Atoms combine with each other to form compounds with different properties from the atoms they contain. The force of attraction between atoms in a compound is a chemical bond.
One type of chemical bond is the ionic bond. This corresponds to a force of attraction between species (ions) with opposite charges. Ions with a positive charge are known as cations; ions with a negative charge are anions. Whether an element is the source of the cation or the anion in an ionic bond depends on several factors, for which the periodic table can serve as a guide.
When forming ionic compounds, elements to the left of the periodic table usually lose electrons, and form a cation that has the same electronic configuration as the nearest noble gas. The loss of an electron from sodium, for example, produces Na+, which has the same electronic configuration as neon.
To subtract an electron from any atom, a large amount of energy, called ionization energy, must be added. The ionization energy of sodium, for example, is 496 kJ/mol (119 kcal/mol).
Energy-absorbing processes are said to be endothermic. Compared to other elements, sodium and the group 1A elements have relatively low ionization energies. In general, the ionization energy increases along a row in the periodic table.
Elements to the right of the periodic table tend to gain electrons to reach the electron configuration of the next higher noble gas. Adding an electron to chlorine, for example, forms the Cl- anion, which has the same full-layer electronic configuration as the noble gas argon.
When a chlorine atom captures an electron, energy is released. Reactions that release energy are described as exothermic, and the energy change for an exothermic process has a negative sign. The energy change for the addition of an electron to an atom is known as its electron affinity and is 349 kJ/mol (83.4 kcal/mol) for chlorine.
The transfer of an electron from a sodium atom to a chlorine atom produces a sodium cation and a chlorine anion, both of which have a noble gas electronic configuration:
If only the ionization energy of sodium (496 kJ/mol) and the electronic affinity of chlorine (349 kJ/mol) are added together, one would conclude that the total process is endothermic by 147 kJ/mol.
The energy released by adding an electron to chlorine is insufficient to satisfy the energy required to remove an electron from sodium. This analysis, however, does not consider the attractive force between the oppositely charged Na+ and Cl- ions, which exceeds 500 kJ/mol and is more than sufficient to make the entire process exothermic. The attractive forces between oppositely charged particles are called electrostatic, or Coulombic, attractions, and is the meaning of an ionic bond between two atoms.
Ionic bonds are very common in inorganic compounds, but rare in organic compounds. The ionization energy of carbon is too large and the electronic affinity too small for carbon to form an anion or cation.
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