4.3 Covalent structure Flashcards
what is the octet rule?
where each outer atom has 8 valence electrons
what is the only exception where a covalent compound does not follow the octet rule?
hydrogen, because it only needs 2 valence electrons
what types of elements follow the octet rule?
- all covalent compounds
- period 1 elements
- period 2 elements
- period 3 elements
what is the difference between an octet rule and an expanded octet rule?
- octet rule is when each outer atom has 8 valence electrons
- expanded octet rule is when the central atom can have more than 8 valence electrons (note that this only applies to the central atom)
when does a structure get termed the word ‘resonance structure’?
when the double bond in a molecule can exist in multiple locations
define electron domains
bonds and lone pairs on the central atom
list the 3 domain geometries and state their molecular geometry
- linear 90
- trigonal planar 120
- tetrahedral 109.5
what are the 2 forms of molecular geometry when a molecule has 3 domains (talk about the lone pairs as well)?
- trigonal planar
- bent (which has 1 lone pair)
what are the 3 forms of molecular geometry when a molecule has 4 domains (talk about the lone pairs as well)?
- tetrahedral 109.5
- trigonal pyramidal 107
- bent
what are allotropes?
different crystalline forms of carbon
what are the 4 different allotropes?
- diamond
- graphite
- graphene
- buckminsterfullerene
describe the structure and properties of diamonds
- giant tetrahedral
- each carbon is thus bonded to 4 other carbons, forming a hexagonal ring
- which makes it very hard
- since it only contains covalent bonds, it has a high melting and boiling point
- no valence electrons = cannot conduct electricity
describe the structure and properties of graphite
- a layered structure composed of carbon atoms bonded with 3 other carbons
- forms a hexagonal ring
- with a sea of delocalized electrons between layers
- forms weak intermolecular forces
- due to their free valence electrons, graphite can conduct within layers but not through them
what is graphene?
- a single layer of graphite
- flexible
- conducts electricity
describe the structure and properties of buckminsterfullerene
- gigantic molecular carbon structure
- each molecule is composed of 60 carbons each bonded to 3 carbons
- forming alternating hexagons and pentagons
- weak intermolecular forces between molecules
- has low melting point
- no valence electrons are free to move so it can’t conduct
why do lone pairs repel bonded pairs and why does it have an affect on the bond angel?
- because a lone pair is not shared by two nuclei, it occupies more space near the central atom than a bonding pair
- decreases bond angel by a bit
write an equation to show the conversion of ozone to oxygen that occurs in the atmosphere. include any conditions required for the reaction to take place
2O3 –> 3O2
requires UV light
explain why the energy required to break the bonds in oxygen is higher in energy than the energy required to break the bonds in ozone
- oxygen has a bond order of 2 (double bond)
- ozone has bond order 1.5
- bonds in oxygen are stronger than those in ozone
describe the main features of the VSEPR theory for predicting shapes of molecules
- pairs of electrons in the outer shell
- repel each other
- so it takes up positions in space to minimise the repulsion
*the electron pairs have a tendency to orient themselves in a way that minimises the electron-electron repulsion between them and maximises the distance between them
explain how oxygen (O2) protects the earth from specific UV radiation. You should name the process that takes places and write an equation to show this
- it is broken down by UV light from the UV-C part of the spectrum via dissociation
- O2 –> 2O
