Drawing Lewis Structures
Drawing a Lewis structure follows a systematic approach that helps visualize how atoms share electrons in covalent molecules. Start by determining the total valence electrons from all atoms in the molecule. Next, create a skeleton structure with the least electronegative element in the center (carbon is always central, hydrogen is always terminal).
After establishing the basic skeleton, add lone pairs of electrons to each atom until they achieve an octet (8 electrons in their outer shell), except for hydrogen which needs only 2 electrons. Remember that boron is another exception that can function with less than an octet.
When you've placed all electrons, count to verify your total matches the valence electrons calculated initially. If you've used more electrons than available, convert some lone pairs into double or triple bonds. Always double-check your final structure to ensure electron count is correct, all electrons are paired, and octets are fulfilled (except for allowed exceptions).
Pro Tip: When determining the central atom, remember this rule of thumb: "C is central, H is terminal, and the least electronegative/least common element usually goes in the middle."
Examples include N₂H₄ (with single bonds between nitrogen atoms and hydrogen atoms attached to each nitrogen), C₂H₂ (with a triple bond between carbon atoms), and compounds like NH₂OH, BeCl₂, PCl₃, and CS₂ each with their unique electron arrangements.