Module 2: Bonding And Structure Flashcards
What is a covalent bond?
An electrostatic attraction between a shared pair of electrons with a positive nuclei
What does a single bond contain?
Two shared electrons and a sigma bond
What does a double bond contain?
Four shared electrons and a sigma bond and a pi bond
What does a triple bond contain??
Six shared electrons and a sigma bond and 2 pi bonds
How is bond length defined?
Defined as the distance between the nuclei of two covalently bonded atoms
How is bond energy (bond enthalpy) defined??
Defined as the energy required to break a bond in every molecule in one mole of gaseous molecules
What does the strength of covalent bonding depend on?
How many electrons are being shared
The bond length
Any additional ionic character
If the bond energy is small, what is the strength of the covalent bond like?
The covalent bond is weak and can easily be broken during a chemical reaction
What is dative covalent bonding??
A type of covalent bonding where both shared electrons are donated by the same atom
What is metallic bonding?
Electrostatic attraction between the sea of delocalised electrons and the lattice of positive ions
Structure of a metallic bond
In metals, the partially filled outer shells all overlap and the outer shell electrons are able to move freely through the metal
Why are the electrons described as delocalised in a metallic bond??
The electrons are not localised within individual bonds
Why are metals good conductors of electricity??
The outer shell electrons are free to move and carry a current
Why are metals good conductors of heat??
When solids get hot, atoms vibrate
In a metal the mobile electrons can carry energy to distant atoms so they can vibrate too so heat can then travel through the metal
Why are metals ductile and malleable??
Because delocalised electrons hold the atoms together without making bonds between specific atoms
Therefore it is relatively easy for layers of atoms to slide past each other
Why are metals shiny and can reflect light??
Incoming light causes electrons to oscillate and they re-emit energy as light
What is an ionic bond??
Electrostatic attraction between a positive ion and a negative ion
Why are most ionic compounds soluble in polar solvents such as water, but some are not??
If ions have large charges then the ionic attraction may be too strong for water to break down the lattic structure
Why does solid sodium chloride not conduct electricity but molten sodium chloride does??
Because when solid, sodium chloride ions are in a fixed lattice but when molten, the sodium chloride ions dissociate and are free to move past each other and conduct a current
List factors affecting the strength of a covalent bond
4
Number of electrons
Bond length
Electron distance from nuclei
Additional ionic character
What is electronegativity??
A measure of the tendency of an atom to attract a bonding pair of electrons
The attraction that a bonding pair of electrons feels for a particular nucleus depends on:
1) number of protons in the nucleus
2) distance from the nucleus
3) amount of screening/shielding by inner electrons
Trends down a group
Values decrease
Atomic radius increases
Increased electron shielding
Outer electrons are further from the nucleus
Less attraction between nucleus and bonding pair of electrons
=
Smaller pulling power
Trends across a period
Values increase
Atomic radius increases
Increased electron shielding
Outer electrons are further from the nucleus
Outer shell electrons are in the same shell so no additional shielding
=
Greater ‘pulling power’
Effect of the number of protons in the nucleus
Electronegativity increases across a period
Effect of distance from the nucleus
Electronegativity falls as you go down a group
Effect of the amount of shielding by inner electrons
Electronegativity falls as you go down a group
What happens in a purely ionic bond
Electrons are completely transferred
What happens in a purely covalent bond
Electrons are equally shared
When is a bond 100% covalent??
When two identical atoms are bonded together
What happens if the atoms that are sharing electrons are different??
The shared electrons will be drawn slightly closer to the more electronegative atom
What are intermolecular (London) forces??
Weak interactions between dipoles of different molecules
What are the three main categories of intermolecular forces in order of strength??
Induced dipole-dipole interactions
Permanent dipole-dipole interactions
Hydrogen bonding
What are induced dipole-dipole interactions??
Weak intermolecular forces that exist between all molecules (non-polar and polar)
When do induced dipole-dipole interactions act??
They act between induced dipoles
Are induced dipole-dipole interactions temporary or permanent
Temporary
What can happen in an instant in an induced dipole-dipole interaction??
In an instant the induced dipole may disappear and the process may happen again between different molecules
What happens when the number of electrons in each molecule increases
The instantaneous and induced dipoles are larger
The dipole-dipole interactions are greater
The attractive forces between molecules are stronger
The induced dipole is larger
There is more energy needed to overcome the intermolecular forces
What is the movement of electrons like in an instantaneous dipole-induced dipole attraction?
The electrons in a molecule are constantly moving
What is the affect of an instantaneous dipole-induced dipole attraction on neighbouring molecules
This dipole will induce dipoles in neighbouring molecules which in turn induce dipoles in their neighbours
What results in a small instantaneous dipole in an instantaneous dipole-induced dipole attraction?
An uneven distribution of charge
Permanent dipole - permanent dipole attractions
Many molecules have differing electronegativities
The separation of charge is called a dipole
The greater the difference in electronegativity, the greater the dipole will be
How is a molecule with an overall dipole described??
Polar
If a molecule has a very small or no dipole then how is it described??
It is described as non-polar
What is a simple molecular substance??
Made up of simple molecules
What structure do simple molecules form in a solid state??
A regular structure called a simple molecular lattice
Simple molecular lattice
Molecules held together by weak intermolecular forces
Atoms within each molecule are bonded together strongly by covalent bonds
What forces are acting on the noble gases
Instantaneous dipole-induced dipole attractions are the only forces acting between atoms
Noble gases group trend
On descending the group, the number of electrons increases so the London forces get stronger
So more energy is needed to separate the atoms
Hence mp and bp increase on descending the group
What is the effect of electron number increasing with noble gases when descending down the group
On descending the group the total number of electrons in each molecule increases so the London forces get stronger so more energy is needed to separate the molecules
As a result mp and bp increases
Hydrogen bonds
Much stronger than the previous two intermolecular forces
Occurs between any molecule that contains a H+ atom covalently bonded to F, O or N
What does the strength of a hydrogen bond depend on??
The strength of an individual hydrogen bond depends on the electronegativity of the atom bonded to H
Is ice more or less dense than water
Ice is less dense than water
Why is ice less dense than water (bond explanation)
When water freezes, more hydrogen bonds establish
As hydrogen bonds are long bonds, the molecules are held apart from each other
In interlocking hexagons
Why does water have an unexpectedly high mp/bp??
Water has hydrogen bonds between molecules which are strong
Other group 6 hydrides have weaker intermolecular forces and so need less energy to break
What is enthalpy change??
The energy required to convert one mole of a substance from liquid to gaseous state
Why does water have an unusually high surface tension??
Because of strong hydrogen bonds across the surface of the water
What is electronegativity??
The ability of an atom to attract the bonding pair of electrons in a covalent bond
Percentage of ionic character equation
% ionic character = 100 x (difference in electronegativity) / 3.3
What is a polar bond??
A covalent bond with a permanent dipole, having partial positive and negative charges on bonded atoms
What is an overall dipole??
Has a shape that results in charge being separated between two ends of the molecule
What is responsible for the anomalously high boiling points of ammonia, water and hydrogen fluoride??
Hydrogen bonds
What are the forces acting between molecules of water?
A hydrogen bond is forming by attraction between delta + and delta - charges on different water molecules
Why are bonds equidistant from each other??
All electron pairs repel equally so each bond is equidistant from each other
What is VSEPR (valence shell electron repulsion theory)???
Molecules adapt the shape that is most stable
This is achieved when the electron pairs are as far apart as possible
Tetrahedral shape explanation
4 identical bonds equally spaced around the central atom
Bond angle of a tetrahedral shape
109.5 degrees
Triganol planar shape explanation
3 identical bonds equally spaced around the central atom
Trigonal planar bonding angle
120 degrees
Pyramidal shape explanation
1 lone pair repels the bonding pairs more, skewing the overall shape
Has 4 electron pairs: 3 bonding pairs and 1 lone pairs
Bond angle of a pyramidal shape
107 degrees
Non-linear shape explanation
2 lone pairs repel bonding pairs further meaning the bond angle is smaller
4 electron pairs: 2 bonding pairs and 2 lone pairs
Non-linear bonding angle
104.5 degrees
What is the general rule for bonding angle decreasing
When a lone pair of electrons is added, the bonding angle decreases by 2.5
Octahedral shape explanation
6 identical bonding pairs
Octahedral bonding angle
90 degrees
Linear shape explanation
2 identical bonding regions as far from each other as possible
Bonding angle of linear shape
180 degrees
Why can elements in period 3 and below have more than 8 electrons in their outer shell
There are empty available d orbitals that can be used
Why can sulfur form 6 covalent bonds??
Sulfur has empty available 3D orbitals in to which 2 of sulfur’s outershell electrons can be promoted
Why does hydrogen chloride have intermediate bonding??
Because they have permanent dipole dipole interactions
The electrons are not equally shared because the hydrogen and chloride atoms have different electronegativities
Structure of ionic bonding
Giant lattice
Properties of ionic bonding
Ionic bonds between ions
Ions held together by strong electrostatic forces between oppositely charged ions
High mp/bp
Strong and hard
Why can ionically bonded molecules conduct when molten or dissolved but not when solid
When molten/dissolved ions are free to move
When solid, ions are fixed in a lattice structure
Metallic structure
Giant lattice
Properties of metallic bonding
Atoms in a metal are held together in a lattice by strong metallic bonds
Close packed arrangement
High mp/bp
Malleable and ductile
Insoluble
Good conductors
Why are metallically bonded molecules good conductors of heat??
Because delocalised electrons are free to move
Covalent structure
Giant lattice
Giant layer lattice
Simple molecular
Simple molecular - hydrogen bonded
Polymers
Properties of covalently bonded molecules (giant lattice)
Covalent bonds between atoms
High mp/bp
Strong and hard
Insoluble
Not conductive
Properties of covalently bonded molecules (giant layer lattice)
Covalent bonds between atoms in layers
Instantaneous dipole:induced dipoles forces between layers
High mp/bp
Soft solids
Insoluble
Conducts electricity
Properties of simple molecular substances
Covalent bonds between atoms in molecules
Instantaneous dipole:induced dipole and maybe permanent dipole-dipole attractions between molecules
Low mp/bp
Solids are soft
Soluble in non-polar solvents
Do not conduct electricity
Properties of hydrogen bonded simple molecular substances
Covalent bonds between atoms in molecules
Hydrogen bonds and id:id attractions between molecules
Relatively low mp/bp
Solids are stronger and harder than other simple molecules due to hydrogen bonding
Soluble in polar solvents
Do not conduct electricity
Properties of polymers
Covalent bonds between atoms within polymer molecules
Instantaneous dipole:induced dipole attractions between polymer molecules
Intermediate mp/bp
Soft And easily moulded
Insoluble in polar solvents
Do not conduct
