Bonding and Structure - Topic 2 Flashcards
What are London forces
The attractive forces between all atoms and molecules
Why does an increase in pi bonds increase the London forces
As the bigger the electron cloud the more uneven electron distribution there is so there would be a much stronger imbalance in charge causing there to be a stronger dipole, so a higher polarizability so stronger LDN forces
covalent bonding
the electrostatic attraction between the bonding shared pair of electrons and the two nuclei
What is expansion of the octet
when an atom is able to have more than 8 valence electrons
What are molecular orbitals
The overlap of atomic orbitals from separate atoms
How many electrons can molecular orbitals hold
2
Types of molecular orbitals
Sigma
Pi
Where are sigma bonds found
Between atoms
Where are pi bonds found
Below and above atoms
what are sigma bonds
the direct overlap of two atomic orbitals
How are pi bonds formed
When p orbitals in separate atoms overlap sidewards forming pi bonds above and below the atoms
Properties of sigma bonds
- all single bonds are sigma bonds
- sigma bonds are free to rotate as rotating each nuclei has no impact on the bonding orbital
- have a high level of attraction between nuclei and shared electrons so are very strong
How many pi and sigma bonds are there in double bonds
x1 sigma
X1 pi
How many pi and sigma bonds are there in triple bonds
x1 sigma
x2 pi
Why are pi bonds weaker than sigma bonds
As the electrons in the pi bond are further away from the nuclei compared to sigma bonds so has there is a weaker level of attraction making the bonds easier to break
What is hybridisation
The concept of mixing w atomic orbitals to create a new type of hybridised orbital which has a different shape and energy level
Number of hybridised and unhybridised orbitals and what is the hybridised orbital called in single bonds
0 unhybridised orbitals
4 hybridised orbitals
Name: sp3 orbitals
Number of hybridised and unhybridised orbitals and what is the hybridised orbital called in double bonds
1 unhybridised p orbital
3 hybridised orbital
Name: sp2
Number of hybridised and unhybridised orbitals and what is the hybridised orbital called in triple bonds
2 unhybridised p orbital
2 hybridised orbitals
Name: sp
In terms of hybridised orbitals explain why carbon has a tetrahedral shape
Each sp3 orbital has the same energy so the electrons will have equal repulsion between all 4 orbitals causing the orbitals to point away from each other at maximum distances at the angle of 109.5 degrees between then
No lone pairs 2 electron pairs geometric shape name
Linear
Bond angle of linear molecule
180 degrees
3 electron pairs no lone pairs name of molecule
Triagonal planar
What are electron domains
Things attached to the central atoms e.g other atoms or lone pairs
What makes up a linear geometric shape
2 electron pairs no lone pairs
What makes up a triagonal planar shape
3 electron pairs no lone pairs
2 bonding pairs 1 lone pair geometrical shape
V shaped
2 bonding pairs 2 lone pairs shape
V shaped
Bond angle for Trigonal planar
120 degrees
Bond angle of v shape
104.5 degrees
What makes up a v shape
2 bonding pairs of electrons and 2 lone pairs
OR
2 bonding pairs and 1 lone pairs of electrons
4 bonding pairs of electrons and no lone pairs geometric shape
Tetrahedral
Tetrahedral bond angle
109.5 degrees
What makes up a tetrahedral shape
4 bonding pairs no lone pairs
3 bonding pairs 1 lone pair geometric shape
Trigonal pyramidal
trigonal pyramidal bond angle
107 degrees
3 bonding pairs 1 lone pairs geometric shape
Trigonal pyramidal shape
6 bonding pairs no lone pairs
Octahedral
What makes up an octahedral
6 bonding pairs
What makes up a triganol bipyramidal shape
5 bonding pairs no lone pairs
Triagonal bipyramidal bond angles
90 and 120 and 180
Octahedral bond angles
90 and 180
why can polar substances
i) molecules containing electronegative atoms
ii) NH3 HF and alcohols
dissolve in water
i) as the positive part of water attracts the electronegative part of the molecule and the negative part of the water attracts to the positive part of the molecule
2) as they all form hydrogen bonds between water molecules
trends of electronegativity
- increases across a period
- decreases down a group
why does electronegativity increase across a period
as the nuclear charge increases causing the atomic radius to decrease but the number of shielding remains constant so there is a stronger attraction between the nucleus and bonding pairs of electrons
why does electronegativity decrease down a group
as the number of shielding electrons increases the atomic radius increases so this means that the shared electron pair is further from the nucleus so less attraction between nucleus and bonding pairs of electrons
why cant non polar substances dissolve in water
These molecules don’t have regions of partial positive or partial negative charge, so they aren’t electrostatically attracted to water molecules and their weak intermolecular forces cannot disturb the strong hydrogen bonding between water molecules
why does solubility decrease as HC chain length increases in alcohols
- As when the length of the hydrocarbon chain increases, the London forces predominate
- the non-polar carbon chain cannot form hydrogen bonds, so as we increase the length a greater part of the molecule is unable to hydrogen bond with water, causing long chain alcohols to be less soluble than short-chain
what is required for a substance to dissolve
- soluble particles must be separated from each other and become surrounded by solvent molecules
- the forces of attraction between solute and solvent molecule must be strong enough to overcome the solvent-to-solvent forces and the solute-to-solute forces
definition for electronegativity
the ability of an atom to attract the bonding pairs of electrons in a covalent bond
what is a saturated solution
a solution that contains as much solute as possible at a particular temperature
definition for solubility
a measure of the concentration of a saturated solution of a solute at a specific temperature
factors affecting electronegativity
- nuclear charge
- atomic radius
- shielding
how does nuclear charge affect electronegativity
more protons stronger attraction between nucleus and bonding pairs of electrons
how does atomic radius affect electronegativity
closer to the nucleus the stronger the attraction between nucleus and bonding pairs of electrons
how does shielding affect electronegativity
the less shielding so the less repulsion and a stronger attraction between the nucleus and bonding pairs of electrons
what are the electronegative atoms
chlorine bromine and iodine
what atoms form hydrogen bonds
O F N
3 types of intermolecular forces
- induced dipole dipole interactions (London forces)
- permanent dipole dipole interactions
- hydrogen bonding
What causes induced dipoles to form
By the random movement of electrons causing an uneven distribution of electrons on one side of the atom. This forms an instantaneous dipole. The negative charge of the instantaneous dipoles repels the electrons in the other atom causing the electrons to move away from the atoms forming an induced dipole. Each atom them experiences a force of attraction between the positive and negative end of the atom, called a London force
How does an increase in electrons affect London forces and the boiling point of an atom
The more electrons there are the stronger the London forces so more energy is required to over come them, so the boiling point increases
how does the shape of a molecule affect the london forces
- the interactions between long thin molecules are stronger than those between short fat molecules because the interactions between long thin molecules can take effect over a larger surface area
- so the more branched the molecule the lower the surface area of contact and therefore the weaker London forces
What causes permanent dipole dipole interactions
This forms when there is an atom with a greater electronegativity so attracts pair of electrons in the covalent bond more strongly causing the electronegative atom to be slightly negative forming a permanent dipole. When two permanent dipoles attract each other they form forming a permanent dipole interaction
Why does tetrachloromethane have no overall permanent dipole
As the molecule is completely symmetrical so the bond polarities cancel out so it has no overall permanent dipole
Why does tetrachloromethane have a higher boiling point than trichloromethane even though it has no overall permanent dipole
As it has more electrons so stronger London forces so more energy is required to overcome the London forces
What causes hydrogen bonding
The electronegative element (nitrogen oxygen or fluorine) strongly attracts the pairs of electrons in the covalent bond causing it to be slightly negative and the hydrogen atom to have a slightly positive charge. The positive hydrogen atom of the other polar molecule (that forms hydrogen bonds) attracts to the lone pair of electrons in oxygen ,fluorine or nitrogen molecule forming a hydrogen bond
How many lone pairs of electrons in fluoride nitrogen and oxygen
3 - fluoride
2 - oxygen
1 - nitrogen
Conditions for hydrogen bonding
- we need a hydrogen atom bonded to a strongly electronegative element
- the electronegative atom has to have at least one lone pair of electrons
Water properties due to hydrogen bonding
- it has a high boiling and melting point due to the hydrogen bonds so requires a lot of energy to overcome hydrogen bonds
- solid form of water is less dense than liquid form
Why is ice less dense than water
As when water reaches its freezing point (0 degrees Celsius) the water molecules arrange themselves into an ordered structure which is stabilised by a network of hydrogen bonds so the water molecules are further apart, which makes ice less dense than liquid water
How does ice having a low density help organisms
Due to its low density it floats on water, insulating the water below preventing the water from freezing completely
definition of hydrogen bonding
a strong intermolecular force between delta-positive hydrogen atom covalently bonded to a F N or O and a lone pair of electrons on the delta negative of the O F or N atom of a nearby molecule
What is dative bonding
When an atom uses a lone pair of electrons to form a covalent bond
How is a dative bond shown in a displayed formula
Using an arrow
Why isn’t a carbon to carbon double bond stronger than a carbon to carbon single bond
As the double bond contains 1x sigma and 1x pi whilst the single bond contains 1x sigma bonds, and its easier to break the pi bond rather than a sigma bond as sigma bonds are stronger
what is ionic bonding
the strong electrostatic attraction between two oppositely charged ions
How are ionic bonds formed
When a metal transfers its electrons to a non metal so that they both have the same electron configuration of a noble gas causing them to be oppositely charged. And the electrostatic attraction between oppositely charged ions is called an ionic bond
What do the square brackets represent in ionic bonding drawings
That the charge is spread over the whole ion
How do ionic compounds/ionic crystals form
When a metal and non metal react the ions produced arrange them self into a giant ionic lattice forming an ionic compound
Properties of ionic compounds
- they have high mp and bp - due to the strong electrostatic forces of attraction so requires a lot of energy to overcome electrostatic forces of attraction
- they are soluble in polar substances such as water - as the water cluster around the ions and bind to them and the energy released when water molecules bind to the ions is enough to overcome the electrostatic forces of attractions holding the ions lattice together
- they don’t conduct electricity when solid - as the ions are in a fixed in place by the electrostatic forces of attraction so they cannot carry a charge
- hard brittle crystalline substances
Why does magnesium oxide have a higher mp than sodium chloride
- magnesium oxide is in group 2 so has a higher charge and a higher charge density so has stronger electrostatic forces of attraction so more energy is needed to overcome so has a higher mp
Why does solubility decrease as the charge of ionic compounds increases
As the hydration energy released when the ions are hydrated (water molecules binding to them) is not large enough to overcome the electrostatic forces of attraction holding the lattice together
isoelectronic definition
molecules and ions with the exact same number and arrangement of electrons
Properties for simple molecular substances
- they have low melting and boiling points - due to the weak London forces between molecules so require little energy to overcome
- do not conduct electricity as they don’t have any delocalised electrons or freely moving ions
- only polar simple substances are soluble in water
Why do giant covalent structures have high mp and bp
- have high melting and boiling points due to the strong covalent bonds between atoms so requires a lot of energy to overcome
How are the atoms arranged in diamond and what is its bond angle
Tetrahedral structure
Bond angle: 109.5
Properties of diamond
- high mp and bp
- does not conduct any electricity as every electron in its covalent bond are fixed between pairs of atoms so no delocalised electrons to act as charge carriers
- insoluble as solvents cannot disrupt the larger number of covalent bonds
- good thermal conductor due to the fixed covalent bonds meaning that the atoms close to the heat get hotter and move faster so the vibrations move quickly throughout the structure heating the whole structure
- very hard due to the strong covalent bonds operating in 3D dimensions
- sublimes at ordinary pressures due to strong covalent bonds
How are the carbon atoms in graphite arrange and what’s the bond angle
Planar hexagonal structure
Bond angle: 120
Properties of graphite
- conducts electricity due to the delocalised electrons from each carbon atom being able to freely move THROUGHOUT the sheets
- high mp and bp (same as diamond)
- can act as a lubricant due to ADSORBED gases on the surface of carbon atoms (gas molecules sticking to the surface) - but lubricating properties are not there in a vacuum
- insoluble in water (same reason as diamond)
- lower density as diamond due to the relatively amount of large space between layers
What is graphene
A single layer of graphite
Suggest why boron nitrate can act as a lubricant in a vacuum whilst graphite cannot
As they don’t need gaseous atoms to be ADSORBED (not absorbed, ADSORBED) onto the surface of layers
Properties of graphene
- conducts electricity
- can be rolled into a carbon nanotube
- one atom thick
Buckminister fullerene formula
C60
Properties of silicone dioxide
- high mp and bp due to strong silicone - carbon covalent bonds
- hard
- doesn’t conduct electricity
- insoluble - as there are no attractions which could occur between solvent molecules and the silicone and oxygen atoms which could overcome the covalent bonds in the structure
what is metallic bonding
The strong electrostatic attraction between positive metal ions and the sea of delocalised electrons
Properties if metallic bonding
- high mp and bp - due to the strong electrostatic forces of attraction between cations and delocalised electrons so requires a lot of energy to overcome these forces and move the metal ions away from their positions in the lattice
- high density - as the atoms are closely packed with little space between them
- can conduct electricity - due to the freely moving delocalised electrons so are attracted to the positive electrode and flow through the metal and this flow is an electrical current
- can conduct heat - when the metal is heated the delocalised electrons move around and conduct heat to the other parts of the metal
- malleable due to metal ions being able to slide over each other when a force is applied (which is known as a slip)
- insoluble
What is the electron pair repulsion theory
That the shape of a molecule is determined by the electron pairs surrounding the central atom as the electron pairs repel other electron pairs so move as far as possible to minimise repulsion
Why do lone pairs reduce the bond angle by 2.5 degrees
As they repel more strongly
why do branched molecules have a lower bp than unbranched molecules
as the molecules dont pack as close together so London forces are reduced as there are fewer contact points
