Formation of ions Knowledge-Based Study

Study notes

1. The Drive for Stable Electron Configurations

   In their ground state, neutral atoms contain an equal number of positively charged protons in the nucleus and negatively charged electrons revolving around it. Because the charges balance, the overall net charge of a neutral atom is zero.

   However, most isolated neutral atoms do not have a fully filled valence shell. Noble gases (Group 18) possess high chemical stability due to their complete valence electron shells—having either two valence electrons in the case of helium (duplet rule) or eight valence electrons (octet rule) for other noble gases. Other main-group elements tend to undergo chemical changes to achieve a stable, noble-gas-like electron configuration. One of the primary pathways to achieve this is the complete transfer of valence electrons, resulting in the formation of ions.

   Loss of Electrons and Cation Formation

   A cation is a positively charged ion formed when a neutral atom loses one or more of its valence electrons. This process predominantly occurs in metallic elements because metals have relatively low ionization energies and low electronegativities.

   When a neutral atom loses electrons, the number of protons in its nucleus remains unchanged, while the total number of electrons decreases. Consequently, the positive nuclear charge exceeds the negative electronic charge, creating a net positive charge:

   $$\text{Charge of ion} = (\text{number of protons}) - (\text{number of electrons})$$

   Example: Sodium ($\text{Na}$) to Sodium Ion ($\text{Na}^+$)

   • A neutral sodium atom has an atomic number of 11: $11\text{ protons}$ and $11\text{ electrons}$. Its electron configuration is $2, 8, 1$.
   • To achieve an octet configuration identical to neon ($2, 8$), sodium loses its single valence electron:

   $$\text{Na} ightarrow \text{Na}^+ + \text{e}^-$$

   The resulting sodium cation ($\text{Na}^+$) retains $11\text{ protons}$ but has only $10\text{ electrons}$, yielding an overall charge of $+1$. Similarly, magnesium (Group 2) loses two electrons to form $\text{Mg}^{2+}$, and aluminum (Group 13) loses three electrons to form $\text{Al}^{3+}$.

   Gain of Electrons and Anion Formation

   An anion is a negatively charged ion formed when a neutral atom gains one or more electrons into its valence shell. Non-metallic elements typically form anions due to their high electronegativity and high electron affinity.

   When a neutral non-metal atom acquires electrons, the number of negatively charged electrons exceeds the number of protons in the nucleus, resulting in a net negative charge.

   Example: Chlorine ($\text{Cl}$) to Chloride Ion ($\text{Cl}^-$)

   • A neutral chlorine atom has $17\text{ protons}$ and $17\text{ electrons}$. Its electron configuration is $2, 8, 7$.
   • To complete its valence shell and attain the noble gas configuration of argon ($2, 8, 8$), chlorine gains one electron:

   $$\text{Cl} + \text{e}^- ightarrow \text{Cl}^-$$

   The resulting chloride anion ($\text{Cl}^-$) possesses $17\text{ protons}$ and $18\text{ electrons}$, giving it a net charge of $-1$. Similarly, oxygen (Group 16) gains two electrons to form the oxide ion ($\text{O}^{2-}$), and nitrogen (Group 15) gains three electrons to form the nitride ion ($\text{N}^{3-}$). Notice that monatomic anions systematically take the suffix -ide.

   Periodic Trends in Ion Formation

   For main-group elements, the position of an element in the periodic table allows straightforward prediction of the type and magnitude of the charge on its common ion:

   • Group 1 (Alkali Metals): 1 valence electron → lose $1\text{ e}^-$ → form $+1$ cations (e.g., $\text{Li}^+$, $\text{Na}^+$, $\text{K}^+$).
   • Group 2 (Alkaline Earth Metals): 2 valence electrons → lose $2\text{ e}^-$ → form $+2$ cations (e.g., $\text{Mg}^{2+}$, $\text{Ca}^{2+}$).
   • Group 13 Metals: 3 valence electrons → lose $3\text{ e}^-$ → form $+3$ cations (e.g., $\text{Al}^{3+}$).
   • Group 15 Non-metals: 5 valence electrons → gain $3\text{ e}^-$ → form $-3$ anions (e.g., $\text{N}^{3-}$, $\text{P}^{3-}$).
   • Group 16 Non-metals (Chalcogens): 6 valence electrons → gain $2\text{ e}^-$ → form $-2$ anions (e.g., $\text{O}^{2-}$, $\text{S}^{2-}$).
   • Group 17 Non-metals (Halogens): 7 valence electrons → gain $1\text{ e}^-$ → form $-1$ anions (e.g., $\text{F}^-$, $\text{Cl}^-$, $\text{Br}^-$).

   Key Takeaway

   Ion formation is fundamentally an electron-transfer phenomenon. The nucleus remains entirely intact during chemical ionization; only valence electrons are lost or gained. Once formed, oppositely charged cations and anions attract each other via strong electrostatic forces, forming the basis of ionic bonds.