Humboldt State University ® Department of Chemistry

Richard A. Paselk

	General Chemistry	Fall 2008
Exercise: Lewis Structures	© R. Paselk 2008
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Lewis Structures

Draw Lewis Structures of the compounds below. Recall that positively charged ions retain the element name in compounds, while negatively charged elemental ions are given the -ide ending. For covalent compounds the more positive (less electronegative) element is usually given the element name, while the more electronegative element is given the -ide ending.

In general we can predict how electrons distribute in compounds using the Hi-Lo-Intermediate electronegativity rules:

1. Compounds composed of a "Hi" and a "Lo" element will be ionic with the respective ions combining to give a neutral compound.
2. All other combinations will give covalent compounds following the "octet rule" whenever possible.

A Periodic Table is provided with Hi (red), Lo (gray) and Intermediate (green) Electronegativities indicated to aid in prediting ionic vs. covalent compounds.

Multiple Bonds

Recall we must show an octet (or duet for Period I) in the outer-most shell (valence electrons). When this does not occur with single electron pairs (bonds) between atoms can sometimes make it happen with multiple bonds. You might find "Clark's Method" useful for determining the bonding patterns of various molecules:

• Clark's Method for determining bonding in covalent Lewis Structures
  • Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge)
    • If S e^- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
    • If S e^- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = D/2 (remember a triple bond is 2 multiple bonds!).
  • If you can draw more than one structure, then chose the most symmetrical.

Periodic Table of the Elements

IA

IIA

IIIA

IVA

VA

VIA

VIIA

VIIIA

H

He

2

Li

Be

B

C

N

O

F

Ne

3

Na

Mg

IIIB

IVB

VB

VI

VIIB

VIIIB

IB

IIB

Al

Si

P

S

Cl

Ar

4

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

Br

Kr

5

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

6

Cs

Ba

Lu

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

1. Phosphide ion

2. Bromide ion

3. Rubidium fluoride

4. Cesium nitride

5. Methane, CH₄

6. Hydrogen sulfide

7. Carbon monoxide

8. Barium iodide

9. Ammonia, NH₃

Set A - Answers 1-9

1. Formaldehyde, CH₂O

2. Strontium nitride

3. Cyanide ion, CN^-

4. Carbon dioxide, CO₂

5. Chloromethane, CH₃Cl

6. Acetic acid, CH₃CO₂H

7. Ozone, O₃

8. Sulfite ion

9. Water, H₂O

10. Carbocation, CH₃⁺

Set B - Answers 1-10

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In addition to these exercises you should familiarize yourself with the materials in your text and lab manual.

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© R A Paselk

Last modified 30 September 2008