Question

Draw the Lewis structure for each of the following molecules: a. \(\mathrm{PCl}_{3}\) b. \(\mathrm{CCl}_{2} \mathrm{F}_{2}\) c. \(\mathrm{CH}_{3} \mathrm{NH}_{2}\)

Short answer

Lewis structures: a) \text{PCl}_3 b) \text{CCl}_2\text{F}_2 c) \text{CH}_3\text{NH}_2

Step-by-step solution

- Understanding Valence Electrons

Determine the total number of valence electrons for each molecule by summing the valence electrons of each atom in the molecule.

- Drawing the Skeleton Structure

Draw a skeleton structure where the central atom is usually the least electronegative element. Connect all atoms with single bonds.

- Distributing Remaining Electrons

Distribute the remaining valence electrons around the atoms to satisfy the octet rule (8 electrons per atom), starting with the outer atoms first.

- Checking Octet Rule

Ensure each atom (except for hydrogen and boron) satisfies the octet rule. For elements like hydrogen, ensure it has 2 electrons.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and they play a crucial role in chemical bonding. For molecules, it's essential to determine the total number of valence electrons. To do this look at the periodic table:
- Each group number often represents the number of valence electrons (e.g., Carbon in group 14 has 4 valence electrons).
- Add up these electrons for all atoms in the molecule.

For example:

• In \(\text{PCl}_{3}\), phosphorus (Group 15) has 5 valence electrons and chlorine (Group 17) has 7 valence electrons. Therefore, \(\text{PCl}_{3}\) has \[5 + (3 \times 7) = 26\] valence electrons.


• In \(\text{CCl}_{2}\text{F}_{2}\), carbon (Group 14) has 4 valence electrons while both chlorine and fluorine (Group 17) each have 7 valence electrons. Thus, \(\text{CCl}_{2}\text{F}_{2}\) has \[4 + (2 \times 7) + (2 \times 7) = 32\] valence electrons.

Understanding valence electrons helps in predicting how atoms bond and the overall structure of the molecule.

Octet Rule

The Octet Rule is a simple guideline in chemistry stating that atoms are most stable when they have eight electrons in their valence shell. This applies mainly to the main group elements. There are some key points to remember:
- Hydrogen is an exception; it is stable with 2 electrons.
- Boron often has fewer than 8 electrons.
After drawing the skeleton structure, distribute remaining electrons to satisfy the octet rule for each atom:

• Start with outer atoms, then move towards the central atom.
• If there are leftover electrons after filling the octets of the outer atoms, place them on the central atom.
• Ensure all atoms follow the octet rule except those with common exceptions.

For example, in \(\text{PCl}_{3}\), once you draw the central phosphorus and attach three chlorines with single bonds, you'll distribute the remaining electrons to ensure each chlorine has 8 electrons around it and fill any remaining on phosphorus.

Skeleton Structure

The skeleton structure of a molecule provides a basic layout showing how atoms are connected. The central atom in the skeleton structure is typically the least electronegative, except for hydrogen, which is never central. The steps are straightforward:

• Identify the central atom, usually the least electronegative (e.g., P in \(\text{PCl}_{3}\) and C in \(\text{CCl}_{2}\text{F}_{2}\)).


• Connect all atoms to the central atom using single bonds. For instance, P is connected to three Cl atoms in \(\text{PCl}_{3}\).

The skeleton structure assures that we establish a framework upon which to build the complete Lewis structure by then distributing remaining electrons and checking the octet rule.