Write Lewis structures that obey the octet rule for each of...

Write Lewis structures that obey the octet rule for each of the following. a. HCN d. $\mathrm{NH}_{4}{ }^{+}$ g. $\mathrm{CO}_{2}$ b. $\mathrm{PH}_{3}$ e. $\mathrm{H}_{2} \mathrm{CO}$ h. $\mathrm{O}_{2}$ c. $\mathrm{CHCl}_{3} \quad$ f. $\mathrm{SeF}_{2}$ i. $\mathrm{HBr}$ Except for $\mathrm{HCN}$ and $\mathrm{H}_{2} \mathrm{CO}$, the first atom listed is the central atom. For $\mathrm{HCN}$ and $\mathrm{H}_{2} \mathrm{CO}$, carbon is the central atom.

Write Lewis structures that obey the octet rule for each of the following.
a. HCN
d. $\mathrm{NH}_{4}{ }^{+}$
g. $\mathrm{CO}_{2}$
b. $\mathrm{PH}_{3}$
e. $\mathrm{H}_{2} \mathrm{CO}$
h. $\mathrm{O}_{2}$
c. $\mathrm{CHCl}_{3} \quad$ f. $\mathrm{SeF}_{2}$
i. $\mathrm{HBr}$
Except for $\mathrm{HCN}$ and $\mathrm{H}_{2} \mathrm{CO}$, the first atom listed is the central atom. For $\mathrm{HCN}$ and $\mathrm{H}_{2} \mathrm{CO}$, carbon is the central atom.

Key Concepts

Central Atom Selection

Determining the central atom in a molecule is essential for correctly arranging the atoms in a Lewis structure. Typically, the least electronegative atom or the atom that can form the most bonds is chosen as the central atom. There are exceptions, such as hydrogen, which is always a terminal atom because it can only form one bond, and in some cases the provided listing indicates a different arrangement.

Formal Charge

Formal charge is a bookkeeping tool used to determine the most stable Lewis structure by assigning a charge to each atom based on the number of electrons in its valence shell as compared to its bonding. The formal charge is calculated using the formula: formal charge = (valence electrons in free atom) – (nonbonding electrons + half the bonding electrons), and structures with the smallest and most evenly distributed formal charges are generally preferred.

Valence Electrons Count

Counting valence electrons is a key step in drawing Lewis structures. It involves summing the number of electrons available from each atom in a molecule, including adjustments for any overall charge. This tally is then allocated among bonding pairs and lone pairs in order to meet the octet rule, ensuring an accurate depiction of the molecule’s electronic structure.

Lewis Structures

Lewis structures are diagrammatic representations of molecules that show how atoms are bonded together using lines for bonds and dots for lone pairs of electrons. They provide a visual method for depicting the arrangement of valence electrons in a molecule, which helps in understanding the bonding, shape, and reactivity of compounds.

Octet Rule

The octet rule is a chemical principle which states that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, mimicking the electron configuration of noble gases. This rule is commonly used to predict the bonding behavior of main-group elements, and it guides the construction of many Lewis structures.

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