4.2 Chemical Bonding and Structure Flashcards
What does ionic bonding involve?
Ionic bonds involve the transfer of electrons from a metallic element to a non-metallic element
Why do elements want to have full outer shells?
Transferring electrons usually leaves the metal and the non-metal with a full outer shell
What electron arrangement do atoms have when they form ions?
Once the atoms become ions, their electronic configurations are the same as a noble gas
What type of forces are in ionic lattices?
Cations and anions are oppositely charged and therefore attracted to each other
Electrostatic attractions are formed between the oppositely charged ions to form ionic compounds
Why do ionic compounds have high mp and bp?
This form of attraction (electrostatic forces) is very strong and requires a lot of energy to overcome
This causes high melting points in ionic compounds
What structures do ionic lattices form?
The ions form a lattice structure which is an evenly distributed crystalline structure
How are ions arranged in a lattice?
Ions in a lattice are arranged in a regular repeating pattern so that positive charges cancel out negative charges
What charge does the lattice have?
Therefore the final lattice is overall electrically neutral
What affects the melting/boiling point of a substance?
Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility
Are ionic compounds weak/strong? Why?
Ionic compounds are strong
The strong electrostatic forces in ionic compounds keep the ions strongly together
Are ionic compounds brittle/malleable?
They are brittle as ionic crystals can split apart
What increases the melting and boiling points of ionic compounds?
The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
Melting and boiling points increase with charge density of the ions due to the greater electrostatic attraction of charges
Mg2+O2- has a higher melting point than Na+Cl–
Are ionic compounds in/soluble in water?
Ionic compounds are soluble in water as they can form ion – dipole bonds
Can ionic compounds conduct electricty?
Ionic compounds only conduct electricity when molten or in solution
Why can ionic compounds only conduct electricity in molten state?
When molten or in solution, the ions can freely move around and conduct electricity
Why do ionic compounds not conduct electricity when solid?
In the solid-state they’re in a fixed position and unable to move around
What elements form an ionic compound?
Ionic compounds are formed from a metal and a nonmetal bonded together
What elements does covalent bonding occur between?
Covalent bonding occurs between two non-metals
What forces are in a covalent bond?
A covalent bond involves the electrostatic attraction between nuclei of two atoms and the electrons of their outer shells
How do the atoms achieve full outer shells in ionic bonding?
No electrons are transferred but only shared in this type of bonding
What is formed during a covalent bond?
When a covalent bond is formed, two atomic orbitals overlap and a molecular orbital is formed
Why does covalent bonding occur?
Covalent bonding happens because the electrons are more stable when attracted to two nuclei than when attracted to only one
How should the electrons in a covalent bond be regarded as?
Covalent bonds should not be regarded as shared electron pairs in a fixed position; the electrons are in a state of constant motion and are best regarded as charge clouds
Will the central atom always have an octet?
NO
Covalent bonds should not be regarded as shared electron pairs in a fixed position; the electrons are in a state of constant motion and are best regarded as charge clouds
What is it known as when the central atom has more than 8 electrons?
Being able to accommodate more than 8 electrons in the outer shell is known as ‘expanding the octet rule’
What is it known as when the central atom has less than 8 electrons?
Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
What can be used to predict whether a bond is ionic or covalent?
The differences in Pauling electronegativity values can be used to predict whether a bond is covalent or ionic in character
How is the electron density shared in diatomic molecules?
In diatomic molecules the electron density is shared equally between the two atoms
Eg. H2, O2 and Cl2
Using electronegativity values, explain why diatomic molecules form covalent bonds?
Both atoms will have the same electronegativity value and have an equal attraction for the bonding pair of electrons leading to formation of a covalent bond
What difference in electronegativity does a covalent bond have?
<1.0
What difference in electronegativity does an ionic bond have?
> 2.0
What difference in electronegativity does a polar covalent bond have?
1.0-2.0
What do simple covalent bonds involve?
In simple covalent bonds the two atoms involved share electrons
What is a dative covalent bond or a coordinate bond?
Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
What is an electron deficient atom?
An electron-deficient atom is an atom that has an unfilled outer orbital
What is an example of a dative bond?
An example of a dative bond is in an ammonium ion
Describe the dative bonding in the ammonium ion NH4 +
The hydrogen ion, H+ is electron-deficient and has space for two electrons in its shell
The nitrogen atom in ammonia has a lone pair of electrons which it can donate to the hydrogen ion to form a dative covalent bond
Is it possible to form a quadruple bond?
NO
It is not possible to form a quadruple bond as the repulsion from having 8 electrons in the same region between the two nuclei is too great
What is bond energy?
The bond energy is the energy required to break one mole of a particular covalent bond in the gaseous states
Bond energy has units of kJ mol-1
How does bond energy affect the strength of the covalent bond?
The larger the bond energy, the stronger the covalent bond is
What is the bond length?
The bond length is internuclear distance of two covalently bonded atoms
(It is the distance from the nucleus of one atom to another atom which forms the covalent bond)
How does bond length change with a higher number of electrons being shared?
The greater the forces of attraction between electrons and nuclei, the more the atoms are pulled closer to each other
This decreases the bond length of a molecule and increases the strength of the covalent bond
Describe a triple bond in terms of strength and length
Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms
Why are triple bonds the strongest and shortest
This increase the forces of attraction between the electrons and nuclei of the atoms
As a result of this, the atoms are pulled closer together causing a shorter bond length
The increased forces of attraction also means that the covalent bond is stronger
When is a covalent bond non-polar?
When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar
When is a covalent bond polar?
When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom
Do the positively charged and negatively charged regions of a polar molecule meet? How does this affect the electron distribution?
The negative charge centre and positive charge centre do not coincide with each other
This means that the electron distribution is asymmetric
How do you determine which part of the molecule is negatively and positively charged?
The less electronegative atom gets a partial charge of δ+ (delta positive)
The more electronegative atom gets a partial charge of δ- (delta negative)
What makes the bond more polar?
The greater the difference in electronegativity the more polar the bond becomes
What is a dipole moment?
The dipole moment is a measure of how polar a bond is
What direction does the dipole moment point in?
The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole:
What are Lewis Structures?
Lewis structures are simplified electron shell diagrams and show pairs of electrons around atoms.
What is the octet rule?
The “octet rule” refers to the tendency of atoms to gain a valence shell with a total of 8 electrons
What elements try to achieve the octet rule?
For elements below atomic number 20 the octet rule states that the atoms try to achieve 8 electrons in their valence shells, so they have the same electron configuration as a noble gas
List elements that are exceptions to the octet rule
H, Li, Be, B and Al
Why is H an exception to the octet rule?
H can achieve a stable arrangement by gaining an electron to become 1s2, the same structure as the noble gas helium
Why is Li an exception to the octet rule?
Li does the same, but losing an electron and going from 1s22s1 to 1s2 to become a Li+ ion
Why is Be an exception to the octet rule?
Be from group 2, has two valence electrons and forms stable compounds with just four electrons in the valence shell
Why are B and Al an exception to the octet rule?
B and Al in group 13 have 3 valence electrons and can form stable compounds with only 6 valence electrons
What are 2 examples of Lewis structures with incomplete octets?
, BeCl2 and BF3
What can explain certain resonance structures?
The delocalization of electrons can explain the structures of some species that don’t seem to fit with a Lewis structure
What are delocalised electrons?
Delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond
What is the “correct” resonance structure for the nitrate ion?
studies of the electron density and bond length in the nitrate (V) ion indicate all the bonds are equal in length and the electron density is spread evenly between the three oxygen atoms
The bond length is intermediate between a single and a double bond
The actual structure is something in between the resonance structures and is known as a resonance hybrid
What are the criteria for forming resonance hybrid structures?
The criteria for forming resonance hybrids structures is that molecules must have a double bond (pi bond) that is capable of migrating from one part of a molecule to another
When can a pi bond be formed that is capable of migrating?
This usually arises when there are adjacent atoms with equal electronegativity and lone pairs of electrons that can re-arrange themselves and allow the double bonds to be in different positions
What are the interactions between bonds and nonbonding electrons?
the electrons in the different bonds and the non-bonding electrons in the outer shell all behave as negatively charged clouds and repel each other
How do atoms minimise this repulsion?
In order to minimise this repulsion, all the outer shell electrons spread out as far apart in space as possible
How can the molecular shapes be determined?
Molecular shapes and the angles between bonds can be predicted by the valence shell electron pair repulsion theory known by the abbreviation VSEPR theory
What are the 3 main rules of VSEPR?
- All electron pairs and all lone pairs arrange themselves as far apart in space as is possible.
- Lone pairs repel more strongly than bonding pairs.
- Multiple bonds behave like single bonds
What are domains?
The regions of negative cloud charge are known as domains and can have one, two or three pairs electrons
What is the bond angle when there are 2 domains?
If there are two electron domains on the central atom, the angle between the bonds is 180o
What is the molecular shape with 2 domains?
Molecules which adopt this shape are said to be LINEAR
What is the bond angle when there are 3 domains?
If there are three electron domains on the central atom, the angle between the bonds is 120o
What is the molecular shape with 3 domains?
Molecules which adopt this shape are said to be TRIANGULAR PLANAR or TRIGONAL PLANAR
What happens when there is a lone pair on the central atom that has 3 electron domains?
If one of these electron domains is a lone pair, the bond angle is slightly less than 120o due to the stronger repulsion from lone pairs, forcing the bonding pairs closer together.
The bond angle is approximately = 118o
What shape is SO2? (3 electron domains and lone pair)
This shape is no longer called triangular planar as the shape names are only based on the atoms present, this molecule is BENT LINEAR
What is the bond angle when there are 4 domains?
If there are four electron domains on the central atom, the angle between the bonds is approx 109o
What is the molecular shape with 34domains?
Molecules which adopt this shape are said to be TETRAHEDRAL
What happens when there is a lone pair on the central atom that has 4 electron domains?
If one of the electron domains is a lone pair, the bond angle is slightly less than 109o, due to the extra lone pair repulsion which pushes the bonds closer together (approx 107o
What are molecules that have 4 electron domains and one lone pair called?
Molecules which adopt this shape are said to be TRIANGULAR PYRAMIDAL or TRIGONAL PYRAMIDAL
What happens when there are 2 lone pairs on the central atom that has 4 electron domains?
If two of the electron domains are lone pairs, the bond angle is also slightly less than 109o, due to the extra lone pair repulsion (approx 104o). E.g. H2O
What are molecules that have 4 electron domains and 2 lone pairs called?
Molecules which adopt this shape are said to be BENT or ANGULAR or BENT LINEAR or V-shaped (when viewed upside down)
(Lone pairs are pulled more closely to the central atoms so they exert a greater repulsive force than bonding pairs)
Are bond polarity and molecular polarity the same concept?
NO
What 2 aspects have to be considered when determining molecular polarity?
The polarity of each bond
How the bonds are arranged in the molecule
If a molecule has polar bonds that does mean it is immediately polar?
NO
Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such way that the individual dipole moments cancel each other out
Why are giant covalent lattices formed?
In some cases, it is not possible to satisfy the bonding capacity of a substance in the form of a molecule; the bonds between atoms continue indefinitely, and a large lattice is formed.
Are there bonds between molecules in a covalent molecule?
There are no individual molecules and covalent bonding exists between all adjacent atoms
What are 4 allotropes of carbon?
Graphite, diamond, fullerene and graphene are allotropes of carbon
What is the structure of diamond?
Diamond is a giant lattice of carbon atoms
Each carbon is covalently bonded to four others in a tetrahedral arrangement
What is the bond angle between c atoms in diamond?
bond angle of 109.5o
What is the resultant bonding in diamond? (strength)
The result is a giant lattice with strong bonds in all directions
How is diamond used and why?
Diamond is the hardest substance known
For this reason, it is used in drills and glass-cutting tools
What is the structure of graphite?
In graphite, each carbon atom is bonded to three others in a layered structure
What do the layers look like in graphite and what are the bond angles?
The layers are made of hexagons with a bond angle of 120o
What happens to the electrons not used in graphite?
The spare electron is delocalised and occupies the space in between the layers
What are the bonds in graphite?
All atoms in the same layer are held together by strong covalent bonds, and the different layers are held together by weak intermolecular forces
What is the structure of fullerene?
It contains 60 carbon atoms, each of which is bonded to three others by single covalent bonds
What is special about the 4th electron in fullerene?
The fourth electron is delocalised so the electrons can migrate throughout the structure making the buckyball a semi-conductor
What is the structure of graphene?
Some substances contain an infinite lattice of covalently bonded atoms in two dimensions only to form layers. Graphene is an example
What is the structure of graphene (c atoms)
Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons
WHat is the structure of silicon (IV) oxide?
Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons
What is each silicon attached to in silicon dioxide?
Each silicon is shared by four oxygens and each oxygen is shared by two silicons
This gives an empirical formula of SiO2
What affects the physical properties of substances?
Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility
What melting and boiling points do giant covalent lattices have?
Giant covalent lattices have very high melting and boiling points
why do giant covalent lattices have high mp and bp?
These compounds have a large number of covalent bonds linking the whole structure
A lot of energy is required to break the lattice
Are giant covalent lattice structures hard or soft?
compounds can be hard or soft
is graphite hard or soft?
Graphite is soft as the forces between the carbon layers are weak
is diamond and SIO2 hard or soft?
Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds
is graphene hard or soft?
Graphene is strong, flexible and transparent which it makes it potentially a very useful material
Are giant covalent lattice structures in/soluble in water?
Most compounds are insoluble with water
Do giant covalent lattice structures conduct electricity?
Most compounds do not conduct electricity however some do
Does graphite conduct electricity?
Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
Does graphene conduct electricity?
Graphene is an excellent conductors of electricity due to the delocalised electrons
Does fullerene conduct electricty?
fullerene is a semi-conducto
Does diamond and SiO2 conduct electricity?
Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom is involved in a covalent bond so there are no free electrons available
Are there covalent bonds between molecules in a covalent compound?
There are no covalent bonds between molecules in molecular covalent compounds.§
How do molecular compounds stay together if there are no covalent bonds? What is broken during melting?
There are, however, forces of attraction between these molecules, and it is these which must be overcome when the substance is melted and boiled
What are the bonds between molecules in molecular covalent compounds called? (general)
intermolecular forces
List 3 intermolecular forces
- london (dispersion) forces
- dipole-dipole attraction
- hydrogen bonding
Are electrons static?
NO
The electrons in atoms are not static; they are in a state of constant motion
Are electrons therefore evenly distributed?
It is therefore likely that at any given time the distribution of electrons will not be exactly symmetrical – there is likely to be a slight surplus of electrons on one side of the atoms
What is the atom known as when the electrons are unequally distributed?
This is known as a temporary dipole
Why is it known as a TEMPORARY dipole?
It lasts for a very short time as the electrons are constantly moving
Temporary dipoles are constantly appearing and disappearing
What is a temporarily induced dipole?
Consider now an adjacent atom. The electrons on this atom are repelled by the negative part of the dipole and attracted to the positive part and move accordingly
What role do temporary dipoles play in London dispersion forces?
There is a resulting attraction between the two atoms, and this known as London (dispersion) forces
Where do london dispersion forces occur?
London (dispersion) forces are present between all atoms and molecules
Are london dispersion forces weak/strong?
they can be very weak
What is significant about london dispersion forces being weak?
They are the reason all compounds can be liquefied and solidified
What two factors affect the strength of London dispersion forces?
the number of electrons in the molecule
Surface area of the molecules
How does a greater number of electrons influence the temporary dipoles?
The greater the number of electrons in a molecule, the greater the likelihood of a distortion and thus the greater the frequency and magnitude of the temporary dipoles
How is the strength of London dispersion forces affected by an increasing number of electrons?
The dispersion forces between the molecules are stronger and the melting and boiling points are larger
How does surface area affect the molecule’s interactions with other molecules?
The larger the surface area of a molecule, the more contact it will have with adjacent molecules
How does a greater surface area affect london dispersion forces/
The greater its ability to induce a dipole in an adjacent molecule, the greater the London (dispersion) forces and the higher the melting and boiling points
How can the change in surface area affecting london forces be illustrated?
This point can be illustrated by comparing different isomers containing the same number of electrons
What are dipole-dipole attractions between?
This is an attraction between a permanent dipole on one molecule and a permanent dipole on another.
What type of bonding occurs between two dipoles?
In addition to the London (dispersion) forces caused by temporary dipoles, molecules with permanent dipoles are also attracted to each other by permanent dipole-dipole bonding
How is boiling point affected by dipole-dipole forces, compared to London forces?
Dipole-dipole bonding usually results in the boiling points of the compounds being slightly higher than expected from temporary dipoles alone
Why are dipole-dipole interactions stronger than london forces?
it slightly increases the strength of the intermolecular attractions
For small molecules with the same number of electrons, what factor affects their melting/boiling points?
dipole/dispersion
dipole-dipole attractions are stronger than dispersion forces
What type of forces does butane have and why?
Butane is a nonpolar molecule and will have only dispersion forces
What type of forces does propanone have and why?
Propanone is a polar molecule and will have dipole-dipole attractions and dispersion forces
Therfore, which molecules (butane vs propanone) has a higher bp?
Therefore, more energy is required to break the intermolecular forces between propanone molecules than between butane molecules
The result is that propanone has a higher boiling point than butane
What is the strongest type of intermolecular force?
Hydrogen bonding is the strongest type of intermolecular force
What is hydrogen bonding a special type of?
Hydrogen bonding is a special type of permanent dipole – permanent dipole bonding
What two factors are needed for H bonding?
A species which has an O or N or F (very electronegative) atom with an available lone pair of electrons
A hydrogen attached to the O, N or F
Why can H bonds only occur with O,N or F?
When hydrogen is covalently bonded to an electronegative atom, such as O or N, the bond becomes very highly polarised
The H becomes so δ+ charged that it can form a bond with the lone pair of an O or N atom in another molecule
What does the number of H bonds depend on?
The number of hydrogen atoms attached to O or N in the molecule
The number of lone pairs on the O or N
Are intermolecular forces stronger/weaker than covalent bonds?
Intermolecular forces are much weaker than covalent bonds, so many covalent substances are liquid or gases at room temperature
What does it mean if a substance is volatile?
Substance with a low melting and boiling point are said to be very volatile (easily evaporate)
What does the strength of intermolecular forces increase with?
the size of the molecule
the increase in the polarity of the molecule
What do non-polar substances dissolve in?
non-polar substances mostly dissolve in non-polar solvents, like hydrocarbons
Why do non-polar substances dissolve in non-polar solvents?
form dispersion forces between the solvent and the solute
Why do polar covalent solvents dissolve in polar solvents?
Polar covalent substances generally dissolve in polar solvents as a result of dipole-dipole interactions or the formation of hydrogen bonds between the solute and the solvent
How is solubility affected by an increase in molecular mass? Why?
As covalent molecules become larger their solubility can decrease as the polar part of the molecule is only a smaller part of the overall structure
Why are polar covalent substances unable to dissolve in non-polar substances?
Polar covalent substances are unable to dissolve well in non-polar solvents as their dipole-ipole attractions are unable to interact well with the solvent
Are giant covalent substances soluble?
NO
Giant covalent substances generally don’t dissolve in any solvents as the energy needed to overcome the strong covalent bonds in the lattice structures is too great
Are covalent substances able to conduct electricity? Why?
As covalent substances do not contain any freely moving charged particles they are unable to conduct electricity in either the solid or liquid state
Is there an exception to covalent substances being bad conductors?
However, under certain conditions some polar covalent molecules can ionise and will conduct electricity
(+ graphite)
What is the structure of metallic bonding?
Metal atoms are tightly packed together in lattice structures
What happens to the electrons in metallic bonding?
When the metal atoms are in lattice structures, the electrons in their outer shells are free to move throughout the structure
WHy are the electrons in metallic bonding referred to as delocalised?
The free-moving electrons are called ‘delocalised’ electrons and they are not bound to their atom
What charge do the ions in metallic bonding have?
When the electrons are delocalised, the metal atoms become positively charged
What keeps the lattice in metallic bonding so neat?
The positive charges repel each other and keep the neatly arranged lattice in place
What forces are present in the metallic lattice?
There are very strong electrostatic forces between the positive metal centres and the ‘sea’ of delocalised electrons
Are metallic compounds malleable/brittle?
Metallic compounds are malleable
i.e when a force is applied, the metal layers can slide
Why are metals malleable?
The attractive forces between the metal ions and electrons act in all directions
So when the layers slide, the metallic bonds are re-formed
The lattice is not broken and has changed shape
Do metals have high/low mp and bp and why?
Due to the strong attractive forces between the metal ions and delocalised electrons
Metals have high melting and boiling points
Can metals conduct electricity?
Metals can conduct electricity when in the solid or liquid state
Why can metals conduct electricity?
As both in the solid and liquid state there are mobile electrons which can freely move around and conduct electricity
Is bonding in metals non/directional and why?
Since the bonding in metals is non-directional, it does not really matter how the cations are oriented relative to each other
The metal cations can be moved around and there will still be delocalised electrons available to hold the cations together
How can the strength of electrostatic attraction be increased?
Increasing the number of delocalised electrons per metal atom
Increasing the positive charges on the metal centres in the lattice
Decreasing the size of the metal ions
(trends seen across a period and down a group)
How does the number of valence electrons change across a period?
number of valence electrons increases
Why is the metallic bonding in aluminium stronger than in for example sodium?
Aluminium ions are also a smaller size than magnesium ions or sodium ions and these two factors lead to stronger metallic bonding which can be seen in the melting points
How does melting point change down group 1? Why
As you go down the group the size of the cation increases so this decreases the attraction between the valence electrons and the metallic lattice, leading to a reduction of the melting point
What are alloys?
Alloys are mixtures of metals, where the metals are mixed together physically but are not chemically combined
How are the atoms bound together in an alloy?
Ions of the different metals are spread throughout the lattice and are bound together by the delocalized electrons
What nature of metallic bonding allows alloys to be formed?
It is possible to form alloys because of the non-directional nature of the metallic bonds
Why do alloys have different properties?
Alloys have distinct properties due to the different packing of the cations in the lattice
What are examples of different properties that alloys may have (3/4)
Alloys often have properties that can be very different to the metals they contain, for example they can have greater strength, hardness or resistance to corrosion or extreme temperatures
Why are alloys tougher than just metals?
Alloys contain atoms of different sizes, which distorts the regular arrangements of cations
This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal
Name 4 alloys
brass
steel
solder
bronze
What is brass made of?
copper and zinc
What special properties does brass have?
strong and resistant to corrosion
What is solder an alloy of?
lead and tin
What is solders special property?
low melting point
