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# Molecular Structure Study Guide (page 2)

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Updated on Sep 24, 2011

#### Example:

Show the resonance structures for nitrate, NO3–. Although the three nitrate resonance structures are written separately, nitrate is a combination of all three structures.

## Formal Charge

Atoms in certain molecules or polyatomic ions may have a formal charge. A formal charge is the difference in the number of valance electrons in the neutral atom (group number) and the number of electrons assigned to that atom in the molecule or polyatomic ion. Mathematically, the equation is

formal charge = group number – number of bonds – number of lone electrons

#### Example:

Give the formal charge for each atom in the nitrate ion, NO3–, and ozone, O3 (yes, even neutral molecules can have elements with formal charges).

### Note

All formal charges must add up to the charge on the polyatomic ion or zero for a neutral molecule.

## Polarity

The electronegativity of an element is its strength and ability to attract paired electrons in a covalent molecule. Electronegativity increases as you move up and over to the right on the periodic table. Fluorine is the most electronegative element. (See Figure 11.2)

A dipole results in a covalent bond between two atoms of different electronegativity. A partial positive (+) and a negative charge (–) develop at both ends of the bond, creating a dipole (i.e., two poles) oriented from the positive end to the negative end. The oxygen atom is more electronegative than hydrogen in water and the result is a dipole. Dipoles are represented by a line with a perpendicular line ( ).

A dipole moment will exist in a molecule if the resulting dipoles do not cancel based on their additive vectors. If the two dipoles of water are added, water has a dipole moment in the "up" direction.

#### Example:

Identify the dipole for each of the following bonds: B-F, Cl-I, N-H.

Predict whether each of the following molecules has a dipole moment: SCl2, CH2Cl2.

SCl2: Sulfur dichloride has a bent structure and a dipole:

CH2Cl2: If drawn (incorrectly!) planar, the molecule shows no dipole:

However, if drawn correct as tetrahedral, the dipole is

Practice problems for these concepts can be found at -  Molecular Structure Practice Questions

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