Electron dot formulation also referred to as Lewis Dot Structures seek to symbolize the atom as a symbol representing what is called the "core" which involves the nucleus and all but the valence outermost electrons. The valence electrons are symbolized as dots clustered around the elemental symbol.
Hydrogen Lewis dot structure would be the letter H with one dot.
All elements in Group 1 called the alkali metals would be represented with their elemental symbol with one dot. They form mono-valent bonding because they only have one unpaired electron in their valence region. Unpaired electrons are considered excellent candidates for bonding electrons. Group 1 elements are mono-valent and generally form ionic bonding with non-metals or metalloids.
All elements in Group 2 called the alkali earth metals would have their electron dot structure represented by their respective elemental symbol with two dots. There are referred to as Di-valent atoms and generally form ionic bonding with non-metals and metalloids.
Group 13 beginning with Boron would have three dots all separated, unpaired, and considered bonding electrons. Group 13 atoms are said to be tri-valent and generally form co-valent bonding associations.
Group 14 headed by Carbon would have four dots representing four valence electrons all unpaired. Group 14 elements particularly Carbon and Silicon are considered tetra-valent capable of forming four covalent bonds with other atoms or group of atoms.
Group 15 beginning with Nitrogen would have five dots two of which would be clustered together as a pair. These paired up electrons would not be candidates for bonding and are referred to as "non-bonding" electrons. Group 15 elements usually form co-valent bonding when combined with other metalloids or non-metals. They generally are considered tri-valent atoms and form three covalent bonding pairs.
Group 16 beginning with Oxygen would have six valence electron consisting of two pairs of dots and two dots unpaired. Group 16 elemenst generally form co-valent bonding when combined with non-metals. They are said to form di-valent covalent bonding where there will be two covalent bonding pairs.
Elements in Group 17 beginning with Fluorine would have seven valence electrons consisting of three pairs of dots and one single dot. This group known as the halogens would be mono-valent with only one bonding pair.
The Inert Gas or better known as the Noble Gases is group 18. They would have eight electrons consisting of four pairs of dots. Since their valence electrons are all paired up, this would suggest that the Noble gases don't normally form bonding associations since each Noble gas are quite satisfied having a complete octet of valence electrons. Their valency is said to be zero, and there are only a handful of compounds formed mostly with Xenon and Radon.
Lewis structures are used to identify symbolically the structure of molecules and the type of co-valent bonding exhibited in molecular substances.
Let's consider the writing of the Lewis structure of C2H5Cl.
The writing of Lewis structures require that we followng the following steps:
For C2H5Cl according to the lewis dot formulas we would have 2 carbons for 8 (4 valence electrons each) and 5 Hydrogens for 5 (one valence electron each) and one Chlorine for a total of seven valence electrons or a total of 20 valence electrons. Since the molecular formula has no charge indicated in the molecular formula then no electron adjustment is needed for the total.
For the C2H5Cl it would take seven bonding pairs (dashes) to connect all the two carbons, five Hydrogens, and one Chlorine together in one unit leaving 6 electrons (3 pairs) electrons left over.
Each Carbon has four bonding pairs surrounding them (8 valence electrons). Each Hydrogen has two electrons, but the Chlorine has only two electrons (one bonding pair)(See Fig 1-b) and so needs six more electrons to complete its octet (8) of valence electrons. The remaining 6 electrons in the valence pool we began with in step 1 will be assigned to the Chlorine as lone (or non-bonding) pairs (3 pairs shown as three pairs of dots in the Lewis Structure). (See Fig 1-c)
Lets try another. What is the Lewis Structure for C3H6?
In this compound there would be 3 carbons for 12 and 6 Hydrogens for 6 more or a total of 18 valence electrons total.
Each Hydrogen obeys the Octet Rule having the required two electrons. However, one or more of the carbons (depending on wheich carbons you attached the Hydrogen atoms to begin with) will not follow the Octet Rule. It is possible to move Hydrogens along with their bonding pairs of electrons from one carbon to another. By placing the two remaining unaccounted for valence electrons between the two carbons can we satisfy the Octet Rule for these carbons. (See Fig 2-c below)
An exception to the Octet Rule would be a carbocation. This is a carbon atom that is one electron deficient such as the CH3+ carbocation. Drawing the Lewis structure for this cation would give the following Lewis Structure:
Notice the positive charge on the formula indicating the necessity of subtracting one electrn from the 7 valence electrons that the one carbon atom and three Hydrogen atoms would have. This would leave 6 valence electrons to assign
This will use up all of the six avaialable valence electrons determined in step 1.(See Fig 3-b)
The Hydrogens are satisfied with two electrons but the carbon has only six and requires eight according to the Octet Rule. Since we have no more valence electrons to assign and there are no lone pairs that can be converted to bonding pairs, this is a clear example of an exception of the Octet Rule.
Now it is your turn to write some Lewis Structures for the following formulas:
May I suggest the following Web site for practice in writing Lewis Structures: Writing Lewis Structure Problem Set (problem sets developed by S.E. Van Bramer for chemistry and environmental science courses at Widener University.)
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Fig 8 below gives the correct answers. Notice there are two different (non-equivalent structures for formula 2 since the Chlorine can be connected to one of the end carbons or to the middle carbon and still be a correct Lewis Structure according to the Octet Rule. These structures are called constitutional isomers to one another.
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