Both H2O and CO2 have 3 atoms, yet one is polar and one is nonpolar. Explain why, using Lewis dot structures in your answer
CO2 is linear.
O=C=O
Symmetrical molecules have no net dipole moment.
H2O is angular.
H
|
O-H
Non-symmetrical molecules have a dipole moment if the electronegativity of the central atom is different from the attached atoms.
To determine the polarity of a molecule using Lewis dot structure, we need to consider the electronegativity of the atoms involved and the molecular geometry.
Let's start with the Lewis dot structure of H2O:
H: •
O: • •
In H2O, the oxygen atom has a higher electronegativity compared to the hydrogen atoms. Each line or dot in the Lewis dot structure represents a valence electron. Oxygen needs to gain two more electrons to achieve a stable octet. On the other hand, the hydrogen atoms only need one electron for stability. To fulfill the octet rule, oxygen shares two electrons with each hydrogen atom, forming two covalent bonds:
H: • ─ O ─ •
|
H: •
The two pairs of electrons shared by oxygen result in a bent or V-shaped molecular geometry. Since oxygen is more electronegative than hydrogen, it attracts the shared electrons more strongly, creating a partial negative charge (δ-) on the oxygen atom and partial positive charges (δ+) on the hydrogen atoms. This uneven charge distribution leads to the H2O molecule being polar.
Now, let's look at the Lewis dot structure of CO2:
O: • ─ C ─ O •
In CO2, the carbon atom and both oxygen atoms have similar electronegativities. Each oxygen atom requires two electrons to complete their octets, while carbon requires four electrons. Thus, carbon forms two double bonds with oxygen, sharing two pairs of electrons with each oxygen atom:
O: • = C = O •
The molecule has a linear molecular geometry, where the carbon atom is in the center, and the oxygen atoms are at opposite ends. Since the electronegativity of carbon and oxygen is similar, the shared electrons are equally distributed between them. Consequently, there is no significant charge separation in the CO2 molecule, making it nonpolar.
In summary, the polarity of a molecule, such as H2O and CO2, is determined by a combination of the electronegativity difference between the atoms and the molecular geometry arising from the arrangement of atoms in a space.