2. Bonding and structure ( COVALENT)
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What is covalent bonding
a chemical bond formed from the electrostatic attraction between the shared pair of electrons and the two nuclei
What 2 structures does covalent bonding form
simple covalent molecule giant covalent structure
Give properties of giant covalent molecules due to their strong covalent bonds
High melting boiling points Hard Insoluable
Why do simple covalent molecules have low melting and boiling points
While they have strong intramolecular forces, there are only weak forces between molecules.
What is graphene
A single layer of graphite
What shape does diamond form
Tetrahedral
What is dative covalent bonding
When one atom donates 2 electrons to an atom or an ion to form a bond – > One of the bonding atoms supplies both the shared electrons to the covalent bond
State the type of bond that joins the two AlCl3 molecules together
Dative covalent bonding
Explain how ammonia and boron trifluoride react together to form NH3BF3 which contains a dative covalent bond
NH3 shares its lone pair of electrons to BF3 as it is electron deficient and only has 6 valence electrons
How do you represent dative covalent bonding in diagrams
Using arrows - arrow points from the atom donating the lone pair to the atom accepting it.
What are valence electrons
electrons in the outer shell of an atom
what are lone pairs
a pair of electrons not used in the covalent bonding
Which elements can expand their octets
Elements after period 3 - utilise their empty d subshells to promote an electron
What are electron deficient molecules ( and so don’t follow octet rule) - give an example
Molecules with too few electrons in outer shell so dont have 8 eg: BF3 - B can only give 3 electrons
What are sigma bonds
Formed from the head on overlap of 2 atomic orbitals: s orbital + s orbital s orbital + p orbital p orbital + p orbital
What is a pi bond
Formed from the side overlap of a p orbitals
What bonds does a double bond contain
Both sigma bond and pi bond
What does the shape of a molecule depend on
Number of bonding pairs and lone pairs
Which repel more: lone pairs or bonding pairs
Lone pairs
By how many degrees do lone pairs reduce the angle of the bonding pairs
For every lone pair present, bond angles get reduced by 2.5 degrees –> lone pairs repel more than bonding pairs so they push bond pairs closer together. Therfore if you have a lone pair the angle goes from 109.5 - 2.5 = 107. If you have 2 bonding pairs 109.5 - 5 = 104.5
Give the 6 steps to work out the shape of a covalent molecule or molecular ion by determining the number of lone pairs and bonding pairs
• Count the number of electrons on the central atom. If the species is charged, add or remove electron(s) to account for the charge. • Each atom forms one bond to the central atom using one of the central atom’s electrons. How many electrons are left and so how many lone pairs are there? • How many electron pairs all together? • Take into account the lone pairs
2BP 0LP
Linear, 180
3BP 0LP
Trigonal planar, 120
2BP 1LP
Bent (non linear), 117.5