Chemical Bonds: Ionic, Covalent, and Their Properties
IONIZATION ENERGY
The energy involved in the process by which a neutral atom of an element X, in a gaseous state, gives up an electron from its outer level and becomes a monopositive ion X+, also in a gaseous state, is called the ionization energy.
ELECTRON AFFINITY
The energy exchanged in the process by which a neutral atom of an element X, in a gaseous state, receives an electron and becomes a mononegative ion X–, also in the gaseous state, is called electron affinity.
ELECTRONEGATIVITY
The electronegativity of an element is the ability of an atom to attract electrons within a molecule of which it is a part.
Topic 13: Chemical Bonds
The forces holding atoms, ions, or molecules (that form the chemical elements and compounds) stably together are called chemical bonds.
Energy and Stability
Atoms join to form larger groups with greater stability and less energy than the individual atoms separately.
Ionic Bond
The ionic bond is the union resulting from the presence of electrostatic attraction between positive and negative ions, leading to the formation of an ionic crystal lattice.
2.1 Structure of Ionic Compounds
Ionic compounds form crystals, which are solid structures formed by cations and anions. In these crystals, the ions are placed in an orderly manner following the three directions of space.
The specific arrangement of ions for each substance depends mainly on the following factors:
- The charge of cations and anions, since there must be a balance of positive and negative charges.
- The electrostatic interactions. There should be the maximum number of electrostatic attractions and the minimum number of repulsions, because attractive electrostatic forces ensure the stability of the crystal.
- The size of the ions, as they are ordered so that there are fewer empty spaces.
Coordination Number
The number of cations that have contact with an anion, or the number of anions that have contact with a cation in an ionic crystal, is the coordination number of the anion or cation, respectively.
Lattice Energy
Lattice energy is the energy exchanged during the formation of one mole of ionic crystal from the corresponding positive and negative ions in the gaseous state. The higher the lattice energy, the more stable the ionic compound.
Covalent Bonds
Ionic substances are solid at room temperature and consist of elements with very different electronegativities. We can deduce that these characteristics are not shared by ionic substances like ammonia, NH3 (g), or water, H2O (l), or substances consisting of a single element, like oxygen, O2, or hydrogen, H2. All the substances mentioned, and many more, are characterized by being constituted by neutral atoms, usually non-metals, and being joined via covalent bonds.
Lewis Model
Shortly after N. Bohr proposed his atomic model, the American chemist G.N. Lewis established the first theory of the covalent bond. According to this theory, the covalent bond is the union that occurs between two atoms by the sharing of one or more pairs of electrons. This sharing means that the electron or electrons contributed by one atom also become part of the outer shell of the other atom. Thus, each atom acquires the external structure of a noble gas by adding the valence electrons contributed by the other atom(s).
Coordinate Covalent Bond
As we just saw, in a covalent bond each atom provides one, two, or three electrons. However, there are some molecules and ions that do not meet this standard. Although it forms a covalent bond, the hydrogen ion is unable to provide any electrons since it has none. When an electron pair is provided entirely by one of the two bonded atoms, the bond is called a coordinate covalent bond. It is as strong as any other simple covalent bond. This type of linkage is present in a large number of molecules and polyatomic ions. The latter include the oxonium ion and ammonium ion.
3.2 Valence Bond Theory
Based on quantum mechanics and its application to atomic models, several theories of chemical bonding were developed. The simplest is the valence bond theory. According to this theory, to form a covalent bond between two atoms, the following is necessary:
- Each atom must have an atomic orbital occupied by a single electron.
- The two half-filled electron orbitals must have opposite spins, i.e., be antiparallel.
- The covalent bond is formed when the two half-filled orbitals overlap to form a common orbital in which the two electrons are paired. The other orbitals remain unbonded.
- The greater the overlap of half-filled orbitals, the more stable the covalent bond.
Observe the process for the H2 molecule in Fig. Each hydrogen atom brings an electron in an orbital. Both electrons have antiparallel spins, which are indicated by arrows. Finally, the electron density is higher between the nuclei. This stabilizes the molecule by decreasing its energy.
In the following examples, we show the formation of covalent bonds in different molecules. These bonds can be represented by a rectangle enclosing the two half-filled orbitals that will form the common orbital in which the two electrons are paired.
Multiple Bonds
Double or triple covalent bonds are formed when two atoms share two or three pairs of electrons, respectively. This involves the superposition of as many pairs of half-filled atomic orbitals, as shown in the following examples.
Coordinate Covalent Bond in Valence Bond Theory
In the same way as Lewis’s theory, a coordinate covalent bond is a covalent bond in which one atom provides both electrons.
In this theory, it is considered that the coordinate covalent bond forms when one atom provides an unoccupied valence orbital, while another atom contributes a valence orbital occupied by two electrons.