3a. Introduction to the covalent bond
- bonding in small molecules
Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A
level US grade
9-12 level Revision Notes
– electron sharing in small or big molecules!
What is a covalent bond? How is it formed?
Covalent bonds are formed by atoms sharing electrons to form
bonds that hold the atoms together in a molecule.
This type of bond usually formed between two non–metallic elements.
The molecules might be that of an
element i.e. one type of atom only OR from different elements chemically combined to form a
Note: The molecular formula is
the summary of all the atoms in a molecule.
The covalent bonding is caused by the mutual
electrical attraction between the two positive nuclei of the two atoms forming the bond, and the
SHARING of a pair of negative electrons between
A COVALENT BOND IS THE SHARING
OF A PAIR ELECTRONS BETWEEN TWO ATOMS
It only involves electrons in
the outer shell i.e. the outermost energy level containing 1–7 electrons,
which can be shared between atoms to form a covalent bond.
One single covalent bond is a sharing of
1 pair of electrons, two pairs of shared electrons between the same two atoms gives a double bond and it is possible for two atoms to share 3 pairs of electrons and give a triple bond.
From the diagram above, and as already
stated, a covalent bond is formed by the mutual electrical charge
attraction of two positive nuclei and the negative electrons in between.
The two nuclei are the centres of the
two atoms bonded together.
The electrons reside where the two energy
levels overlap so the electrons are shared in the formation of a
Note: In the examples of covalent
bonding it is
assumed YOU can work out the electron configuration (arrangement
in shells or energy levels)
given the atomic number from the Periodic Table.
This kind of bond or
electronic linkage, does act in a particular direction i.e. along the 'line'
between the two nuclei of the atoms bonded together, this is why covalent molecules have
a particular shape.
In the case
of ionic or metallic bonding, the electrical attractive forces act in all
directions around the particles involved.
Which electronic structures are
the most stable?
because is this what atoms will try to get to electronically!
(atomic number) and
When atoms SHARE ELECTRONS in a
covalent bond, they try to attain the electron structure (electron
configuration) of the electronically very stable atoms of the Group 0 Noble
The electron structures of helium neon
and argon are shown above.
eg helium (2), neon (2.8) or argon (2.8.8), that is,
a full outer
shell of electrons (full highest energy level).
Hydrogen (1 proton, 1
electron) needs one shared electron to become electronically stable
Many other elements become stable by
sharing electrons to give an outer shell of 8 electrons, like Ne and
Quite simply, this is because
these are the most stable electron arrangements and have a full outer shell of
electrons (full highest energy level).
The number of bonds formed
depends on the number of electrons that needs to be shared so that any pair
of atoms in a molecule forming a covalent bond attain the electron
arrangement of a noble gas (i.e. 2, 2.8 or 2.8.8 etc.)
Note that hydrogen and helium
only have one shell, so when referring to the full outer shell of
hydrogen, it is the one and only shell, but the descriptive word 'outer'
is much more crucial when describing the electronic structures of any
element with at least two shells e.g. when describing covalent bonding in
molecules containing carbon, oxygen, nitrogen and chlorine etc.
In advanced level chemistry you
will encounter examples of electronic structures of atoms in covalent
molecules that are NOT those of a Noble Gas.
An example of COVALENT BONDING
a covalent bond is formed by two atoms sharing electrons so that the atoms
combine to form molecules.
The bond is usually
formed between two non–metallic elements combine to form a molecular compound. The two positive nuclei (due to the positive protons
of both atoms are mutually attracted to the shared negative electrons between
them forming the covalent bond in the molecule. They share the electrons in a way that gives a
stable Noble Gas electron arrangement like helium (2) or neon (2.8) etc..
This kind of bond or
electronic linkage does act in a particular direction i.e. along the
'line' between the two nuclei of the atoms bonded together, this is why
molecules have a particular shape.
and oxygen atoms share electrons to give covalent O–H bonds to form
molecules of the covalent compound water
has a 'bent' shape 2 bond pairs (2 non-bonding pairs)
How to draw diagrams of small covalent
molecules and work out a formula from valencies
Covalent Bonding: small molecules & properties
bonding and structure notes
Perhaps of interest?
Advanced A level Notes on
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