Structure 2.2
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When are covalent bonds formed?
When electrons from different atoms are shared so that each atom attains a noble gas electron configuration (full valence shell). Known as the octet rule.
What are covalent bonds due to?
The electrostatic attraction between shared pairs of electrons and the positively charged nucleus of each atom.
Single covalent bond
When one pair of electrons is shared between two atoms
Double covalent bond
Two pairs of shared electrons
Triple covalent bonds
Three pairs of shared electrons
Bond strength ___ as bond length decreases, as the number of shared pairs of electrons ___
increases, increases
The greater the number of shared pairs of electrons, the ___ the attraction between the two covalently bonded atoms
greater
Coordination covalent bonds
Covalent bonds where both the shared electrons come from the same atom
Arrows in lewis diagrams signify…
coordination covalent bonds
What is an incomplete octet?
An exception to the octet rule where atoms are stable with fewer than 8 electrons in their valence shell. This usually happens with elements like boron (6val e) and beryllium (4val e), they are said to be electron deficient.
Electron deficient
Electron deficient describes molecules or atoms that don’t have enough electrons to achieve a full octet around one or more atoms. These molecules often contain atoms like boron or beryllium, which are stable with fewer than 8 electrons.
Electronegativity
A relative measure of the attraction an atom has for a shared pair of electrons when it is covalently bonded to another atom
Trend in electronegativity values
They increase across a period and up a group. Most electronegative elements are F, O and N. H is similar to C
What is the polarity of a covalent bond between two atoms of the same element?
Non-polar as the bonding pair of electrons is shared equally between the two atoms
What results in a polar bond?
A covalent bond between two different types of atoms. The more electronegative atom will attract the bonding pair of electrons making that atom the negative dipole.
What happens if the difference in electronegativity between the two covalently bonded atoms is greater than 1.8?
The bonding pair of electrons moves completely towards the more electronegative atom and the bonding becomes ionic.
What does VSEPR stand for?
Valence shell electron pair repulsion
VSEPR theory
States that pairs of electrons are arranged around the central atom in a simple molecule or ion so that they are mutually repulsive as possible (as far apart as possible).
To find the shape of a molecule/ion what needs to be considered?
• The number of pairs of e’s • All electrons in a multiple bond count as 1 pair of e’s or 1 e domain • order of repulsion
Order of repulsion
non-bonding : non-bonding > non-bonding : bonding > bonding : bonding
Covalent network structures
A lattice of atoms all covalently bonded together to form essentially one giant molecule held together by very strong covalent bonds.
Properties of covalent network structures
High melting and boiling points. Poor conductors (except for graphite) as their electrons are localised.
Why is graphite a good conductor
Because it has delocalised electrons
Allotrope
different forms of the same element that have distinct physical structures but the same chemical composition