Monatomic ions are formed by the gain or loss of electrons to the valence shell (the outer-most electron shell) in an atom . The inner shells of an atom are filled with electrons that are tightly bound to the positively charged atomic nucleus, and so do not participate in this kind of chemical interaction. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.
Atoms can be ionized by bombardment with radiation, but the more usual process of ionization encountered in chemistry is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell") electronic configurations. Atoms will gain or lose electrons depending on which action takes the least energy.
For example, a sodium atom, Na, has a single electron in its valence shell, surrounding two stable, filled inner shells of 2 and 8 electrons. Since these filled shells are very stable, a sodium atom tends to lose its extra electron and attain this stable configuration, becoming a sodium cation in the process
Na
⟶
Na
+
+
e
−
{\displaystyle {\ce {Na -> Na+ + e-}}}
On the other hand, a chlorine atom, Cl, has 7 electrons in its valence shell, which is one short of the stable, filled shell with 8 electrons. Thus, a chlorine atom tends to gain an extra electron and attain a stable 8-electron configuration, becoming a chloride anion in the process:
Cl
+
e
−
⟶
Cl
−
{\displaystyle {\ce {Cl + e- -> Cl-}}}
This driving force is what causes sodium and chlorine to undergo a chemical reaction, wherein the "extra" electron is transferred from sodium to chlorine, forming sodium cations and chloride anions. Being oppositely charged, these cations and anions form ionic bonds and combine to form sodium chloride, NaCl, more commonly known as table salt.
Na
+
+
Cl
−
⟶
NaCl
{\displaystyle {\ce {Na+ + Cl- -> NaCl}}}
Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as a proton, H+, in neutral molecules. For example, when ammonia, NH3, accepts a proton, H+—a process called protonation—it forms the ammonium ion, NH+4. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration, but ammonium has an extra proton that gives it a net positive charge.
Ammonia can also lose an electron to gain a positive charge, forming the ion NH+3. However, this ion is unstable, because it has an incomplete valence shell around the nitrogen atom, making it a very reactive radical ion.
Due to the instability of radical ions, polyatomic and molecular ions are usually formed by gaining or losing elemental ions such as H+, rather than gaining or losing electrons. This allows the molecule to preserve its stable electronic configuration while acquiring an electrical charge.
Liquids with low dielectric constant (below 10) are not quite suitable for ions formation for several reasons. First of all, electrostatic attraction between cation and anion is much stronger than in water, which requires well developed solvating layer for preventing their immediate reaggregation. However, molecules of nonpolar liquids cannot create such layer due to lack of dipole moments. In addition, many electrolytes are not soluble in nonpolar liquids.
Nevertheless, pioneering works by Onsager, Fuoss, Kraus in 20th century proved that ionization in nonpolar liquids is possible.
Recent series of studies conducted by Dukhin and Parlia with wide variety of liquids and solutes confirmed this conclusion and allowed formulation of the following concept for ionization in non-polar liquids, which is distinctively different from aqueous solutions. The solute substance must be amphiphile consisting of hydrophobic tail and polar head in order to create ions in nonpolar liquid. Existence of hydrophobic tail ensures solubility. Existence of polar head provides source for initial ions creation by dissociation. The most peculiar feature is formation of solvation layer around ions almost immediately after dissociation.
Solvent molecules cannot build up such solvating layers. However, the neutral molecules of the solute do have some dipole moments at their polar heads. These dipoles would be attracted by primary ions right after dissociation. This attraction creates a layer of the neutral solute molecules around central ions, which can be considered as solvation layer. Such solvated ions look like charged inverse micelles. Basically, solute amphiphilic molecules in nonpolar liquids are source of both, dissociation and self-solvation, which distinguishes this ionization from aqueous solutions dramatically.
This concept led to creation of conductivity (non-aqueous) theory that fits experimental data for wide variety of nonpolar systems within up to 7 orders of magnitude.
The energy required to detach an electron in its lowest energy state from an atom or molecule of a gas with less net electric charge is called the ionization potential, or ionization energy. The nth ionization energy of an atom is the energy required to detach its nth electron after the first n − 1 electrons have already been detached.
Each successive ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium has one valence electron in its outermost shell, so in ionized form it is commonly found with one lost electron, as Na+. On the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as Cl−. Caesium has the lowest measured ionization energy of all the elements and helium has the greatest. In general, the ionization energy of metals is much lower than the ionization energy of nonmetals, which is why, in general, metals will lose electrons to form positively charged ions and nonmetals will gain electrons to form negatively charged ions.
