covalent bond in which two pairs of electrons are shared between two atoms

molecule that contains an odd number of electrons

molecule containing at least one main group element that has more than eight electrons in its valence shell

diagram showing lone pairs and bonding pairs of electrons in a molecule or an ion

symbol for an element or monatomic ion that uses a dot to represent each valence electron in the element or ion

two (a pair of) valence electrons that are not used to form a covalent bond

guideline that states main group atoms will form structures in which eight valence electrons interact with each nucleus, counting bonding electrons as interacting with both atoms connected by the bond

bond in which a single pair of electrons is shared between two atoms

bond in which three pairs of electrons are shared between two atoms

← Back to Chemistry 2e

Chemistry 2e · 7.3 Lewis Symbols and Structures

Lewis structures: practice problems

11 problems from the book, with its answers. Check your working, or have any of them explained to you on the whiteboard.

How to approach these problems

Count the valence electrons first: the group number of each main-group atom, plus one for each negative charge, minus one for each positive charge. Put the least electronegative atom in the center (never hydrogen), connect the others with single bonds, and give the outer atoms lone pairs until each has an octet. Leftover electrons go on the central atom. If the central atom still has fewer than eight, turn lone pairs from its neighbours into double or triple bonds. Check the total: it must equal your count. The usual mistakes: miscounting the electrons of an ion, giving hydrogen more than two, and forgetting that atoms from period 3 down, like Se and Xe, can hold more than eight.

Exercises 23
Write the Lewis symbols for each of the following ions:
(a) As³⁻
(b) I⁻
(c) Be²⁺
(d) O²⁻
(e) Ga³⁺
(f) Li⁺
(g) N³⁻
Sign up free to have it checked
Exercises 25
Write the Lewis symbols of the ions in each of the following ionic compounds and the Lewis symbols of the atom from which they are formed:
(a) MgS
(b) Al₂O₃
(c) GaCl₃
(d) K₂O
(e) Li₃N
(f) KF
Sign up free to have it checked
Exercises 27
Write the Lewis structure for the diatomic molecule P₂, an unstable form of phosphorus found in high-temperature phosphorus vapor.
Sign up free to have it checked
Exercises 29
Write Lewis structures for the following:
(a) O₂
(b) H₂CO
(c) AsF₃
(d) ClNO
(e) SiCl₄
(f) H₃O⁺
(g) $NH_{4}^{+}$
(h) $BF_{4}^{-}$
Sign up free to have it checked
Exercises 31
Write Lewis structures for the following:
(a) SeF₆
(b) XeF₄
(c) $SeCl_{3}^{+}$
(d) Cl₂BBCl₂ (contains a B–B bond)
Sign up free to have it checked
Exercises 33
Correct the following statement: “The bonds in solid PbCl₂ are ionic; the bond in a HCl molecule is covalent. Thus, all of the valence electrons in PbCl₂ are located on the Cl⁻ ions, and all of the valence electrons in a HCl molecule are shared between the H and Cl atoms.”
Sign up free to have it checked
Exercises 35
Methanol, H₃COH, is used as the fuel in some race cars. Ethanol, C₂H₅OH, is used extensively as motor fuel in Brazil. Both methanol and ethanol produce CO₂ and H₂O when they burn. Write the chemical equations for these combustion reactions using Lewis structures instead of chemical formulas.
Sign up free to have it checked
Exercises 37
Carbon tetrachloride was formerly used in fire extinguishers for electrical fires. It is no longer used for this purpose because of the formation of the toxic gas phosgene, Cl₂CO. Write the Lewis structures for carbon tetrachloride and phosgene.
Sign up free to have it checked
Exercises 39
The arrangement of atoms in several biologically important molecules is given here. Complete the Lewis structures of these molecules by adding multiple bonds and lone pairs. Do not add any more atoms.
(a) the amino acid serine:
(b) urea:
(c) pyruvic acid:
(d) uracil:
(e) carbonic acid:
Sign up free to have it checked
Exercises 41
A compound with a molar mass of about 42 g/mol contains 85.7% carbon and 14.3% hydrogen by mass. Write the Lewis structure for a molecule of the compound.
Sign up free to have it checked
Exercises 43
How are single, double, and triple bonds similar? How do they differ?
Sign up free to have it checked

Key terms · 9

double bond: covalent bond in which two pairs of electrons are shared between two atoms
free radical: molecule that contains an odd number of electrons
hypervalent molecule: molecule containing at least one main group element that has more than eight electrons in its valence shell
Lewis structure: diagram showing lone pairs and bonding pairs of electrons in a molecule or an ion
Lewis symbol: symbol for an element or monatomic ion that uses a dot to represent each valence electron in the element or ion
lone pair: two (a pair of) valence electrons that are not used to form a covalent bond
octet rule: guideline that states main group atoms will form structures in which eight valence electrons interact with each nucleus, counting bonding electrons as interacting with both atoms connected by the bond
single bond: bond in which a single pair of electrons is shared between two atoms
triple bond: bond in which three pairs of electrons are shared between two atoms

Learn 7.3 on the whiteboard

The tutor teaches the whole section, drawing it out step by step and answering your questions as you go.

Read 7.3 in the textbook · This section on openstax.org

Lewis structures: practice problems

11 problems from the book, with its answers. Check your working, or have any of them explained to you on the whiteboard.

How to approach these problems

Exercises 23

Write the Lewis symbols for each of the following ions:

(a) As³⁻

(b) I⁻

(d) O²⁻

(e) Ga³⁺

(f) Li⁺

(g) N³⁻

Exercises 25

Write the Lewis symbols of the ions in each of the following ionic compounds and the Lewis symbols of the atom from which they are formed:

(a) MgS

(b) Al₂O₃

(d) K₂O

(e) Li₃N

(f) KF

Exercises 27

Write the Lewis structure for the diatomic molecule P₂, an unstable form of phosphorus found in high-temperature phosphorus vapor.

Exercises 29

Write Lewis structures for the following:

(a) O₂

(b) H₂CO

(d) ClNO

(e) SiCl₄

(f) H₃O⁺

(g) $NH_{4}^{+}$

(h) $BF_{4}^{-}$

Exercises 31

Write Lewis structures for the following:

(a) SeF₆

(b) XeF₄

(d) Cl₂BBCl₂ (contains a B–B bond)

Exercises 33

Correct the following statement: “The bonds in solid PbCl₂ are ionic; the bond in a HCl molecule is covalent. Thus, all of the valence electrons in PbCl₂ are located on the Cl⁻ ions, and all of the valence electrons in a HCl molecule are shared between the H and Cl atoms.”

Exercises 35

Methanol, H₃COH, is used as the fuel in some race cars. Ethanol, C₂H₅OH, is used extensively as motor fuel in Brazil. Both methanol and ethanol produce CO₂ and H₂O when they burn. Write the chemical equations for these combustion reactions using Lewis structures instead of chemical formulas.

Exercises 37

Carbon tetrachloride was formerly used in fire extinguishers for electrical fires. It is no longer used for this purpose because of the formation of the toxic gas phosgene, Cl₂CO. Write the Lewis structures for carbon tetrachloride and phosgene.

Exercises 39

The arrangement of atoms in several biologically important molecules is given here. Complete the Lewis structures of these molecules by adding multiple bonds and lone pairs. Do not add any more atoms.

(a) the amino acid serine:

A Lewis structure is shown. A nitrogen atom is single bonded to two hydrogen atoms and a carbon atom. The carbon atom is single bonded to a hydrogen atom and two other carbon atoms. One of these carbon atoms is single bonded to two hydrogen atoms and an oxygen atom. The oxygen atom is bonded to a hydrogen atom. The other carbon atom is single bonded to two oxygen atoms, one of which is bonded to a hydrogen atom.

(b) urea:

A Lewis structure is shown. A nitrogen atom is single bonded to two hydrogen atoms and a carbon atom. The carbon atom is single bonded to an oxygen atom and another nitrogen atom. That nitrogen atom is then single bonded to two hydrogen atoms.

A Lewis structure is shown. A carbon atom is single bonded to three hydrogen atoms and another carbon atom. The second carbon atom is single bonded to an oxygen atom and a third carbon atom. This carbon is then single bonded to two oxygen atoms, one of which is single bonded to a hydrogen atom.

(d) uracil:

A Lewis hexagonal ring structure is shown. From the top of the ring (moving clockwise), three carbon atoms, one nitrogen atom, a carbon atom, and a nitrogen atom are single bonded to each another. The top carbon atom is single bonded to an oxygen atom. The second and third carbons and the nitrogen atom are each single bonded to a hydrogen atom. The next carbon atom is single bonded to an oxygen atom, and the last nitrogen atom is single bonded to a hydrogen atom.

(e) carbonic acid:

A Lewis structure is shown. A carbon atom is single bonded to three oxygen atoms. Two of those oxygen atoms are each single bonded to a hydrogen atom.

Exercises 41

A compound with a molar mass of about 42 g/mol contains 85.7% carbon and 14.3% hydrogen by mass. Write the Lewis structure for a molecule of the compound.

Exercises 43

How are single, double, and triple bonds similar? How do they differ?

Key terms · 9

double bond: covalent bond in which two pairs of electrons are shared between two atoms
free radical: molecule that contains an odd number of electrons
hypervalent molecule: molecule containing at least one main group element that has more than eight electrons in its valence shell
Lewis structure: diagram showing lone pairs and bonding pairs of electrons in a molecule or an ion
Lewis symbol: symbol for an element or monatomic ion that uses a dot to represent each valence electron in the element or ion
lone pair: two (a pair of) valence electrons that are not used to form a covalent bond
octet rule: guideline that states main group atoms will form structures in which eight valence electrons interact with each nucleus, counting bonding electrons as interacting with both atoms connected by the bond
single bond: bond in which a single pair of electrons is shared between two atoms
triple bond: bond in which three pairs of electrons are shared between two atoms

Learn 7.3 on the whiteboard

The tutor teaches the whole section, drawing it out step by step and answering your questions as you go.

Read 7.3 in the textbook · This section on openstax.org