: (a) HF (e) SeCl (b) CFCl (f) CH (c) PCl (d) OF (g) NH
The Correct Answer and Explanation
Here are the correct Lewis formulas for the molecules listed. In these diagrams, a line represents a single covalent bond (a pair of shared electrons), and dots represent lone pair electrons.
(a) HF (Hydrogen Fluoride)
H – F̈:
¨
(The Fluorine atom has three lone pairs of electrons.)
(b) CF₃Cl (Chlorotrifluoromethane)
:F̈:
|
:F̈ – C – C̈l:
| ¨
:F̈:
¨
(The central Carbon atom is bonded to three Fluorine atoms and one Chlorine atom. Each halogen atom has three lone pairs.)
(c) PCl₃ (Phosphorus Trichloride)
..
:C̈l – P – C̈l:
¨ | ¨
:C̈l:
¨
(The central Phosphorus atom has one lone pair, and each Chlorine atom has three lone pairs.)
(d) OF₂ (Oxygen Difluoride)
..
:F̈ – O – F̈:
¨ ¨
(The central Oxygen atom has two lone pairs, and each Fluorine atom has three lone pairs.)
(e) SeCl₂ (Selenium Dichloride)
..
:C̈l – Se – C̈l:
¨ ¨
(The central Selenium atom has two lone pairs, and each Chlorine atom has three lone pairs.)
(f) C₂H₆ (Ethane)
H H
| |
H – C – C – H
| |
H H
(There are no lone pairs in this molecule. Each carbon has four bonds, and each hydrogen has one bond.)
(g) N₂H₄ (Hydrazine)
H H
| |
.. H – N – N – H ..
(Each Nitrogen atom has one lone pair of electrons.)
A Lewis formula, also known as a Lewis structure, is a two-dimensional diagram that illustrates the covalent bonding between atoms in a molecule, as well as any lone pairs of electrons. The process for creating a Lewis formula follows several key steps. The first step is to sum the valence electrons of all atoms present in the chemical formula. This total represents all electrons available for bonding and for non-bonding lone pairs.
Next, you determine the arrangement of the atoms. The least electronegative atom typically serves as the central atom, with the other atoms, called terminal atoms, positioned around it. Hydrogen is always a terminal atom. A skeletal structure is then drawn, connecting the terminal atoms to the central atom with single bonds, where each bond represents two shared electrons. The number of electrons used in these bonds is subtracted from the total valence electron count. The remaining electrons are then distributed as lone pairs, first to the terminal atoms to satisfy the octet rule (eight valence electrons), and then any leftover electrons are placed on the central atom.
If the central atom still lacks a full octet after all electrons are distributed, lone pairs from terminal atoms are moved to form double or triple bonds with the central atom until its octet is complete. For example, in PCl₃, phosphorus is central and has 26 total valence electrons. After forming three P-Cl single bonds (6 electrons), the remaining 20 electrons are placed as three lone pairs on each chlorine (18 electrons) and one lone pair on the central phosphorus atom (2 electrons). This completes the octets for all atoms. For C₂H₆, the 14 valence electrons are all used in the seven single bonds of the skeleton, leaving no lone pairs and satisfying the octet for each carbon.
