Most Lewis dot structures are guided by the Octet Rule, which states that bonded atoms must have eight valence electrons. However, there are exceptions to the octet rule. Some atoms, such as hydrogen, helium, and boron, have less than an octet. Many atoms, such as those past sulfur on the periodic table, can have an expanded octet, meaning more than eight valence electrons. Watch this video, which outlines these cases.
Source: Khan Academy, https://youtu.be/Tkb1OemB2KI
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