CHEMISTRY 4
CHAPTER 13
THE STRUCTURE AND SHAPE
DRAWING LEWIS DIAGRAMS
Procedure for drawing Lewis diagrams for substances that “obey” the octet rule.
1 Count the total number of valence electrons
2 Place the least electronegative atom in the center of the molecule
3 Draw a tentative diagram. Join atoms by single bonds.
4 Determine the number of electrons available for lone pairs by subtracting twice the number of single bonds from the total number of electrons available.
5 Starting at the outer atoms, distribute the available lone pair electrons to complete the octet around each atom except hydrogen which requires a duet.
6 If there are not enough electrons to complete all octets, move one or more lone pairs from an outer atom to form a double or triple bond with the central atom until all atoms have an octet.
Example 1 Water molecule H2O
Total number of valence electrons: 6+1+ 1 = 8
Tentative diagram: Hydrogen can have only two electrons, so it can form only one bond.
H-O-H
Number of electrons available for lone pairs : 8 - 2 x 2 = 4
Put two lone pairs around oxygen. Oxygen has eight electrons around it.
Example 2 . Ammonia molecule NH3
Total number of valence electrons: 5+3 = 8
Tentative diagram:
H-N-H
H
Number of electrons available for lone pairs : 8 - 3 x 2 = 2
Put the lone pair around nitrogen. Nitrogen has eight electrons around it.
Example 3 . Ammonium ion NH4+
An ammonium ion is produced when a hydrogen ion is bonded to the unshared electron pair of the nitrogen atoms in an ammonia molecule. The bond formed when one atom contributes both electrons is called coordinate covalent bond.
Total number of valence electrons: 5 + 3 = 8
Tentative Lewis diagram
H
H -N- H
H
No electrons available for lone pair.
There are single bonds between the nitrogen and the three hydrogens. The charge at each hydrogen atom is zero.
The electron count around the nitrogen gives a total of 6 electrons ( two core electrons+ 1/2 x8 bonding electrons) . Nitrogen’s nuclear charge is 7. Hence the positive charge is placed on nitrogen.
Example 4 Hydronium ion H3O+
Hydrogen cation does not exist in water. It reacts with water molecule to from hydronium ion.
Total number of valence electrons 6+2 = 8
Tentative diagram:
H-O-H
H
Number of electrons available for lone pairs : 8 - 3 x 2 = 2
H-O-H
H
The lone pair is placed on oxygen. The formal charge on oxygen is
8 (nuclear charge) - 2 (core electrons)-2 (lone pair) -1/2 ( 6 bonding electrons) = 1
H-O-H
H
Example 5 Methane CH4
Total number of valence electrons 4 + 4 = 8
Tentative Lewis diagram
H
H -C- H
H
The molecule does not have lone pair
Example 6 Hydrogen carbonate ion HCO3-
Total number of valence electrons in the molecule 1 + 4 + 3 x 6 + 1 = 24
Tentative Lewis diagram :
H-O-C-O
O
Number electrons available for lone pairs : 24 - 2 x 4 = 16
Place lone pairs around oxygen atoms. Carbon has only three electrons pairs.
H-O-C-O
O
A lone pair may be moved from either of the outer oxygens to from a double bond with carbon. The negative charge is on the other outer oxygen.
H-O-C-O <------> H-O-C-O
O O
The two Lewis diagrams are equivalent and are called resonance structures. It is customary to place a two -headed arrow between resonance diagrams. On paper, the bonds between the carbon and the two outlying oxygens look different, but in fact they are identical. All C-O bonds are of equal length , between that of a double and a single bond. The negative charge is evenly distributed over two oxygen atoms; it is said to be delocalized. We say that hydrogen carbonate is a resonance hybride of the two resonance structures.
Example 7 Carbonate ion CO32-
Total number of valence electrons 4 + 3 x 6 + 2 = 24
Tentative Lewis diagram :
O-C-O
O
Number electrons available for lone pairs : 24 - 2 x 3 = 18
Place lone pairs around oxygen atoms. Carbon has only three electrons pairs.
O-C-O
O
A lone pair may be moved from one of the three outer oxygens to from a double bond with carbon. The negative charge is on the two other outer oxygen atoms
O-C-O <------O-C-O <-----> O-C-O
O O O
Carbonate ion has three resonance structures.
THE VALENCE SHELL ELECTRON PAIR REPULSION THEORY (VSEPR).
The VSEPR applies primarily to substances in which a second period atom is bonded to two, three, and four other atoms.
ELECTRON PAIR REPULSION
Molecules that have only single bonds.
To apply the VSEPR method we simply count the number of lone electron pairs and the number of bonding electron pairs around the central atom. We will call the total number of electron pairs the steric number, SN. The basic idea of VSEPR is that the electrons pairs in Lewis diagrams repel each other, and distribute themselves in positions that are far away from each other as possible. These are the locations of lowest energy. This arrangement of electrons pairs is called electron pair geometry.
The geometries are summarized below:
A If the central atom has two electron pairs ( as in BeF2 ) the electron pair geometry is linear. The electron pair angle is 1800
B If the central atoms is surrounded by three electron pairs. The geometry is trigonal planar. The electron pair angle is 1200
C If the central atom is surrounded by four electron pairs, The geometry is tetrahedral, and the electron pair angle is 109.50
MOLECULAR GEOMETRY
Molecular geometry describes the shape of a molecule and the arrangement of atoms around a central atom.
Two electron pairs, two bonded atoms. Molecule is linear. Example BeF2
Three electron pairs, three bonded atoms. trigonal planar. Example BF3
Four electron pairs, four bonded atoms. tetrahedral Example CH4
Four electron pairs, three bonded atoms. Trigonal pyramidal. Example NH3 ( 107.50 )
Four electron pairs, two bonded atoms. Angular or bend. Example H2O ( 104.50 )
GEOMETRY OF MULTIPLE BONDS
Two or three electrons pairs in a multiple bond behave as a single electron pair in establishing geometry.
For example, HCN and CO2 are linear.
POLARITY OF MOLECULES
A polar molecule is one in which there is an unsymmetrical distribution of charge, resulting in + and - poles ( dipole). Example HF, HCl, H2O
If the central atom has no lone pairs and all atoms bonded to it are identical, the molecule are nonpolar. Example CCl4
In an electric field, polar molecules are oriented with the positive end toward the direction of the field.
THE STRUCTURES OF SOME ORGANIC COMPOUNDS
Carbon atoms have four valence electrons and can form four covalent bond with other atoms. One carbon atoms can be bonded to other carbon atoms forming a long chain.
HYDROCARBONS
Hydrocarbon are binary compounds of carbon and hydrogen.
ALCOHOLS AND ETHERS
Alcohol is an alkane in which a hydrogen atom is replaced by a hydroxyl group.
CARBOXYLIC ACID
Carboxylic acids contain the carboxyl group -COOH
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