2.2.2 Electronegativity + Bond Polarity Flashcards
Why aren’t all covalent bonds symmetrical (rarely equally shared)
As atoms have different electronegativities
What’s electronegativity
The power of an atom to withdraw electron density toward itself from a covalent bond (pull electrons toward itself in a covalent bond)
How does EN increase on the periodic table
Going up a group
going right across a period
What part of the periodic table has the most electronegativity
What’s the element with most EN
Top right
F (4.0)
How does electronegativity increase
Why
From left -> right
As the nuclear charge increases, the shielding of outer electrons remains the same but the attraction of the bond pair to the nucleus increases
What are non-polar bonds
If 2 atoms at the end of a covalent bond have similar electronegativities (small difference), electrons mostly stay in the middle as there’s little electron density distortion
What’s a polar bond
A covalent bond where the electron pair distribution isn’t symmetrical
How’s a polar bond formed
If 1 atom in a covalent bond is more EN than the other, the more En atom draws the electron density towards itself and get partially negatively charged (d-).
The other atom becomes partially positively charged (d+) as it’s now short of electron density
What can a polar bond be shown as
What is this
A ‘bond dipole’
Combination of a + and an arrow
+ - d+
Arrow - d-
What charge does the more EN atom get in a polar bond
Partially negative charge (d-)
What 4 elements covalently bonded to another atom usually form a polar bond
N
O
F
Cl
What are dipole moments
Directions in which polar bonds move (overall polarity)
Why would CO2 and CCl4 not have dipole moments
As the bonding dipole effect acts with equal force in opposite directions, cancelling out
What leaves open the possibility of permanent dipole-dipole forces forming
If there’s a large difference in electronegativity of 2 atoms in a covalent bond , the bond will be polar and contain d- and d+ regions
What directions to dipole moments go in
d+ -> d-
What molecules do intermolecular forces of attraction only occur between
E.g
Simple covalently-bonded molecules
E.g ethanol
What happens to the IMFs when a substance melts/boils
What bonds can’t do this as they’re too strong
IMFs break
Covalent bonds are too strong
What are IMFs of attraction like compared to covalent bonds
Very weak
What must be refer the first 2 types of IMFs as instead of bonds and why
What are they
Must refer to them as interactions as they’re so weak
Induced dipole-dipole interactions (very,very weak)
Permanent dipole-dipole interactions (very weak)
What are the 3 types of IMFs in order of weakest to strongest
Induced dipole-dipole interactions
Permanent dipole-dipole interactions
Hydrogen bonding
What are IDDI
The d- region of 1 atom weakly attracting the d+ region of another
What IMF is present in all atoms + molecules all the time
IDDI
What atom has the strongest IDDI
Why
Larger atom
As will have more electrons
What IMFs are only temporary
IDDI
What molecules do permanent dipole-dipole interactions act between
E.g
Molecules with permanent dipoles (dipole moments)
E.g HCl, H2O
What are permanent dipole-dipole interactions like compared to IDDI
A covalent bond
Stronger than IDDI
Very weak compared to a covalent bond
When’s does hydrogen bonding occur
When hydrogen is directly bonded to a very EN atom
3 atoms hydrogen bonding occurs between
N
O
F
What must there be on the N, O or F for hydrogen bonding to occur
A lone pair
What’s hydrogen bonding formed from
The lone pair of electrons on 1 molecule to a H on an adjacent molecule
What’s the strongest IMF
Hydrogen bonding
What happens in water when it boils
The O-H covalent bonds don’t break, the hydrogen bonds between water molecules do
What does water turn to when it’s boiled and hydrogen bonds are broken
Stream
3 factors that affect electronegativity
Nuclear charge (no. of protons)
Shielding
Atomic radius
How does nuclear charge affect EN
More protons -> stronger attraction of electrons to positive nuclei
How does shielding affect EN
More shells -> weaker attraction
How does atomic radius affect EN
Smaller atomic radius (closer bonding pair to nucleus) -> stronger attraction
Why does En decrease down a group (2)
More shielding so less positive attraction from nuclei can be felt
Atom gets bigger
Why does EN increase across a period (3)
Nuclear charge increases (protons)
Atom gets smaller
Stronger attraction between bonding pair and nucleus
What does hydrogen bonding affect
Boiling points
What molecules are easier to boil
Non-polar molecules that don’t contain N, O or F
What’s the boiling point like for a molecules with hydrogen bonding
High
What’s water MP + BP like and why (2)
High due to:
It’s size + Mr
The hydrogen bonds that need more energy to break
What formations are molecules in ice arranged in
Tetrahedral formations
When do hydrogen bonds become fixed in water
Not when waters a liquid only when it cools to ice
Why does ice float on water
As it’s less dense than liquid water due to having long hydrogen bonds that cause water molecules in the ice lattice to pack further apart
What are hydrogen bonds responsible for in biology
For holding the 2 helices in DNA
Strong - hold 2 strands together between replications
Weak - to break 1 by 1 to allow it to unwind for copying during transcription
What are the 2 types of covalent structures
Simple (molecular) lattice structures Giant covalent (macromolecular) lattice structures
Common example of a simple (molecular) lattice structure
Iodine - I2
How strong are simple (molecular) lattice structures
Soft and easy to break
What’s the melting and boiling point of simple (molecular) lattice structures like
Low melting point and boiling point
Sublimed easily with barely any heat (solid->gas)
Do simple (molecular) lattice structures conduct electricity? Why
No
There aren’t any charged particles to carry current
What are the forces and bonds like in simple (molecular) lattice structures
Long IDDI (London forces) Short covalent bonds
3 examples of giant covalent (macromolecular) lattice structures
Diamond
Graphite
Silicon dioxide (sand)
What’s the structure of diamond like (3)
3D
Each carbon bonds to 4 others
Each carbon has a tetrahedral structure
What’s the strength and melting point like of diamond? Why?
Hard
High MP as many strong covalent bonds must be broken to separate atoms (macromolecular)
Does diamond conduct electricity? Why?
Doesn’t conduct electricity as particles are fixed and don’t have a charge
What’s the structure of graphite like (2)
Has layers of carbon atoms
Each C covalently bonded to 3 others
What is between the layers of graphite?
What does this allow them to do
Weak London forces
Allow them to side over each other and behave as lubricant
Does graphite conduct electricity
Why
Yes
As it has delocalised electrons between layers
What’s the strength + melting point like of graphite
Why
High MP due to strong covalent bonds (macromolecular)
Soft due to weak IDDI
What’s graphene and its structure
A single layer of graphite
Hexagonally arranged carbon atoms linked by strong covalent bonds
What 2 things does graphene conduct
What’s its thickness and strength like
Conducts electricity + copper
Thinnest + strongest material ever made
What structure is silicon dioxide (sand) similar to
What’s the exception
Similar to diamond structure
Except oxygen atoms are covalently bonded between silicon atoms
What properties is silicon dioxide similar to
Diamond
What structure won’t dissolve in water and will conduct at all states
Metallic
What happens to particles in a solid when heating it
They vibrate more about a fixed position, causing temperatures to rise
Slightly expand as particles move apart
What does heat break when turning a solid to a liquid
So what’s the temperature like at melting point
Heat breaks forces between particles
Temperature is constant at melting point
Why does temperature rise more when heating a liquid
Why does expansion occur again
At heat supplied gives liquid particles more kinetic energy to move faster causing rise in temperature
Expansion occurs as particles move apart slightly more
When a liquid is turning to a gas what does the heat do
What’s temperature like at boiling point
Breaks all intermolecular forces between particles
So temperature is constant at boiling point
Why does temperature rise further when heating a gas
As heat supplied is used to increase speed (kinetic) energy of gas particles
Expanding gas rapidly as particles again move apart
