Bonding, Shapes of molecules and Types of intermolecular forces Flashcards
Ionic bond
Electrostatic force of attraction between oppositely charged ions in a lattice as a result of electron transfer
Why is a sodium ion smaller than a sodium atom
Na+ is smaller than Na since it has one less shell of electrons
The proton to electron ratio is greater in Na+
Na+ has a greater effective nuclear charge than Na so the remaining electrons are more strongly attracted and are closer to the nucleus
Structure of an ionic bond
Giant ionic lattice
Isoelectronic
Same number and arrangement of electrons
Suggest 3 forces of repulsion which exist in an ionic lattice
Between ions of the same charge
Between electron clouds of the ions
Between the nuclei of the ions
What are the properties of ionic compounds
High melting temperatures
Solids at room temperature
Soluble in water (most ionic compounds)
Conduct electricity when molten or dissolved in water
Why do ionic compounds have a high temperature
They have a giant ionic lattice.
There are strong electrostatic forces of attraction between the ions.
A large amount of energy is needed to break the strong ionic bonds
Why can ionic compounds conduct electricity when molten or dissolved in water
The ions are free to move and carry the charge
Explain why sodium iodide has a lower melting temperature than that of sodium chloride. The charge on both anions is the same, -1, but the I- is bigger than Cl-
I- has a smaller charge density than Cl-
The electrostatic forces of attraction between Na+ and I- ions are weaker therefore less energy is needed to break the weaker ionic bond in NaI
What are major factors that affect the strength of an ionic bond
Radius/size of the ion
Charge of the ion
What happens to ionic radius as you go down a group
It increases
What happens to ionic radius as you go across a period
It decreases
Why does ionic radius increase as you go down a group
Outer electrons are further away from the nucleus.
Although the nuclear charge increases down a group, the outer electrons are more shielded by the inner electron shells.
There is a weaker attraction between the nucleus and the outer electrons as you go down a group so ionic radius increases
Why does ionic radius decrease as you go across a period
The ratio of protons to electrons increases across a period so nuclear charge increases
There is no change in shielding of the outer shell of electrons.
There is a stronger attraction between the nucleus and the outer electron as you go across a period so ion become smaller
What is a covalent bond
The attraction between a shared pair of electrons and the nuclei of the bonded atoms. Each atom gives one electron to the shared pair
What is average bond enthalpy a measurement of
Covalent bond strength
What happens to the strength of a covalent bond as average bond enthalpy increases
Covalent bond becomes stronger
Properties of simple molecular structure (covalent)
They are liquids or gases at room temperature
They have low melting points and boiling points
They do not conduct electricity
Many do not dissolve in water
Why do covalent compounds have low melting and boiling points
They have a simple molecular structure and are made up of small molecules with weak Van der Waals forces between the molecules. Not much energy is needed to break these weak intermolecular forces
Why do covalent compounds not conduct electricity
There are no free electrons since the outer electrons are all fixed in covalent bonds
Why does iodine, I2, have a low melting point
Iodine is made up of small molecules with weak Van der Waals forces between the molecules. Not much energy is needed to break these weak intermolecular forces.
Dative covalent bond
The attraction between a shared pair of electrons and the nuclei of the bonded atoms. The shared pair of electrons is given by the SAME atom
What is a co-ordinate bond/ dative bond represented by
An arrow showing the direction of donation
An aluminium chloride molecule reacts with a chloride ion to form the AlCl4- ion.
Name the type of bond formed in this reaction and explain how this bond is formed in AlCl4-.
type of bond : Dative covalent bond
Explanation : The electron pair on Cl- is donated to the Al atom in AlCl3
Why can element in periods 1 and 2 not expand their octet
They have no 2d orbitals
Why can elements in period 3 and above expand their octet
They have an empty 3d orbital
Explain why the melting point of BCl3 is much lower than that of NaCl
BCl3 is made up of small molecules with weak Van der Waals forces between the molecules as it is covalently bonded. Not much energy is needed to break these weak intermolecular forces.
NaCl has a giant ionic lattice with strong electrostatic forces of attraction between the ions. A large amount of energy is needed to break these strong ionic bonds
What are the allotropes of carbon
Diamond, graphite and graphene
Allotropes
Different forms of the same element in the same physical state
Structure of diamond
Giant covalent lattice
Why does diamond have a high MP
it has a giant covalent lattice with many strong covalent bonds between the atoms.
A large amount of energy is needed to break so many bonds
Why is diamond a poor conductor of electricity
There are no free electrons. The outer electrons in diamond are all fixed in covalent bonds
Structure of graphite
giant covalent lattice
Why is graphite soft
There are weak Van der Waals forces between the layers of carbon atoms so the layers can slide over each other
Explain how solid graphite conducts electricity
In graphite each carbon has one free electron. Graphite conducts electricity because it has delocalised electrons that move along the layers and carry the charge
What are the 4 types of crystal structure
ionic
metallic - magnesium and sodium
macromolecular (giant covalent) - diamond, graphite
molecular - iodine, ice
Metallic bond
Electrostatic force of attraction between the delocalised electrons and positive metal ions arranged in a lattice
Structure of metals
Positive metal ions are arranged in a regular lattice and held together by electrostatic attraction to delocalised electrons. Metals have a giant metallic lattice
Why are metals malleable/ductile
There are no rigid, directed bonds in the metal so the layers of ions can slide over each other when a force is applied to the metal
How do pairs of electrons in the outer shell of atoms arrange themselves
as far apart as possible to a position of minimal repulsion
What is the order of pair repulsion
lone pair-lone pair repulsion > lone pair-bond pair repulsion > bond pair-bond pair repulsion
Lone electron pair
2 electrons that do not form a covalent bond
How do you work out the total number of bonding pairs and lone pairs for molecules and ions with single bonds only
- Work out the number of outer shell electrons on the central atom by looking at what group it is in e.g group 5 has 5 electrons. DON’T FORGET ABOUT THE CHARGES.
- Work out how many atoms are bonded to the central atom (number of bonding pairs) e.g. NH3 would have 3 as there are 3 hydrogens
- Work out how many lone pairs there are by doing 1/2( outer electrons on central atom - number of atoms bonded to central atom)
- Add the bonding pairs and lone pairs
What is the shape of the molecule and its bond angles if it has 2 bonding pairs and 0 lone pairs
Linear and 180 degrees
What is the shape of the molecule and its bond angles if it has 3 bonding pairs and 0 lone pairs
Trigonal planar and 120 degrees
What is the shape of the molecule and its bond angles if it has 4 bonding pairs and 0 lone pairs
tetrahedral and 109.5 degrees
What is the shape of the molecule and its bond angles if it has 3 bonding pairs and 1 lone pair
Trigonal pyramidal and 107 degrees
What is the shape of the molecule and its bond angles if it has 2 bonding pairs and 2 lone pairs
Bent ( V shaped) and 104.5 degrees
What is the shape of the molecule and its bond angles if it has 5 bonding pairs and 0 lone pairs
Trigonal bipyramidal and 120, 90 degrees
What is the shape of the molecule and its bond angles if it has 4 bonding pairs and 1 lone pair
Trigonal bipyramidal and 119,89 degrees
What is the shape of the molecule and its bond angles if it has 3 bonding pairs and 2 lone pairs
Trigonal bipyramidal and 89 degrees
What is the shape of the molecule and its bond angles if it has 6 bonding pairs and 0 lone pairs
Octahedral and 90 degrees
Why is the beryllium chloride molecule linear
There are 2 bonding pairs and no lone pairs of electrons around the central beryllium atom. These electron pairs repel each other as far apart as possible to a position of minimum repulsion. The BeCl2 molecule is linear with bond angles of 180 degrees
What is the shapes and bond angles of CO2 and HCN
Linear with bond angles of 180 degrees
Why is the CO2 molecules linear
There are two regions of electron density around the central C atom. These electron regions repel each other as far apart as possible to a position of minimum repulsion. The CO2 molecule is linear with bond angles of 180 degrees. ( USE AS AN EXAMPLE FOR ALL EXCEPTIONS E.G NO3-)
How are double and triple bonds treated when predicting the shape of a molecule
As a single bond
Why is the AlCl3 molecule trigonal planar
There are 3 bonding pairs and no lone pairs of electrons around the central Al atom. These electron pairs repel each other as far apart as possible to a position of minimum repulsion. The AlCl3 molecule is therefore trigonal planar with bond angles of 120 degrees
What are the shapes and bond angles of NO3- and CO3 2-
Trigonal planar with bond angles of 120 degrees
What does a straight line represent when drawing the shape of a molecule
A bond in the plane of the paper
What does a triangle represent when drawing the shape of a molecule
A bond in front of the plane of the paper
What does a dashed line represent when drawing the shape of a molecule
A bond behind the plane of the paper
What is the shape and bond angle of SO4 2-
Tetrahedral with bond angles of 109.5
State and explain the shape and bond angle(s) of the ammonia molecule and
There are 3 bonding pairs and one lone pair of electrons around the central N atom. These electron pairs repel each other as far apart as possible to a position of minimum repulsion. The NH3 molecule is pyramidal.
Since lone pair bonding-pair repulsion is greater than bonding pair-bonding pair repulsion, the H-N-H bond angle is lesser than the tetrahedral angle and so is 107 degrees.
What is the shape and angle of SO3 2-
Pyramidal with bond angles of 107 degrees
Electronegativity
The power/ability of an atom to attract the pair of electrons in a covalent bond
What is the most electronegative element
Fluorine
What are the trends in electronegativity in the periodic table
Non-metals have a higher electronegativity than metals
Electronegativity increases across a period of the periodic table as atom gets smaller across a period
Electronegativity decreases as you go down a group as atom gets bigger down a group
In terms of atomic structure, give 2 reasons why oxygen is more electronegative than carbon
Oxygen atom has a greater nuclear charge than carbon atom because it has more protons.
Oxygen atom has a smaller atomic radius than the carbon atom
How do partial charges arise when 2 different atoms are joined by a covalent bond
The more electronegative atom attracts the bonding pair of electrons more and as a result has a delta negative charge whilst the other atom has a delta positive charge
When is a bond non-polar
The bonded atoms are the same or the bonded atoms have the same/similar electronegativity
Dipole
Separation of opposite charges
Permanent dipole
A dipole in a polar covalent bond which does not change
Why is CCl4 not polar
The CCl4 molecule is symmetrical and the dipoles cancel.
What happens in terms of permanent dipoles, if the molecule is symmetrical
Any dipoles will cancel and molecule will NOT have a permanent dipole
What happens in terms of permanent dipoles, if the molecule is not symmetrical
Dipoles do not cancel and molecule has a permanent dipole
Why might a molecule have polar bonds but not necessarily be a polar molecule
The molecule is symmetrical and dipoles cancel
What shaped molecules form polar molecules
Bent, pyramidal and trigonal bipyramidal
What are the 3 types of intermolecular forces
Van der Waals forces
Permanent dipole-dipole interactions
Hydrogen bonding
Which type of intermolecular forces is the strongest
Hydrogen bonding > permanent dipole-dipole interactions > Van der Waals forces
Where do Van der Waals forces exist
Between non-polar molecules / all molecular substances and noble gases OR exist between all atoms or molecules
Explain how Van der Waals forces arise between non-polar molecules
Due to the movement of electrons there is an uneven distribution of electrons in a molecule. This gives rise to an instantaneous dipole.
This instantaneous dipole induces a dipole on the neighbouring molecule.
The induced dipole induces further dipoles on neighbouring molecules which then attracts one another
What is the main factor affecting the size of Van der Waals forces
The number of electrons.
How does the number of electrons affect the size of Van der Waals forces
As the number of electrons per molecule increases, the strength of the forces increases. LArger molecules have larger electrons clouds meaning stronger Van der Waals forces
Where do permanent dipole-dipole interactions occur
Between polar molecules
Explain what happens in permanent dipole-dipole interactions
Polar molecules have a permanent dipole.
Polar molecules are attracted to each other by permanent dipole-dipole interactions.
The negative end of one molecule is attracted the positive end of another.
How is a hydrogen bond formed
By the attraction between the hydrogen atom of one molecule and the lone pair of an N, O or F atom of a different molecule
What are the conditions for a hydrogen bond to form
A hydrogen atom is attached to a small electronegative atom such as N, O or F
An electronegative atom with a lone pair of electrons
Explain why a hydrogen bond forms between 2 water molecules
Oxygen atom is more electronegative than the hydrogen atom.
Oxygen atom attracts the bonding pair of electrons more and has a delta negative charge hydrogen atom has a delta positive charge
A hydrogen bond is formed by the attraction of the delta positive hydrogen atom of one water molecule and the lone pair on the small delta negative oxygen atom on a different water molecule
Why does H2O have a higher BP than NH3
Each water molecule form 2 hydrogen bonds but each ammonia molecule can only form one. More energy is needed to break the hydrogen bonds between water molecules.
Why does HF have a higher BP than NH3
Fluorine is more electronegative than nitrogen so the H-F bond is more polar than the N-H bond.
The hydrogen bonds between hydrogen fluoride molecules are stronger.
More energy is needed to break the hydrogen bonds between HF molecules
Polar
The electrons are shared unequally in a bond. One atom has a partial positive charge and the other atom has a partial negative charge
Explain the structure of ice
With 2 hydrogen atoms and 2 lone pairs on the oxygen atom, each water molecule can form 4 hydrogen bonds. The hydrogen bonds extend outwards, holding the water molecules slightly in a 3 dimensional tetrahedral lattice.
This arrangement creates an open lattice structure which decreases its density, hence ice is less dense than water
Why is the boiling point of HF greater than HCl, HBr and HI
There are hydrogen bonds between molecules of hydrogen fluroride
There are Van der Waals forces between molecules of the other hydrogen halides.
Hydrogen bonds are stronger than VdW forces therefore more energy is needed to separate molecules of HF
Why is the BP of HI greater than HCl
HI has more electrons per molecule than HCl, therefore the VdW forces between HI molecules are stronger and more energy is needed to separate it.
What is the trend in BP for the noble gases
It increases as you go down the group since there are more electrons per atom therefore the strength of the VdW forces increases.
What happens to the boiling point as the molecule becomes more branched and why
BP decreases because there is less contact with molecules with a smaller SA .There are more VdW forces between linear molecules compared to branched or spherical molecules
What is the trend in boiling points of the hydrogen halides from HF to HI
HF has the greatest boiling point and then it HI, HBr and finally HCl
HF>HI>HBr>HCl
Why is the boiling temperature of HF greater than that of HCl, HBr or HI
There are hydrogen bonds between molecules of hydrogen fluoride.
There are Van der Waals forces between molecules of the other hydrogen halides.
Hydrogen bonds are stronger than Van der Waals forces therefore more energy is needed to separate molecules of HF
Why is the boiling point of HI greater than HCl
Hydrogen iodide has more electrons per molecule than hydrogen chloride, therefore the Van der Waals forces between hydrogen iodide molecules are stronger.
More energy is needed to separate molecules of hydrogen iodide.
State and explain whether the electronegativity of fluorine is greater than, similar to or less than that of bromine.
Hence explain why HF can form hydrogen bonds but HBr cannot
Fluorine is more electronegative than bromine because it is smaller.
The bonding electrons in HF are closer to the nucleus of the fluorine atom.
The H-F bond is polar, but the H-Br bond is not polar enough, so hydrogen bonds cannot form between HBr bonds
What is the trend in boiling points of the noble gases and why
The boiling points increase as you go down the group.
The number of electrons per molecule increases down the group so VdW forces get stronger .
More energy is therefore needed to separate the atoms.
What is the shape and bond angles of a molecules with 2 bonding pairs and 1 lone pair
Bent, 118 degrees
