Which three of the following molecules have one or more lone pairs on the central atom?
[tex]$H _2 O$[/tex]
[tex]$CH _2 F_2$[/tex]
HCN
[tex]$NBr _3$[/tex]
[tex]$SeO _2$[/tex]
[tex]$SiH _4$[/tex]
Answer (1)
Analyze each molecule's Lewis structure to determine the number of valence electrons and bonding electrons of the central atom.
Calculate the number of lone pairs on the central atom using the formula: Lone pairs = (Valence electrons - Bonding electrons) / 2.
Identify the molecules with one or more lone pairs on the central atom.
The molecules with one or more lone pairs on the central atom are H 2 O , NB r 3 , and S e O 2 .
Explanation
Problem Analysis We need to identify three molecules from the list that have one or more lone pairs on the central atom. To do this, we will analyze the Lewis structure of each molecule and count the lone pairs on the central atom.
H 2 O Analysis For H 2 O , Oxygen is the central atom. Oxygen has 6 valence electrons. It forms 2 bonds with Hydrogen. The number of lone pairs is calculated as (6-2)/2 = 2.
C H 2 F 2 Analysis For C H 2 F 2 , Carbon is the central atom. Carbon has 4 valence electrons. It forms 4 bonds (2 with H and 2 with F). The number of lone pairs is calculated as (4-4)/2 = 0.
H CN Analysis For H CN , Carbon is the central atom. Carbon has 4 valence electrons. It forms 4 bonds (1 with H and 3 with N). The number of lone pairs is calculated as (4-4)/2 = 0.
NB r 3 Analysis For NB r 3 , Nitrogen is the central atom. Nitrogen has 5 valence electrons. It forms 3 bonds with Bromine. The number of lone pairs is calculated as (5-3)/2 = 1.
S e O 2 Analysis For S e O 2 , Selenium is the central atom. Selenium has 6 valence electrons. It forms 4 bonds with Oxygen (2 double bonds). The number of lone pairs is calculated as (6-4)/2 = 1.
S i H 4 Analysis For S i H 4 , Silicon is the central atom. Silicon has 4 valence electrons. It forms 4 bonds with Hydrogen. The number of lone pairs is calculated as (4-4)/2 = 0.
Conclusion From the analysis, the molecules with one or more lone pairs on the central atom are H 2 O , NB r 3 , and S e O 2 .
Examples
Understanding lone pairs is crucial in predicting molecular geometry and reactivity. For instance, the presence of lone pairs on the oxygen atom in water ( H 2 O ) influences its bent shape and polarity, making it an excellent solvent. Similarly, in ammonia ( N H 3 ), the lone pair on nitrogen affects its ability to act as a base. These concepts are fundamental in fields like drug design, where molecular interactions determine a drug's efficacy.
