Structure and Bonding, Shapes and IM forces (5.2,5.3, Chapter 6 + 7.3) Flashcards
What is covalent bonding?
The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What is the name for a covalent compound with less than 8 electrons in the outer shell e.g. BF3 with 6 electrons?
Electron deficient compounds
What happens to the electronic configuration in BF3?
It stays the same, 1s2, 2s2, 2p1
What is actually happening in BF3?
The 2s and 2p orbitals become hybridised orbitals called “sp2” with one electron in each orbital
What is the name for a covalent compound with more than 8 electrons in its outer shell e.g. PCL5 or SF6?
Expanding the octet
Which elements can expand their octets and why?
Period 3,4 and 5 elements because they have low-lying d-orbitals (in the 3rd or more shell) which the extra electrons can be in, there can be up to 18 electrons in the outer shell
e.g. P, S and Cl
What is the difference between a lone pair and a bonding pair of electrons?
A lone pair is a pair of electrons not bonded to another atom (and often becomes a dative bond) e.g. ••
A bonding pair is pair of electrons bonded to two atoms in a covalent bond e.g. •x
What are double/triple covalent bonds?
- When the electrostatic attraction is between two/three shared pairs of electron and the nuclei of bonded atoms -
- Bonds that contain more than one shared pair of electrons between two atoms e.g. O2, N2 and CO2
What is a dative covalent bond/coordinate bond?
- A covalent bond in which the shared pair of electrons has been supplied by one of the bonding atoms only
- The shared electron pair was originally a lone pair of electrons from one of the bonded atoms
When does dative bonding occur?
When one atom or compound has at least one lone pair of electrons (or just a full outer shell) is covalently bonded to another atom or compound which needs electrons
What happens when NH3 becomes NH4+?
NH3 has a lone pair of electrons on the nitrogen
An H+ ion has no electrons in its outer shell
The H+ is datively bonded to the lone pair of electrons on the nitrogen to form NH4+
NH4+ has a positive charge because hydrogen has lost one electron
How do you draw a dative bond in a display formulae?
Instead of a line, an arrow is drawn starting at the atom which is donating the pair of electrons (at the lone pair) and goes towards the atom receiving the pair of electrons
What is the theory that describes the shapes of molecules?
VSEPR theory
Valence Shell Electron Pair Repulsion Theory
What is the shape and bond angle of a molecule with 2 electron pairs?
Linear, 180 e.g. CO2
What is the shape and bond angle of a molecule with 3 electron pairs?
Trigonal planar, 120 e.g. BF3
What is the shape and bond angle of a molecule with 4 electron pairs?
Tetrahedral, 109.5 e.g. CH4
What is the shape and bond angle of a molecule with 6 electron pairs?
Octahedral, 90 e.g. SF6
How do electron pairs interact?
Electron pairs repel each other as far apart as possible
What is the repulsion order for electron pairs?
L.p - L.p > L.p. - B.p > B.p - B.p
What happens when lone pairs are present?
The lone pair(s) repels the bonding pairs more so the bond angles are smaller as the bonding pairs are pushed closer together
What is the shape and and bond angle of a molecule when there is 1 lone pair and 3 bonding pairs?
Pyramidal, 107
What is the shape and bond angle of a molecule when there are 2 lone pairs and 2 bonding pairs?
Non-linear (bent), 104.5
What is the process for working out the shape and bond angle of a molecule?
1) dot and cross diagram
2) count the total number of electron pairs (bonding and lone)
3) based on the number of electron pairs, decide the overall base shape and starting bond angle
4) subtract 2.5 from the starting bond angle for each lone pair to get the bond angle of the molecule
What is electronegativity?
The ability of an atom to attract the bonding electrons in a covalent bond
What factors determine electronegativity?
Atomic radius and nuclear charge
What are the three most electronegative elements in decreasing order?
Fluorine, oxygen and nitrogen
What is a polar bond?
- When the bonded electron pair is shared unequally between the bonded atoms
- A covalent bond between two atoms of different electronegativities where the shared, bonding electrons are pulled towards the more electronegative atom
What is a permanent dipole caused by?
The difference in electronegativity between the two atoms in a polar bond
What is a dipole?
- The separation of opposite charges
- An uneven distribution of electrons in a covalent bond causing a difference in charge between the atoms. The more electronegative atom therefore has a slightly negative charge and the less electronegative atom has a slightly positive charge
What makes a bond more polar?
A greater difference in electronegativity
Why are the covalent bonds in diatomic elements non-polar? (pure covalent bond)
The bonded atoms come from the same element and the electron pair is shared equally
What happens in ionic substances?
The electrons move over entirely to the negative ion as there is such a large difference in electronegativity
What determines whether or not a molecule has an overall (molecular) dipole?
The arrangement of the polar bonds
When does the molecule have no overall (molecular) dipole and is non-polar?
When the polar bonds are arranged symmetrically so that the dipoles cancel each other out
When does a molecule have an overall dipole and is polar?
When the polar bonds are arranged so that they don’t cancel each other out so charge is arranged unevenly across the whole molecule
Why can only bonds between atoms of a single element, like diatomic gases, be purely covalent?
Because electronegativity difference between the atoms is zero and so the bonding electrons are arranged completely evenly within the bond
What are the three types of intermolecular forces?
Induced dipole-dipole interactions (London forces)
Permanent dipole-dipole interactions
Hydrogen bonds
What happens during induced dipole-dipole interactions?
1) Random movement of e- causes an instantaneous dipole in one molecule (but its position is constantly shifting)
2) The instantaneous dipole induces a dipole in neighbouring molecules
3) Opposite partial charges cause weak electrostatic attraction between neighbouring particles
4) The induced dipole induces further dipoles on neighbouring molecules, which then attract one another
Why do induced dipole-dipole interactions increase as you go down the group?
As you go down a group, there are more electrons so there are stronger dipoles therefore stronger intermolecular forces (induced dipole-dipole interactions) and therefore more energy is needed to overcome these forces
What are permanent dipole-dipole interactions?
The slight positive and slight negative charges on polar molecules cause weak electrostatic forces of attraction between molecules
How does surface area affect London forces?
Molecules with a greater surface are have stronger London forces because they have a bigger exposed electron cloud
What happens when you boil a liquid?
You need to overcome the intermolecular forces so that the particles can escape from the liquid surface
Why is ice less dense than water?
- Hydrogen bonds hold water molecules apart in an open lattice structure - each water molecule can form 4 H-bonds
- The water molecules in ice are further apart than in water because the open lattice structure holds the molecules further apart as the water forms more H-bonds
- Therefore, solid ice is less dense than liquid water and floats
Why does water have relatively high melting and boiling points?
- Water has hydrogen bonds as well as London forces
- A Large amount of energy is needed to break the hydrogen bonds in water, so water has a much higher than expected boiling point/melting point
What is the order of strength of intermolecular forces from weakest to strongest?
Induced dipole-dipole interactions
Permanent dipole-dipole interactions
Hydrogen bonding
What is a hydrogen bond?
A special type of permanent dipole-dipole interaction found between molecules containing:
1) an electronegative atom with a lone pair of electrons
2) a hydrogen atom attached to an electronegative atom
Which intermolecular force is hydrogen bonding an ‘extreme’ of?
Permanent dipole-dipole interactions
Where does the hydrogen bond form?
Between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule
What do molecules which have hydrogen bonding usually contain?
-OH or -NH groups
What effect does hydrogen bonding have on molecules?
They are soluble in water
They have higher melting and boiling points than molecules of a similar size that are unable to form hydrogen bonds e.g. H20, NH3 and HF generally have the highest boiling points if you compare them with other hydrides in their groups, because more energy is needed to break the hydrogen bonds
If two molecules have similar induced dipole-dipole interactions then what determines which molecule has a high boiling point?
If one of the substances has molecules that are more polar than the other, it will have permanent dipole-dipole interactions and so a higher boiling point as more energy is needed to overcome the stronger forces
What is an ionic bonding?
The electrostatic attraction between positive and negative ions
- it holds together cations and anions in ionic compounds
Why does an ionic lattice form?
Because each ion is electrostatically attracted in all directions to 6 ions of the opposite shape
What do ionic compounds have high melting and boiling points?
Because a lot of energy is needed to overcome the strong electrostatic attractions between oppositely charged ions in the giant ionic lattice
Why do ionic compounds conduct electricity when molten or aqueous but not when solid?
Because in a liquid (or dissolved water), the solid ionic lattice breaks down and the ions are now free to move as mobile charge carriers
- But in a solid the ions are fixed in position in the giant ionic lattice and there are no mobile charge carriers
Why are ionic compounds soluble in water?
Because water is a polar molecule so the positive ions are attracted to the slightly negative oxygen atoms and the negative ions are attracted to the slightly positive hydrogen atoms, breaking the giant structure
Why do simple covalent compounds have low melting and boiling points?
Because not a lot of energy is needed to overcome the weak intermolecular forces
As intermolecular forces get stronger, melting and boiling points increase
Why are polar molecules soluble?
Water is a polar molecule so only tends to dissolve other polar substances
Compounds with hydrogen bonds can form hydrogen bonds with water molecules, so will be soluble
Molecules that only have London forces will be insoluble
Why don’t simple covalent compounds conduct electricity?
Because even though some covalent molecules have permanent dipoles, overall covalent molecules are uncharged so can not conduct electricity
What scale is electronegativity measured on?
The Pauling Scale - the greater an element’s Pauling value, the higher its electronegativity
Why are the melting points of ionic lattices containing ions with greater ionic charges higher?
Because there is a stronger attraction between the ions
What happens when ionic compounds dissolve in water?
Polar water molecules break down the lattice and surround each ion in solution
When might an ionic compound not be very soluble?
In a compound made of ions with large charges - the ionic attraction may be too strong for water to be able to break down the lattice
What does the solubility of an ionic compound in water depend on?
The relative strengths of the attractions within the giant ionic lattice and the attractions between ions and water molecules
Between what atoms does covalent bonding occur?
- Non-metallic elements
- Compounds of non-metallic elements
- Polyatomic ions
What is a covalent bond?
The overlap of atomic orbitals, each containing one electron, to give a shared pair of electrons
What are two key features of a covalently bonded molecule?
- The shared pair of electrons is attracted to the nuclei of both the bonding atoms
- The bonded atoms often have outer shells with the same electron structure as the nearest noble gas
What are lone pairs?
Paired electrons that are not shared
What is the average bond enthalpy?
A measurement of covalent bond strength
- the larger the value of the average bond enthalpy, the stronger the covalent bond
Explain electron-pair repulsion
- Electron pairs around the central atom repel each other as far apart as possible
- The greater the number of electron pairs, the smaller the bond angle
- Lone pairs of electrons repel more strongly than bonded pairs of electrons
What is the electronegativity difference in a fully covalent bond?
0
What is the electronegativity difference in a polar covalent bond?
0-1.8
What is the electronegativity difference in a fully ionic bond?
More than 1.8
What is a non-polar bond?
When the bonded electron pair is shared equally between the bonded atoms
When is a bond non-polar?
When the bonded atoms are the same or have the same/similar electronegativity
What are intermolecular forces?
Weak interactions between dipoles of different molecules
What kinds of molecules have London forces?
ALL MOLECULES
Why does larger number of electrons mean larger induced dipoles?
- The more electrons in a molecule, the larger the instantaneous and induced dipoles
- Therefore, the greater the induced dipole-dipole interactions and the stronger the attractive forces between molecules
Why do molecules with permanent dipole-dipole interactions have higher boiling points than molecules with only London forces?
- They are polar so have London forces and permanent dipole-dipole interactions between molecules
- Extra energy is needed to break the additional permanent dipole-dipole interactions
What is a simple molecular substance made up of?
Simple molecules - small units containing a definite number of atoms with a definite molecular formula
What do simple molecules form in the solid state?
A regular structure called a simple molecular lattice
Describe the simple molecular lattice
- The molecules are held together by weak IM forces
- The atoms within each molecule are bonded together strongly by covalent bonds
Why do simple molecular substances have low melting and boiling points?
Because only the weak IM forces break, NOT the covalent bonds
What happens when a non-polar simple molecular compound is added to a non-polar solvent e.g. hexane?
- IM forces form between the molecules and the solvent
- The interactions weaken the IM forces in the simple molecular lattice - the IM forces break and the compound dissolves
What happens when a non-polar simple molecular compound is added to a polar solvent?
- There is little interaction between the molecules in the lattice and the solvent molecules
- The IM bonding within the polar solvent is too strong to be broken
What happens when a polar simple molecular compound is added to a polar solvent?
- They may dissolve as the polar solute molecules and polar solvent molecules can attract each other
What does the solubility of polar molecular compounds depend on?
The strength of the dipole - some molecules have both polar and non-polar parts of their structure so can dissolve in both polar and non-polar solvents
Why are simple molecular structures non-conductors of electricity?
- There are no mobile charged particles in simple molecular structures
- With no charged particles that can move, there is nothing to complete an electrical circuit
What is metallic bonding?
The strong electrostatic attraction between cations and delocalised electrons
Why are electrons delocalised in a solid metal structure?
Each atom has donated its negative outer-shell electrons to a shared pool of electrons, which are delocalised (spread out) through the whole structure
What do the positive ions in a metal structure consist of?
The nucleus and the inner electron shells of the metal atoms
Describe the structure of the cations in the metal structure
The cations are in a fixed position, maintaining the structure and shape of the molecule
Describe the structure of the electrons in the metal structure
The delocalised electrons are mobile and are able to move throughout the structure
What happens in metals with 2+ cations?
There are twice as many electrons to balance the charge
What are billions of metal atoms held together in?
A giant metallic lattice
What are the properties of most metals?
- Strong metallic bonds
- High electrical conductivity
- High melting and boiling points
Why can metals conduct electricity in solid and liquid states?
When a voltage is applied across a metal, the delocalised electrons can move through the structure, carrying charge
Why do most metals have high melting and boiling points?
- The MP depends on the strength of the metallic bonds holding together the atoms in the giant metallic lattice
- For most metals, high temperatures are necessary to provide the large amount of energy needed to overcome the strong electrostatic attraction between the cations and electrons
- This strong attraction means most metals have high MPs and BPs
Why do metals not dissolve?
- There is no interaction between polar solvents and charges in the metallic lattice
- Interactions would lead to a reaction, rather than dissolving
What is a giant covalent lattice?
Many billions of atoms held together by a network of strong covalent bonds
Which elements have a giant covalent lattice?
The non-metals Boron, Silicon and Carbon (diamond/graphite)
Why do diamond and silicon form a giant covalent lattice?
- They are in Group 14/4 and their atoms have 4 electrons in the outer shells
- They used these 4 electrons to form covalent bonds to other carbon or silicon atoms
- The results is a tetrahedral structure with angles of 109.5 degrees
Why to giant covalent structures have high melting and boiling points?
Because the covalent bonds are very strong and high temperatures are necessary to provide the large quantities of energy needed to break them
Why are giant covalent lattices insoluble?
Because the covalent bonds holding together the atoms in the lattice are too strong to be broken by interactions with the solvent
Why are giant covalent lattices non conductors (except graphite and graphene)?
- In diamond and silicon, all four outer shell electrons are involved in the covalent bonding, therefore none are available for conducting electricity
What are graphite and graphene?
Giant covalent structures of carbon based on planar hexagonal layers with bond angles of 120 degrees by electron pair repulsion
Describe the bonding in graphite?
- Only 3 of the 4 outer shell electrons are used in covalent bonding
- The remaining electron is released into a pool of delocalised electrons shared by all atoms in the structure
Describe the structure of graphite
Composed of parallel layers of hexagonally arranged carbon atoms - layers bonded by weak London forces
Why can graphite and graphene conduct electricity?
The bonding in the hexagonal layers only used 3 of carbon’s 4 outer shell electrons and the spare electron is delocalised between the layers
What is graphene?
A single layer of graphite
Describe the periodic trend in melting points across period 2 and 3
1) The MP increases from Group 1 to Group 14/4
2) There is a sharp decrease in MP between Group 14/4 and Group 15/5
3) The MPs are comparatively low from Group 15-18
Explain why there is sharp decrease in MP between Group 14/4 and Group 15/5
- It marks a change from giant to simple molecular structures
- On melting, giant structures have strong forces to overcome so have high MPs
- Simple molecular forces have weak forces to overcome and therefore much lower MPs
- Can also see the start of the diagonal divide between metals and non-metals
