Lewis structure

• The valence shell of an atom is highest principal energy level $n$ of the atom that contains at least one electron in its ground state
• The valence electrons (אלקטרוני ערכיות) of an atom are the electrons residing in its valence shell. The rest of electrons are called the core electrons (or inner-shell electrons).
• number of valence electrons
• valence (or valency) of an atom
• The number of bonds an element can form

Lewis structures

• Lewis structures (or electron dot structures)

• count and add total valence electrons

• central atom:

  • the least electronegative atom is usually central
  • hydrogen is almost never a central atom
  • halogens are usually terminal
  • carbon is very commonly the central atom in organic molecules

• each single (or double or triple) bond corresponds to 2 (or 4 or 6) electrons

• complete octets of terminal atoms distribute remaining electrons as lone pairs on outer atoms until each has an octet (or 2 for H)

• The formal charge ($q^{\ast }$) (of an atom in a molecule) is the hypothetical charge the atom would have if all the atoms in the molecule had the exact same electronegativity

  • $q^{\ast }=V-\left(L+\frac{1}{2}B\right)$
    • $V$ is the number of valence electrons of the neutral atom its ground state
    • $L$ is the number of lone pair electrons
    • $B$ is the total number of bonding electrons shared with other atoms through covalent bonds

Resonance

• resonance (or mesomerism) (מזומריות)
  • resonance form (or resonance structures, contributing structure) (מבנה-תהודה)
  • resonance hybrid (מכלוא-רזוננס) – average of all the individual resonance forms
  • The dominant (or major contributing) resonance structure
    • The dominant resonance structure is the one where formal charges are as close to zero as possible
    • If formal charges cannot be avoided, any negative charges should be placed on the most electronegative atoms
  • aromatic compound (תרכובת ארומטית)
  • aromaticity (ארומטיות)
  • delocalized electrons (אלקטרונים לא-מאותרים)
  • delocalization (אל-איתור)

Electron counting

rule	elements	valence orbitals
duet rule	$\mathrm{H}$ and $\mathrm{He}$	$1s$
octet rule	main-group elements	$ns,np$
18-electron rule	transition metals	$ns,(n-1)d,np$

• octet rule
  • ”atoms usually react in such a way as to obtain a noble gas configuration” (Denniston, 2022)
  • “elements in groups 1A to 7A (1, 2, 13 to 17) react with other elements by forming ionic or covalent bonds to produce a stable electron arrangement, usually eight electrons in the outer shell.” (Timberlake, 2017)
  • “In covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs.” (Jones, 2016)
  • Exceptions:
    • Molecules or polyatomic ions with an odd total number of valence electrons are unable to satisfy the octet rule (e.g. $\mathrm{NO}$)
    • incomplete octet: A valence shell of an atom that has fewer than eight electrons. (e.g. $\mathrm{BF}{}_3$)
    • expanded octet (or expanded valence shell): A valence shell that has more than eight electrons. (e.g. $\mathrm{PF}{}_5$)
      • hypervalent molecule
• duet rule (or duplet rule)
• 18-electron rule

Electron pairing

• unpaired electron (אלקטרון לא-מזווג)
  • “an electron that occupies an orbital of an atom singly, rather than as part of an electron pair” (Wikipedia)
• unshared electron (אלקטרון לא-משותף)
  • valence electron that belongs exclusively to a single atom and is not part of a chemical bond
• shared electron (אלקטרון משותף)
  • an electron that occupies a bonding molecular orbital between two atoms
• electron pair (or Lewis pair)
  • lone pair (or unshared pair, non-bonding pair, or electron pair) (זוג לא-קושר)
    • “Two paired electrons localized in the valence shell on a single atom. Lone pairs should be designated with two dots. The term ‘nonbonding electron pair’ is more appropriate, and is found in many modern text books.” (IUPAC Gold Book)
    • pair of unshared electrons
  • shared pairs (or bonding pairs) (זוג קושר)
    • pair of shared electrons
• radical (or free radical)
  • “an atom, molecule, or ion that has at least one unpaired valence electron” (Wikipedia)
• biradical
  • ”a molecule with two unpaired electrons” (Jones, 2016)

References

• Denniston (2022). General, Organic, and Biochemistry. McGraw-Hill Education.
• Timberlake, Karen C. (2017). Chemistry: An Introduction to General, Organic, and Biological Chemistry. Pearson.
• Jones, Loretta (2016). Chemical Principles. W. H. Freeman.