Chemical Bonding

Chemical Bonding

Chemical bonding is a force exerted between atoms that is strong enough to combine two or more atoms together as a single unit (molecule). If just two elements combine to form

a molecule, it is known as a binary compound. All compounds consisting of two or more different elements follow the same rules of bonding. These types of chemical reactions are interactions of electrons that “join” the shells surrounding the atoms of different elements. How these electrons of different atoms combine to form molecules is the essence of the sci- ence of chemistry. There are three major energy reactions related to ionic and covalent types of chemical bonding:

1. Ionization energy is the energy required to remove an electron from a neutral atom, changing the atom into a positive ion. The alkali metals on the left side of the periodic table exhibit the least ionization energy because they have only one electron in their outer shell. In general, the ionization energy increases from left to right for the elements on the periodic table. This means that the noble gases with a closed shell exhibit the maximum ionization energy.

2. Electron affinity occurs when a neutral atom attracts an electron to become a negative ion. Electron affinity is related to the energy changes when an electron is added to a neutral atom, changing it into a negative ion.

3. Electronegativity is the ability (in terms of structure and energy level of electrons) of either neutral atoms or molecules to attract bonding electrons to them. In essence, electronega- tivity is a measurement of how effectively an atom within a molecule is able to attract bonding electrons to itself.

Let’s define the difference between an atom and an ion. The atom has a neutral charge because of the equal number of negative electrons and positive protons. Ions that have nega- tive charges are called anions, and positive ions are called cations. The charge on an atom that transforms the atom into an ion results from the gain or loss of one or more electrons. Radicals also act as ions, or rather polyatomic ions, in chemical reactions because of their electrical charge. Some of these types of ions are chemicals known as “free radicals” that are fragmented molecules having one or more unpaired electrons, thus “free” electrons. They are short-lived and highly reactive. In 1815, the French scientist Joseph-Louis Gay- Lussac (1778–1850) experimented with gases used to fly balloons. He discovered one gas

that turned out to be poisonous, which he named “cyanogen” (C 2 N 2 ). He determined that

Atomic Structure | 19 tight bonds between the C and N atoms, and thus the group acted together as single charged

atoms in chemical reactions. This knowledge advanced the field of organic chemistry. These groups or fragments of molecules became known as “organic radicals.” There are three types of free radicals:

1. simple single atoms such as O - , F, Cl;

2. inorganic free radicals, such as OH, SO 4 -- , NH 4 + ; and 3. organic free radicals, such as CH 3 + ,C 6 H 6 ++ .

Generally, free radicals are formed when the stable bonds in molecules are ruptured, resulting in two molecular fragments, each with unpaired electrons. Radicals can be single-charge atoms or just separated parts of a molecule that have one or more “free” electrons. Most free radicals are extremely reactive, but short-lived. These characteristics cause some concern with regard to their negative role in health and aging.

Chemical compounds are composed of two or more atoms formed into molecular arrange- ments wherein the total energy of the new molecule is less than the total energy of the constituent atoms of the molecule. Atoms have the unique capacity to maintain electrons in their outer shells, each having a complete number of the required electrons (2, 8, 18, 32). For instance, if an atom of an element has a single electron in its outermost shell, it will reactively give that electron up or share it with another atom that requires an electron to complete its outer shell. A simple example: The element sodium is a very reactive pure metal with a single electron in its outermost shell that it wants to give up so that it can have a complete shell at the next lower level. It will react with an element, usually a nonmetal such as chlorine that has only seven electrons in its outer shell. Chlorine “wants” to gain one electron to complete its outer shell at eight. Thus, both elements react to complete their outer shells. Sodium gives up an electron, and chlorine accepts it, creating a new molecule of sodium chloride. When this occurs, sodium has a 1 + charge, and chlorine has a 1 - charge, and the joining between the two is known as an ionic bond. In other words, the ionic bond exists where one or more electrons that form a neutral atom are moved to another atom, which in turn results in both a positive and negative ion in a new molecule. In this example, ionic bonds hold the atoms together to form the new molecule of a compound called sodium chloride, or table salt, which has none

of the characteristics of the two original elements: (2Na + Cl 2 = 2Na + Cl).

Another type of bond is the covalent bond, in which one, two, or more pairs of electrons are shared by two or more atoms. Unlike ionic bonds, covalent bonds involve the sharing of electrons by atoms. The simplest example of covalent bonding occurs when two atoms of

hydrogen bond to form a diatomic hydrogen molecule (H• + •H yield H:H, or H 2 ). There are several other specialized types of bonding as well:

1. Polar covalent bonds (in which the electrons spend more time with one atom or another atom in a molecule)