3.3 VALENCE CONCEPT – ACTUAL AN PROPOSED
In chemistry, valence is a measure of the number of chemical bonds formed by the atoms of a given element.
In an atomic structure, the electrons situated on the last shell are called valence electrons. In accord with the octet rule, and to become more energetically stable, atoms gain, lose, or share valence electrons in an effort to obtain a noble gas configuration in their outer shell. The configuration of electrons in an atom's outer shell determines its ability and affinity to enter into chemical reactions.
The valence number of an element can be determined by using a few simple rules relating to an element's location on the periodic table. In ionic compounds (formed between charged atoms or groups of atoms called ions) the valence of an atom is the number of electrons that atom will gain or lose to obtain a full outer shell. In group one of the periodic table, elements are assigned a valence number of 1. A valence number of 1 means that a group one element will generally react to lose one electron to obtain a full outer shell. Group two elements are assigned a valence number of 2 that means elements which generally react to lose two electrons to obtain a full outer shell. Group 17 elements are assigned a valence number of negative one (-1). A valence number of 17 means that a group two element will generally react to gain one electron to obtain a noble gas electron configuration. Reflecting an inability to react with other elements, Nobel gases, already maintaining a stable arrangement of electrons, are assigned a valence of zero (0).
In covalent compounds the valence number gives an indication of the number of bonds formed by each atom.
If the situation would be like up presented, a new theory will be useless. According to definition of valence, the capacity of combination of an element must have a single value because for achieving a noble gas structure a fixed number of electrons must be loosed, gained or shared. Experimentally, this fact is respected only for alkaline metals. Other elements can form a variable number of bounds, and the noble gas structure is respected only in some combination of respective elements. As was presented there are combinations with deficit of electrons (like BF3) and compound with excess of electron (PCl5) reported to a noble gas structure. More strange is that even noble gases react with other elements and form quite stable compounds.
In reality the structure of noble gas is attaint for atoms in particular cases, in special for a series of simple compounds located in principal groups of actual periodic system.
The concept of valence must be modified in order to reflect the tendency of elements to form variable number of bounds. Experimentally is observed that an element can participate at a maxim number of bound equal with the number of electrons from the last shell according to the new configuration proposed. Consequently the valence is an expression of the total number of electron uncoupled magnetically. An atom can have a primary valence due to the arrangement of electron as pairs in atoms in a ground state, and can have another’s secondary valences due to the perturbation of already coupled electrons from the last shell.
For example: chloride in a ground state, present only one electron single and has a primary valence equal to 1. But if in reaction condition another two electrons from the last shell are perturbed the valence of Chloride will be 3. If another two electrons are decoupled they became capable again to form bounds so the valence will be 5. If last two electrons from the last shell are perturbed the valence of chloride will be 7. So the chloride can form seven bounds without any transfer of electrons. But chloride can also gain an electron and in this case a eight bound can be formed. The secondary valences appear in the case of covalent and coordinative bonds due to the reciprocal perturbation and rearrangement of electrons atomics magnetic moments. In the ionic bonding the element participate generally with primary valence at crystal formation.
Sulphur as second example, can make two covalent bonds due to the primary valence, but also can form four or six covalent bonds due to the decoupling of other electrons from last shell.
A more detailed analysis will be made in a further book related to the implication of this theory in chemistry.