The three dimensional structural arrangement of different atoms in a molecule is called as Molecular Geometry. There are different types of molecular structure formations depending on the number of covalent bonds. The VSEPR theory which also means valence shell electron pair repulsion theory is used to decide the geometrical structure of the given molecule.
According to the VSEPR theory the number of valence electrons on the central atom decide the molecular structure of the compound. The central atom can either form bond pairs or lone pairs with its valence electrons. The bond pairs are formed when the central atom shares the electrons with another atom. The lone pair are the electrons which belong to the central atom in a molecule and are not shared with any other atom. Lone pairs are also called as non-bonding pair of electrons. Here is how the number of bond pairs and lone pairs can be calculated for a given atom.
Number of bonding electrons = (Total possible valence shell electrons) - (Valence shell electrons of the atom)
Number of non-bonding electrons = (Total possible valence shell electrons) – 2 x (number of bonding electrons)
Using the formulas for finding the bonding and non-bonding electrons for some atoms:
|Atom||Total number of valence electrons possible.||Number of valence electrons||Number of bonding electrons||Number of non-bonding electrons|
|Carbon C||8||4||8 - 4 = 4||8 -2 (4) = 0|
|Nitrogen N||8||5||8 - 5 = 3||8 – 2 (3) = 2|
|Oxygen O||8||6||8 - 6 = 2||8 – 2 (2) = 4|
|Fluorine F||8||7||8 - 7 = 1||8 – 2 (1) = 6|
|Neon Ne||8||8||8 - 8 = 0||8 - 2 (0) = 8|
According to the table mentioned above here is how the atoms will look like:
Depending on the number of bonding pairs and the lone pairs the molecular geometry of atoms can be predicted. Here is the table mentioned by the VSEPR theory:
|Bonding Electron pairs||Lone pairs||Shape of the Molecule||Angle|
|3||1||Trigonal Pyramidal||< 109.50|
|5||0||Trigonal Bipyramid||900, 1200, 1800.|
|4||1||Seesaw||900, 1200, 1800|
|5||1||Square Pyramidal||900, 1800|
|4||2||Square Planar||900, 1800|
For the Carbon di-oxide molecule, Carbon is the central atom and it forms covalent bonds with the two oxygen atoms. The first step is to write the electronic configuration for the carbon atom and check for the number of valence electrons it has.
· The Carbon atom has an atomic number of 6.
· The electronic configuration of Carbon is 1s2, 2s2, 2p2.
· The number of valence electrons of carbon is 4.
· As already shown in the table above Carbon has 4 bonding electrons and no lone pairs.
· The Carbon atom needs 4 more electrons to reach the stable state configuration. Hence it forms 4 covalent bonds.
· The Oxygen atom has the atomic number of 8.
· The electronic configuration of Oxygen atom is 1s2, 2s2, 2p4.
· The number of valence electrons for Oxygen is 6.
· As already shown in the table above Oxygen has 4 non-bonding electrons (2 lone pairs) and 2 bonding electrons.
· The Oxygen atoms needs two more electrons to reach the stable sate configuration. Hence it forms two covalent bonds.
Using the VSEPR theory for Molecular Geometry, CO2 has linear shape. Central atom carbon forms double bonds with each Oxygen atom [C=O]. Due to the sharing of electrons Carbon and Oxygen now have 8 electrons in the outermost shell.
As the molecular Structure for the Carbon di-oxide CO2 molecule is linear, it has an angle of 1800. Since the Oxygen atom has 2 lone pairs, it pulls the shared bond pair of electrons towards itself due to which there is an electronegativity generated in the carbon oxygen bond formation. This is the reason carbon oxygen double bond is polar covalent. However as there are two oxygen atoms on both sides they form symmetry due to which the CO2 molecule is non-polar.