Chemical Bonding Fundamentals: Electron Shells, Valence Electrons, and Ion Formation

Periods and Electron Shells

• The period number on the periodic table directly corresponds to the shell where the outermost electrons are located.
• First Period: Corresponds to the first shell.
  • This shell holds up to $2$ electrons.
  • Examples: Hydrogen ($1$ electron), Helium ($2$ electrons).
• Second Period: Corresponds to the second shell.
  • This shell holds up to $8$ electrons, accounting for $8$ elements (e.g., Carbon).
• Third Period: Corresponds to the third shell.
  • Holds up to $8$ or $18$ electrons (historically related to elements in period $3$).
• Subsequent shells hold $18, 32$, and $32$ electrons respectively.
• Shells vs. Orbitals: Shells are comprised of multiple orbitals, allowing them to hold greater numbers of electrons (e.g., $2, 8, 8, 18, 32, 32$ electrons across different shells).

Valence Shell and Valence Electrons

• The valence shell is the outermost electron shell of an atom.
• Valence electrons are the electrons located in the valence shell that are free to move between atoms or be shared in chemical bonding.
• These electrons are critically important for an atom's chemical behavior.
• For the first three periods, the outer shell is the valence shell (with the exception of some later elements).
  • Period $1$ valence shell holds only $2$ electrons.
  • Other elements (excluding noble gases) ideally aim for $8$ electrons in their valence shell for stability.

Noble Gases and Chemical Stability

• Noble gases (Group $18$) are exceptional because they already possess $8$ electrons in their valence shell (except for Helium, which has $2$).
• Key Chemical Property: Noble gases are chemically inert; they do not readily undergo chemical reactions.
• This is due to their stable electron configuration – they have a