what?
Lone pair - Lone pair > Lone pair - bond pair > Bond pair - Bond pair
Example?
NH3 with one lone pair + 3 bonding pairs, repulsion between the lone pair + bond pairs = greater than that between the bond pairs themselves so HN-H angle is closed up from 109 to 10
VSEPR theory?
VALANCE SHELL REPULSION THEORY
What does it do for us?
Lets us predict the shape of similar molecules in which bonded atoms = arranged to form a central atom
Valance shell?
electron shell in which bonding occurs
VSEPR?
number of electron pairs is first found to give the general shape of the molecule since Repelling basis keeps as far away from one another in the spaces as possible
Linear?
0 Lone pairs
2 Bonding pairs
180 degrees bond angle
BECl2, CO2
Trigonal planar?
0 lone pairs
3 Bonding pairs
120 degrees Bond angle NH4+, CH4
Trigonal bipyrmidal?
0 lone pairs
5 bonding pairs
120 /90 degrees Bond angle
PCl5
Octahedral?
0 lone pairs
6 Bonding pairs
90 degrees Bonding angle
SF6
Trigonal pyrimidal?
1 Lone pairs
3 Bonding pairs
107.5 degrees Bond angle
NH3, H30+
V shaped?
2 lone pairs
2 bonding pairs
104.5 degrees bond angle
H2O,F2O
What is the normal tetrahedral bond angle in water?
water with 2 lone pairs
normal bond = 109 degrees for
H-O-H repelled down to 104 degrees by lone pair and lone pair - bond repulsion
Shape of molecule?
governed by number of electrons in valance shell
electron pairs arrange themselves as far as part as possible, so minimum repulsion
