Lewis Structures: Mastery Test

In a molecule of methane, carbon (having four valence electrons) binds to four hydrogen atoms. How many lone pairs of electrons will be represented in the Lewis structure of methane?

• four

• two

• one

• zero

Answer explanation

In methane (CH4), carbon shares its four valence electrons with four hydrogen atoms, forming four bonds. Since all electrons are involved in bonding, there are zero lone pairs in the Lewis structure.

What is the first step in creating a Lewis structure for a molecule?

• counting the number of lone pairs of electrons

• counting the number of electron orbitals

• counting the number of single bonds

• counting the number of valence electrons

Answer explanation

The first step in creating a Lewis structure is counting the number of valence electrons. This determines how many electrons are available for bonding and lone pairs in the molecule.

Select the correct answer. How many lone pairs of electrons are represented in the Lewis structure of a nitrate ion (NO₃⁻)?

• 4

• 8

• 12

• 24

Answer explanation

In the Lewis structure of the nitrate ion (NO₃⁻), there are 4 oxygen atoms, each contributing 2 lone pairs. Thus, 4 oxygen x 2 lone pairs = 8 lone pairs total. Therefore, the correct answer is 8.

Select the correct answer. What is the total number of electrons shown in this Lewis structure of carbon dioxide?

• 8

• 12

• 16

• 24

Answer explanation

In the Lewis structure of carbon dioxide (CO2), each oxygen atom contributes 6 electrons and carbon contributes 4 electrons. Thus, 2(6) + 4 = 16 electrons in total.

Select the correct answer. Which is the correct Lewis structure for fluorine, which is a group 7A element?

• A

• B

• C

• D

Answer explanation

The correct Lewis structure for fluorine (Group 7A) shows 7 valence electrons, represented as 6 dots around the symbol 'F' and one pair of dots indicating a lone pair. Choice A accurately depicts this arrangement.

Select the correct answer. Which is the correct Lewis structure for carbon tetrabromide (CBr₄), in which a central carbon atom is bound to four atoms of the group 7A element bromine?

• A

• B

• C

• D

Answer explanation

The correct Lewis structure for carbon tetrabromide (CBr₄) is choice B, which shows a central carbon atom bonded to four bromine atoms, each with three lone pairs, fulfilling the octet rule for all atoms.

Select the correct answer. Which is the correct Lewis structure for carbon monoxide?

• A

• B

• C

• D

Answer explanation

The correct Lewis structure for carbon monoxide (CO) is choice B, which shows a triple bond between carbon and oxygen, fulfilling the octet rule for both atoms. Choices A, C, and D do not accurately represent the bonding in CO.

Select the correct answer. Which is the correct Lewis structure for carbononitridic chloride (CNCl)?

• A

• B

• C

• D

Answer explanation

The correct Lewis structure for carbononitridic chloride (CNCl) is A. It accurately represents the bonding between carbon, nitrogen, and chlorine, with the appropriate number of valence electrons and formal charges.