Chemistry > Chemical Bonding and Molecular Structure > 2.0 Lattice Energy
Chemical Bonding and Molecular Structure
1.0 Ionic Bond or Electrovalent Bond
2.0 Lattice Energy
3.0 Characteristics of Electrovalent Compounds
4.0 Covalent Bond (By Mutual Sharing of Electrons)
5.0 Characteristics of Covalent Compounds
6.0 Fajan’s Rule
7.0 Hydrogen Bonding
8.0 Coordinate Bond
9.0 Valence Shell Electron Pair Repulsion (VSEPR) Theory
10.0 Valence Bond Theory
11.0 Sigma and Pi Bonds ($\sigma $ and $\pi $ Bonds)
12.0 Hybridisation
12.1 Types of hybridization and spatial orientation of hybrid orbitals
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
13.0 Molecular Orbital Theory
2.1 Formation of Ions with Higher Charges
12.2 Method of predicting the Hybrid state of the central atom in covalent molecules of polyatomic ions
Formation of a cation with unit positive charge is easy if the first ionization energy is low as in the case of alkali metals. Alkaline earth metals ionizes in two successive steps. $$\begin{equation} \begin{aligned} Mg \to M{g^ + } + {e^ - } \\ M{g^ + } \to M{g^{2 + }} + {e^ - } \\\end{aligned} \end{equation} $$But energy needed to ionize alkaline earth metals are higher than alkali metals.
However, bipositive ions like $Mg^{2+}$, $Ca^{2+}$, $Sr^{2+}$ and $Ba^{2+}$ are quite common. Formation of a tripositive ion like $Al^{3+}$ requires much more energy ($= 5138\ kJ$) which is not available ordinarily. Successive ionization energies of aluminium are: $$\begin{equation} \begin{aligned} Al\mathop \to \limits^{{E_1}} A{l^ + } + {e^ - }\quad {E_1} = 577kJ \\ A{l^ + }\mathop \to \limits^{{E_2}} A{l^{2 + }} + {e^ - }\quad {E_2} = 1816kJ \\ A{l^{2 + }}\mathop \to \limits^{{E_3}} A{l^{3 + }} + {e^ - }\quad {E_3} = 2745kJ \\\end{aligned} \end{equation} $$
It is on this account that most of aluminium compounds are covalent. In solution, however, aluminium is known to give hydrated ions ${\left[ {Al.6{H_2}O} \right]^{3 + }}$. This is possible because of the high heat of hydration of $Al^{3+}$. The energy liberated during hydration of ions is sufficient for ionization.
Similarly, anions with unit negative charge (e.g. $Cl^{–1}$, $Br^{–}$, $I^{–}$) are very common. This is because the electron affinity of these atoms is positive and quite high.
Formation of anions carrying two units of negative charge (e.g. $S^{2–}$, $O^{2–}$) is not so easy as their electron affinities are negative i.e., energy is needed to add second electron. Formation of anions carrying three units of negative charge (e.g. $N^{3–}$, $P^{3–}$) is almost rare.
