9.2.2 Predicting Lewis Dot Structures

Question 1

Predicting Lewis Dot Structures

Answer 1

• Lewis dot structures of elements in the same column (group) of the periodic table are the same.
• Some stable molecules or ions have an odd number of valence electrons.
• Elements in the third row (period) of the periodic table and beyond form molecules using expanded octets.
• A triple bond is shorter and stronger than a double bond, which is shorter and stronger than a single bond.

Question 2

note

Answer 2

• Lewis dot structures of elements in the same column (group) of the periodic table are the same.
• In the example, carbon bonds with four fluorine atoms in the same way that it bonds with four chlorine atoms. Fluorine and chlorine are in the same column of the periodic table, and both have a single unpaired electron.
• In the same way, sulfur and oxygen can be expected to
  combine with other elements similarly, as will nitrogen and phosphorus.
• Some stable molecules or ions have an odd number of valence electrons. These molecules are exceptions to the octet rule.
• Examples of such molecules are nitric oxide, nitrogen dioxide and the superoxide ion. In each of these there exists an atom with an unfilled octet.
• Molecules with an unpaired electron are also called radicals or free radicals. Radicals are very reactive and can be damaging to living systems.
• Elements in the third row (period) of the periodic table and beyond form molecules using expanded octets.
• These elements demonstrate exceptions to the octet rule. Examples of these are phosphorus and sulfur, which are able to form compounds such that they contain 10 and 12 valence electrons, respectively.
• Covalent bonds can be represented using a line between atoms that signifies the presence of a shared pair of electrons. Double and triple bonds are represented with two and three lines joining the bonded atoms, respectively.
• A triple bond is shorter and stronger than a double bond, which is shorter and stronger than a single bond.

Question 3

Which of the following molecules has an odd number of valence electrons?

Answer 3

NO2

Question 4

Which molecule has the strongest and shortest carbon-carbon bond?

Answer 4

The carbon-carbon bond in this molecule, ethyne (acetylene), is a triple bond, which is the strongest carbon-carbon bond.

Question 5

Which of the following is the correctly drawn Lewis dot structure of BCl3?

(Note that boron, like some other elements of Groups IA and IIIA on the periodic table, only provides 3 valence electrons, and thus forms an incomplete octet of 6 electrons when fully bonded.)

Answer 5

The boron atom provides 3 valence electrons, and each chlorine atom provides 7 valence electrons, for a total of 24 valence electrons. The boron atom has a single bond to each chlorine atom.

Question 6

Which Lewis dot structure of the phosphorous anion, [PO3 ]3−, is correct?

Answer 6

The phosphorus atom provides 5 valence electrons, each oxygen atom provides 6 valence electrons, and the 3− charge adds 3 electrons, for a total of 26 valence electrons. In this Lewis dot structure, the phosphorus atom has an expanded set of 10 valence electrons.

Question 7

Which Lewis dot structure of the phosphorous anion, [PO3 ]3−, can be drawn in which the phosphorus atom has an octet set of 8 valence electrons?

Answer 7

The phosphorus atom provides 5 valence electrons, each oxygen atom provides 6 valence electrons, and the 3− charge adds 3 electrons, for a total of 26 valence electrons. In this Lewis dot structure, the phosphorus atom has an octet set of 8 valence electrons.

Question 8

The beryllium (Be) atom has 2 valence electrons, and when beryllium is the central atom of a molecule such as beryllium dichloride, BeCl2, it has only 4 valence electrons instead of 8 according to the octet rule. Which of the following is the correctly drawn Lewis dot structure of BeCl2?

Answer 8

The beryllium atom provides 2 valence electrons, and each chlorine atom provides 7 valence electrons, for a total of 16 valence electrons. The beryllium atom has a single bond to each chlorine atom.

Question 9

Which of the following is the correctly drawn Lewis dot structure of the phosphate anion, [PO4 ]3−? In this molecule, the phosphorus atom has an expanded octet of 10 valence electrons.

Answer 9

The phosphorus atom provides 5 valence electrons, each oxygen atom provides 6 valence electrons, and the 3− charge adds 3 electrons, for a total of 32 valence electrons. The phosphorus atom has an expanded set octet of 10 valence electrons.

Question 10

Which of the following is the correctly drawn Lewis dot structure of sulfuric acid, H2SO4? In this molecule, the sulfur atom has an expanded octet of 12 valence shell electrons.

Answer 10

The sulfur atom provides 6 valence electrons, each oxygen atom provides 6 valence electrons, and each hydrogen atom provides 1 electron, for a total of 32 valence electrons. In this Lewis Dot structure, the sulfur atom has an expanded octet of 12 valence electrons.

Question 11

Which is the lowest numbered period in the periodic table in which atoms can have expanded octets?

Answer 11

3

Question 12

Which of the following molecules has an even number of valence electrons?

Answer 12

O2