Lewis Structures

Lewis structures are visual representations of the valence electron arrangement within molecules and polyatomic ions. They help us understand bonding and predict molecular geometry. The fundamental goal is to arrange atoms and valence electrons so that each atom achieves a stable electron configuration, typically an octet (eight valence electrons), resembling a noble gas. The first step in constructing a Lewis structure is to count the total number of valence electrons for all atoms in the molecule or ion. For ions, remember to add electrons for negative charges and subtract for positive charges.

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Next, you need to determine the central atom. This is usually the least electronegative atom (excluding hydrogen, which is always terminal). If there are multiple atoms of the same element, the single atom of that element is often central (e.g., carbon in CH₄). Once the central atom is chosen, draw a skeletal structure by connecting all other atoms (terminal atoms) to the central atom with a single bond. Each single bond represents two shared valence electrons. Subtract these bonding electrons from your total valence electron count.

Now, distribute the remaining valence electrons as lone pairs to satisfy the octet rule for all terminal atoms first. Hydrogen atoms only need two electrons (a duet) and are satisfied with their single bond. After satisfying the terminal atoms, place any leftover electrons as lone pairs on the central atom.

Finally, check if all atoms have an octet (or a duet for hydrogen). If the central atom (or any other atom) does not have a complete octet, you may need to form multiple bonds. Move one or more lone pairs from a terminal atom to form double or triple bonds with the central atom until all atoms achieve an octet. Remember to recalculate formal charges if you are evaluating different resonance structures, though for basic construction, ensuring octets is the primary goal.