covalent bonding Flashcards
definition of a covalent bond (2m)
electrostatic attraction between a shared pair of electrons and the positively charged nuclei, forming either polar or non - polar bonds
Covalent bonding occurs in elements with
non - metals, high electronegativity
define lewis formula, lone pairs and the octet rule
Formula that shows all the valence electrons (bonding and lone pairs)
Lone pairs are electrons not involved in the reaction
Octet Rule: Atoms are stable with 8 electrons in their valence shell
What are 2 exceptions to the octet rule and why
Berrylium and Boron
because both have fewer than eight valence electrons available for bonding:
Chromatography - def
explain how it works
Separates the the components of a mixture based on their relative attractions involving intermolecular forces to mobile and stationary phases
If the substance has stronger intermolecular forces with water in the mobile phase it will travel further up the paper
If the substance has stronger intermolecular forcesd with paper in the stationary phase
retardation factor
distance moved by component / distance moved by solvent front
order of strength of bonds and length of bonds
what is the relationship and why
triple > double > single
length is vice versa because
Double bonds are shorter because they have two pairs of bonding electrons, while single bonds only have one pair. The two pairs of electrons in double bonds pull the atoms closer together, resulting in shorter and stronger bonds
Coordination bond w example
Both the electrons in the covalent/ionic bond come from the same atom
e.g: hydrogen cation + water = hydronium H30+
ligand:
ion or molecule that binds to a central metal atom to form a coordination complex
this occurs in transition metals
VSEPR 3 principles
1) Electron pairs repel each other and therefore arrange themselves as far apart from each other as possible
2) Lone pairs occupy mmore space than bonding pairs
3) Double and triple bonds occupy more space than single bonds
3ED Possibilities
3BP - Trigonal Planar
2BP - Bent
2ED + explanation
Linear geometry
Electron pairs repel each other and adopt positions at 180 degrees
wedges and dashes
wedges: bonds cmg out of the plane
dashes: going into the plane
3ED explanation + angles
Trigonal Planar - electron pairs adopt positions at 120 degrees from each other, the domains form a triangle while the atoms lie flat on the plane
In bent on v shaped (2BP) the angle will be less than 120 degrees because the lone pair exerts a strong repulsion and takes up more space
4ED Explanation
Electron pairs adopt positions at 109.5 degrees from each other, the ends of the domains form the corners of teh tetrahedron
5ED possibilities
5BP - Trigonal bipyramidal
4BP - Seeasw
3BP - T shaped
2BP - Linear
5ED explanation
Trigonal bipyramidal bonding features 5 electron domains with 3 equatorial and 2 axial positions, forming 120° and 90° bond angles
Seesaw bonding arises when 1 lone pair occupies an equatorial position in a trigonal bipyramidal structure,
T-shaped bonding results from 2 lone pairs occupying equatorial positions, leaving 3 bonded atoms in a 90° and 180° arrangement,
6ED Possibilities
6BP - Octahedral
5BP - Square pyramidal
4BP - Square Planar
3BP - T shape
Formal charge formula
FC = Valance electrons - Non Bonding Pairs - (1/2 x Bonding pairs)
Formal charge function
and how we choose
Helps us decide which lewis structure is preferable when there is more than one option
It is preferably when all atoms have a formal charge closest to zero
or the most electronegative atom should have the negative formal charge
Sigma and pi bonds
+ electron density
Sigma Bonds: head on overlap of atomic orbitals + electron density conecentrated along the bond axis
Pi bonds: Sideways overlap of atomic orbitals and electron density is concentrated on opposite sides of the bond axis
Polarity
How electrons are distributed within bonds, results from the difference of electronegativity of the bonded atoms
One can have a partially positive charge / partially negative charge
Pure covalent
Atoms involved in the formation of the bond are identical
the charge is distributed evenly so there is no polarity or dipole moment (dipoles can cancel each other out)
2 conditions for polarity
- Molecules are non - polar when all the bonds are non polar
- Molecules are polar when their bond dipoles do not cancel each other out and vice versa
Covalent networks / Giant covalent definition and 3 properties
Atoms held together by covalent bonds in a continous 3D lattice
- High melting and boiling points
- Insoluble
- Don’t conduct electricity
Allotropes
Elements with different structural bonds
Graphite (5)
- Layers of carbon sheets bonded in a hexagonal arrangement
- Trigonal planar geometry around each carbon
- One delocalised electron per atom - conducts electricity
- Strong covalent bonds between carbons, Weak LDF between sheets
- Sheets can be seperated easily making it a good lubricant + soft
Graphene (4)
- 2D
- One sheet of graphite
- Strong, light, flexible and transparent
- Conducts electricity just like graphite
Fullerenes
C60
Atoms arranged in interlinking 20 hexagonal and 12 pentagonal rings
Used in nanotech
Silicon Dioxide
bonding
2 properties
plus crystalline form
3D lattice each silicon bonded to 4 other silicon atoms in a tetrahedral arragement
Very strong and semi conductor of electricity
Crystalline form of si02: quartz
LDF Forces & what affects them
l
Experienced by all molecules from temporary instaneous dipoles.
These dipoles occur due to the random movement of electrons around the molecules
- Effect increases with increasing number of electrons in the atom as they increase polarizability and hence strength of LDF and molecular shape / size
Dipole - induced dipole
Permanent dipole in the polar molecule induces a temporary dipole in the non polar molecule
Dipole dipole forces
Permnanent dipoles experienced between polar molecules
Hydrocarbon chains (phillic / phobic)
They are non polar and do not form hydrogen bonds with water
Long chains are hydrophobic
Soap and detergent have a hydrophobic tail and a hydrophillic head: called surface active agents
- Volatility
- Electrical conductivity
- Solubility
- High for molecular substances like CH4, low for covalent network structures like diamond
- Low because they lack delocalised electrons except graphite
- Low except compounds that form hydrogen bonds with water molecules like C2H5OH and NH3
Resonance structures occurance and properties
- Occur when there is more than one possible position for a double bond in a molecule or ion
- They have an identical length and strength as the intermediate between a single and double bond
- There will always be delocalised electrons present in a resonance structure, as this brings additionally stability
- Pi electrons of the double bond become delocalised and spread out between the possible bonding positions
Benzene property
Pi electrons become delocalised and spread out over all the carbon-carbon bonding positions
They form 2 rings above and below the plane of the molecule
The bonds have an order of 1.5
The bonds are intermediate in length and strength between single and double bonds
Kekule structure & why it was wrong
Shows alternating single and double bonds
This is not correct because the bonds would have different length whereas it was proven that the bonds all have the same intermediate length
Physical evidence for benzene (2)
Equal carbon carbon bond lengths and strengths with an order of 1.5
Regular hexagonal structure
Bond order and relation to length
Stregth of the bond, inversely proportional to length
Chemical evidence for benzene (3)
Substitution > Addition – Benzene favors substitution due to delocalized electrons, maintaining aromatic stability.
No Bromine Water Reaction – Benzene does not decolorize bromine water as it lacks true double bonds.
One Isomer in Substitution – Delocalization ensures all C-C bonds are equivalent, forming only one isomer. Double bonds would result in more than 1 isomer
Hydrogenation Less Exothermic – Enthalpy change is less negative than predicted, proving extra stability from aromatic delocalization and lack of 3 double bonds
4ED Possibilities & bonding angle trend
4BP - Tetrahedral
3BP- Trigonal Pyramidal
2BP - Bent
bonding angle decreases as u go down because lone pairs increase
