Topic D: Models of Bonding and Structure - Covalent Flashcards
Definition of covalent bond
Electrostatic attraction between a shared pair of electrons and the nuclei of the atoms being bonded
What are some exceptions to the octet rule
Molecules whose central atoms have fewer than 8 electrons after bonding (usually compounds in Group 2 and 3)
Molecules whose atoms have more than 8 electrons after bonding (usually compounds in period 3 onwards)
Molecules with an off number of electrons
What factors affect bond strength
Bond length
Bond multiplicity
Bond polarity
How does bond length affect bond strength
The longer the bond length, the further from the shared pair of electrons the nuclei are, thus the bond energy and strength will be lower
How does bond multiplicity affect bond strength
Bond multiplicity refers to the presence of multiple covalent bonds between 2 atoms
The presence of multiple bonds increase the overall bond strength between the 2 atoms
How does bond polarity affect bond strength
An increase in bond polarity means an increase in the ionic character of the bond, which usually results in an increase in bond strength and energy
What is a coordination bond
A covalent bond in which the shared pair of electrons is contributed by only one of the bonded atoms. This atom donates both electrons to the shared pair
What are the criteria for the formation of a coordination bond
Donnor atom must possess an unshared pair of electrons
Acceptor atom must possess an empty orbital in its valence shell
How is the VSEPR theory applied
Any atom in a molecule or ion bonded to more than one atom is known as the central atom (eg in H2O, O is the central atom)
Each bonded atom or lone pair of electrons joined to the central atom constitutes an electron domain
A multiple bond is to be considered as 1 electron domain for the purpose of determining the overall geometry
Electron pairs around the central atom arrange themselves as far as possible to minimise repulsion
What is the order of the strength of repulsion in the VSEPR theory
From strongest to weakest
Lone pair - lone pair repulsion
Lone pair- bond pair repulsion
Bond pair - bond pair repulsion
Definition of electronegativity
The relative attraction that an atom has for the shared pair of electrons in a covalent bond
How does electronegativity change going across a period
Going across a period, the nuclear charge increases but the shielding effect remains relatively constant, so the effective nuclear charge increases
Thus atoms get smaller, resulting in a decreased distance between bonding electrons and the nuclei
Electrostatic attraction between the bond and nuclei increases, so electronegativity increases
How does electronegativity change going down a group
Going down the group, atoms get larger, resulting in an increased distance between bonding electrons and the nuclei
The increase in distance results in a decrease in electrostatic attraction between bonding electrons and the nuclei of the atoms, so electronegativity decreases
How does the difference in electronegativity of two atoms affect its polarity
When the difference in electronegativity increases, the polarity of the covalent bond increases
It is possible for the difference in electronegativity to increase until a stage that the covalent bond turns into an ionic bond
How are electrons shared in a polar bond
Electrons are shared unequally because of the difference in electronegativity
Valence electron density distorts towards the atom with the higher electronegativity
How does a dipole moment form
In a polar covalent molecule, the atom with the higher electronegativity will have a higher electron density and thus a partial negative charge
The atom with the lower electronegativity will have a lower electron density and thus a partial positive charge
The separation of charges creates a dipole
What are the 3 types of dipoles
Permanent dipole
Instantaneous induced dipole (temporary)
Induced dipole
What is a permanent dipole
Exists in all polar molecules as a result of the difference in electronegativity of bonded atoms
What is an instantaneous induced dipole (temporary)
In non-polar molecules, there are no permanent dipoles because electron clouds are symmetrically distributed around the molecule
However due to fluctuations of electron clouds, positive and negative charges can arise temporarily, giving rise to instantaneous induced dipoles
What is an induced dipole
When a molecule having a dipole comes close to a non-polar molecule, a temporary dipole will be induced on the non-polar molecule
What is the criteria for a covalent molecule to be polar
There must be at least one polar bond
The polar bonds must not cancel, as there should be a net dipole
What are physical properties of giant covalent structures
Hard
High melting and boiling points
Insoluble in polar solvents and water
Low electrical conductivity
What are the properties of simple covalent structures
Usually soft or brittle
Low melting and boiling points
Solubility in water and organic solvents depends on its polarity
Low electrical conductivity
What are the 3 types of intermolecular forces
London Dispersion force
Dipole-dipole forces
Hydrogen bonding
How are London dispersion forces formed
The electron density on a non-polar molecule is usually evenly distributed.
However, the electrons in a molecule are in continual motion, so temporary fluctuations in the electron density of the molecule result in a non-symmetrical electron cloud and an instantaneous induced dipole.
This temporary dipole can induce a similar dipole in neighbouring molecules so that molecules attract each other
Why are London dispersion forces weak
These attractions tend to be temporary and fluctuating in nature, thus making them generally weak compared to covalent bonds
What factors affect the strength of London dispersion forces
When the number of electrons increases, the size of the electron cloud also increases, causing London dispersion forces to become stronger.
When the size of the molecule increases, the size of the electron cloud also increases, causing London dispersion forces to become stronger.
How are dipole-dipole forces formed
There is an electrostatic attraction between the oppositely charged ends of two polar molecules with permanent dipoles
This is called the dipole-dipole forces
What factors affect the strength of dipole-dipole forces
The polarity of the molecule
The greater the electronegativity of between the bonded atoms, the larger the bond dipoles and the stronger the attraction between oppositely charged dipoles
What are the requirements for hydrogen bonding to occur
For one molecule, a hydrogen atom is attached to a highly electronegative atom such as N, O or F
For the other molecule, there is at least one lone pair of electrons on the electronegative atom
How does hydrogen bonding occur
When a hydrogen atom is bonded to a highly electronegative atom such as N, O or F, the bonding electron pair is drawn towards the more electronegative atom
The electron density of the hydrogen is reduced and it becomes slightly positively charged. The highly electronegative atom of another molecule possessing lone pairs of electrons can share its electron density with the positively charged hydrogen atom
What does the number of hydrogen bonds formed per molecule depend on
Number of lone pairs available on the electronegative atom
Number of H atoms covalently bonded to the electronegative atom
What is the order of the strength of intermolecular forces of attraction from strongest to weakest
Hydrogen bonding
Dipole-dipole
London dispersion
What are factors affecting the degree of polarisation
Charge and size of cation
Charge and size of anion
How does the charge and size of cation affect the degree of polarisation
The higher the cationic charge, the greater is the polarising power
The smaller the ionic radius of the cation, the greater the polarising power
How does the charge and size of anion affect the degree of polarisation
The higher the anionic charge, the greater the polarisability
The larger the ionic radius of the anion, the greater the polarisability
