Partial negative and positive charges?

Which molecule from this list should have the largest partial positive and negative charges: NO, CO2, O2, CH4, NH3. Why?

Answer:
The bond in O2 is covalent, but it is not a polar covalent bond. It is a nonpolar covalent bond.

In the last two cases one of the atoms is more electronegative that the other atom. The most electronegative atom gets more than its share of the electrons so it has a partial negative charge.
In the C- N bond the N is more electronegative than the C so the N gets more than its share of the electrons. Sometimes we indicate this difference in charge distributions by writing,

(d +)C- N(d- ).

The d + means that the C atom has a partial positive charge. (This is not a full unit charge, but only a fraction of a unit charge.) The d- means that the N atom has an equal and opposite partial charge.

In the C- H bond the C is more electronegative than the H so this time the C has the partial negative charge. Write,

(d- )C- H(d +).
Polar covalent bonds form a dipole. We say that they have a dipole moment.

A dipole moment is a charge multiplied by a distance (the distance between the separated charges).

d + ¬ d ® d-

(Recall that d + and d- are equal and opposite partial charges.)

Dipole moment = d ´d

Dipole moment is a vector quantity. That is, it has a magnitude and a direction.

When we add dipole moments we must include the direction information.

Molecules with a dipole moment are called polar molecules.

Examples:
HF
(The most electronegative element is the negative end.)
H2

O2

N2

CO2

H2O

NH3

CH4
So far we have only been talking about the polarity of individual bonds. What is of most importance to us is the polarity of a molecule.
You need to look at the electronegativity differences between the atoms that are bonded together in each compound, that is, you need to determine the polarity of each bond in each molecule. Then you also need to think about whether the differences in electronegativity balance each other out in a 3-dimensional molecule, or whether they reinforce each other.

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