This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: LEWIS DOT DIAGRAMS AND ASSIGNING FORMAL CHARGES Bonds and Lone Pairs: Each covalent bond between atoms usually contains 2 electrons. Between 2 atoms we can have any of the following: a single bond (2 electrons) a double bond (4 electrons i.e., 2 2-electron bonds) a triple bond (6 electrons i.e., 3 2-electron bonds) In addition, we can have electrons on atoms which are not shared in bonds: a lone pair of electrons (2 electrons, one with spin up and one with spin down) a lone electron (a single electron happens less often, usually only when necessary) The Octet Rule: Noble gas configurations are especially stable. Ignoring any d-type electrons, (we will focus on the non-metals, which have either no d-electrons or a completely filled set of d-electrons in either case, the d electrons will usually not participate in bonding.) That means we would expect either 2 electrons, (a 1s 2 He configuration), or 8 electrons, (a ns 2 np 6 Ne, Ar, Kr, Xe, or Rn noble gas configuration), to be especially stable. When a bond is formed, both atoms can benefit by sharing the electrons in the bond. For example, H 2 O would look like this H : O : H Each H atom feels the presence of 2 electrons in its vicinity the 2 it shares with the O atom. Each O atom feels the presence of 8 electrons in its vicinity 2 shared with the left H atom, 2 shared with the right H atom and 4 more in 2 lone pairs which it shares with no other atom. Both have achieved a noble gas configuration without having to form ions to do it. Preferred octets: For elements in groups IVA (or 14 in more modern tables), or higher, an s 2 p 6 configuration is preferred. Thus these elements desire 8 electrons in their vicinity, either shared or in lone pairs. For elements in lower numbered groups, too many electrons would need to be added to reach a total of 8. For these elements, a maximum of twice the number of valence electrons is desired, no more. Table 3-1: # Valence Electrons and Preferred Octet by Group Number Group 1 2 3 (13) 4 (14) 5 (15) 6 (16) 7 (17) 8 (18) # valence e 1 2 3 4 5 6 7 8 octet 2 4 6 8 8 8 8 8 Note, for example, that boron will only choose to have 6 electrons in its vicinity, i.e., twice the number of its valence electrons. Determining the Number of Bonding and Lone Pair Electrons: The following simple formula is generally foolproof. When it fails, it alerts us that one or more atom will end up with more than or less than an octet, thus still providing important information. S = N A where N = total # electrons n eeded by all atoms to have a n oble gas configuration; (2 for each H atom, usually 8 for each other atom, but see preceding table.) A = total # valence electrons a ctually a vailable from the atoms, (including the charge, if any: add 1 for each negative charge on the whole formula, subtract 1 for each positive charge on the whole formula). This is the number of electrons which must be shown in the...
View Full Document