Lewis Structures - Workshop Outline
The PowerPoint presentation as well as this workshop outline will be available online at the SLC’s wikispace page.
Check us out at sesciencelab.wikispaces.com
The absolute value of the difference in electronegativity (ΔEN) of two bonded atoms provides a rough measure
of the polarity to be expected in the bond and, thus, the bond type.
When the difference is very small or zero, the bond is covalent and nonpolar. When it is large, the bond is polar covalent or ionic.
Which of the following compounds are Ionic and which are Covalent?
a) Cl2CO, b) MnO, c) NCl3 d) CoBr2 *(one exception) e) K2F f) HI g) MnI2 *(another exception)
Writing Lewis Structures
What are the Lewis structures of hydrogen cyanide (HCN), ethane (H3CCH3), acetylene (HCCH), and ammonia (NH3)? Solution
Step 1. Calculate the number of valence electrons.
HCN: (1 × 1) + (4 × 1) + (5 × 1) = 10
H3CCH3: (1 × 3) + (2 × 4) + (1 × 3) = 14
HCCH: (1 × 1) + (2 × 4) + (1 × 1) = 10
NH3: (5 × 1) + (3 × 1) = 8
Step 2. Draw a skeleton and connect the atoms with single bonds. Remember that H is never a central atom.
Step 3. Where needed, distribute electrons to the terminal atoms:
HCN: six electrons placed on N
H3CCH3: no electrons remain
HCCH: no terminal atoms capable of accepting electrons
NH3: no terminal atoms capable of accepting electrons
Step 4. Where needed, place remaining electrons on the central atom:
HCN: no electrons remain
H3CCH3: no electrons remain
HCCH: four electrons placed on carbon
NH3: two electrons placed on nitrogen
Step 5. Where needed, rearrange electrons to form multiple bonds in order to obtain an octet on each atom:
HCN: form two more C–N bonds
H3CCH3: all atoms have the correct number of electrons
HCCH: form a triple bond between the two carbon atoms
NH3: all atoms have the correct number of electrons
Formal Charge: The charge that a given atom would have if all the atoms in a molecule had the same electronegativity (equal sharing of electrons in all bonds).
If different Lewis structure can be drawn for a given compound, all the Lewis structures are referred to as resonance structures. Not all resonance structures are equal.
The most likely Lewis structure is the one that:
1) Has all formal charges as close to zero as possible.
2) If a negative charge must be present on the Lewis structure, the best resonance structure will place the negative charge on the most electronegative element.
= −/ −NON
Valence electrons are the total number of electrons in the outermost shell.
Bonded and non-bonded electrons of the atom can be determined from the Lewis Structure.
Example:
Atom N C S
Valence Electron 5 4 6
½ Bonded Electrons -3 -4 -1
Nonbonding Electrons -2 -0 -6
Formal Charge on atom 0 0 -1
Practice:
1. Write the Lewis structure for sulfuric acid, H2SO4, which has two oxygen atoms and two OH- groups bonded to the sulfur.
2. Write resonance forms that describe the distribution of electrons in each example:
(a) Selenium dioxide, SeO2 (b) The Formate ion:
3. Write resonance forms that describe the distribution of electrons in each of these molecules or ions:
(a) Pyridine: (b) The allyl ion:
4. Sodium nitrite has been used to preserve bacon and other meats, is an ionic compound. Write the resonance forms of the nitrite ion, NO2–
5. In terms of the bonds present, explain why acetic acid, CH3CO2H, contains two different types of carbon-oxygen bonds, whereas the acetate ion, formed by loss of a hydrogen ion from acetic acid, only contains one type of carbon-oxygen bond.
6. Draw all possible resonance structures for each of these compounds. Determine the formal charge on each atom in each of the resonance structures:
(a) SO2
(b) NO2−
(c) NO3−
7. Based on formal charge considerations, which of the following would likely be the correct arrangement of atoms in nitrosyl chloride: ClNO or ClON?
8. Based on formal charge considerations, which of the following would likely be the correct arrangement of atoms in hypochlorous acid: HOCl or OClH?
9. Iodine forms a series of fluorides. Write Lewis structures for each of the four compounds and determine the formal charge of the iodine atom in each molecule:
(a) IF (b) IF3 (c) IF5 (d) IF7
10. Draw Lewis dot formula including resonance structures and formal charge. If applicable circle the most likely Lewis structure: a) SiCl4 b) NH4+ c) CS2 d) SeO3 e) N2O4 f) NCl3
2