bonding and hybridisation - building blocks of life

what are lone pairs

pairs of electrons occupying the same orbital in the valence shell

what is the octet rule

to reach maximum stability, main group elements seek to gain 8 electrons in their valence shell

what is hypervalency

a phenomenon where main elements (in the s and p block) accommodate more than 8 electrons in their valence shell

examples of hypervalent atoms

the phosphate anion and sulfur hexafluoride

how is hypervalency explained

ionic model - where some bonds are ionic - further explored by resonance (bonds switch around)

what is a covalent bond

forms when 2 atoms share a pair of electrons

• the two electron clouds overlap and this attractive force decreases potential energy of system

what are the two main bonding theories

valence bond theory and molecular orbital theory

what is the valence bond theory

electrons in a chemical bond are localised between the two atoms, bonds are formed when valence orbitals overlap

pros/cons of valence bond theory

• enables us to readily understand the shape and reactivity of biological molecules

• cannot explain photochemical reactions

what is the molecular orbital theory

atomic orbitals combine to form molecular orbitals, the atoms interact with eachother like waves, can be constructive or destructive interference

what does constructive interference lead to

bonding molecular orbitals

what does destructive interference lead to

anti-bonding molecular orbitals

pros/cons of molecular bond theory

• conceptually challenging

• allows to rationalise curly arrow notation

• widely used in theoretical chemistry

what does the combination of 2 atomic orbitals form

a bonding molecular orbital (low energy) and an antibonding molecular orbital (high energy)

what are the two types of molecular orbitals

sigma and pi

what is a sigma orbital

formed by end-on overlap of atomic orbitals

what is a pi orbital

formed by side-on overlap of atomic orbitals

what is electronegativity

measure of attraction an atom has for electrons in a bond

influences the energy of valence electrons

what does an electronegativity difference of under 0.5 indicate

non-polar covalent bond

what does an electronegativity difference between 0.5 and 2 indicate

polar covalent bond

what is an ionic bond

a electrostatic bond often formed between nonmetals and metals which relies on the attraction between opposite charges

what is formed when a metal donates an electron to a non-metal

cation

what is formed when a non-metal accepts an electron

anion

what is a dative bond

a bond where both of the shared electrons come from one atom

what are coordination complexes

compounds consisting of a coordination centre (usually a metal ion) surrounded by molecules (ligands) capable of coordinating with the central atom through dative bonding.

what are ligands

molecules that can bond to a central metal atom through dative bonding

what are examples of ligands

ammonia, water, chloride ions

what is the coordination number

total number of ligands attached to central metal, determines the geometry of the complex

coordination number 2 shape

linear

coordination number 4 shape

tetrahedral

coordination number 6 shape

octahedral

what is a chelating agent

a ligand capable of forming more than 1 coordinate bond

what is the complex called when the ligand forms more than 1 coordinate bond

chelate

what is conjugation

the ability of electrons in a system of double bonds to spread over the whole system, rather than staying localised on a specific double bond

what does conjugation do

increase stability

example of conjugated system

alternating single double bonds or aromatics

what is resonance

a method of describing the delocalised electrons in molecules where bonding cannot be rationalised by Lewis structures

examples of resonance structures

• peptide bond (no free rotation around C-N)

• carbonate ion

• benzene

what is homolytic fission

the process where a bond is broken evenly and each atom gains one electron - forms radicals

what is heterolytic fission

the process where a bond is broken unevenly and one atom gains both electrons - forms opposite ions

what is the VSEPR theory based on

electrons repel each other and place themselves as far away as possible to maximise separation

what do non-bonding pairs do in VSEPR

repel each other more, which can distort expected geometry

what is steric number

the sum of the atoms attached to central atom and the number of lone pairs

shape and bond angle for steric number 2

linear - 180

shapes and bond angles for steric number 3

0 lone pairs - trigonal planar - 120

1 lone pair - angular - >120

shapes and bond angles for steric number 4

0 lone pairs - tetrahedral - 109

1 lone pair - trigonal pyramidal - <1109

2 lone pairs - angular - <<109

what is hybridisation

the mixing of atomic orbitals which combine to form a new set of degenerate hybrid orbitals

3 sets of carbon hybrid orbitals

sp3, sp2, sp

what shape does sp3 form

tetrahedral

what shape does sp2 form

trigonal planar

what shape does sp form

linear