Each H atom group 1 has 1 valence electron, and the O atom group 16 has 6 valence electrons, for a total of 8 valence electrons. This responsibility can be a major challenge when there is no clear principle involved or where there is a new situation not encountered before. Summary Lewis structures for polyatomic ions follow the same rules as those for other covalent compounds.
Each Cl atom interacts with eight valence electrons: the six in the lone pairs and the two in the single bond. To obtain an octet, these atoms form three covalent bonds, as in NH3 ammonia. This suggests the best Lewis structure has three B—F single bonds and an electron deficient boron.
Chemistry faces the same challenge in extending basic concepts to fit a new situation. A well-known example is BF3: The third violation to the octet rule is found in those compounds with more than eight electrons assigned to their valence shell.
Figure 3. Rearrange the electrons to make multiple bonds with the central atom in order to obtain octets wherever possible. Placing a bonding pair of electrons between each pair of bonded atoms gives the following: Six electrons are used, and 6 are left over. The Octet Rule The other halogen molecules F2, Br2, I2, and At2 form bonds like those in the chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom.
Richard Smalley —a professor of physics, chemistry, and astronomy at Rice University, was one of the leading advocates for fullerene chemistry. If the central atom has fewer electrons than an octet, use lone pairs from terminal atoms to form multiple double or triple bonds to the central atom to achieve an octet.
An exception is that hydrogen is almost never a central atom.