Abstract
The recently introduced even-odd rule has been shown to successfully represent chemical structures of ions and molecules. While comparing available drawings in the scientific literature with the list of compounds predicted by the even-odd rule, it became however obvious that existing compounds are fewer than expected. Several predicted compounds involving many covalent bonds have apparently never been experimentally observed. Neutral oxygen for instance is expected to have 6 valence electrons, whereas oxygen can only build a maximum of two bonds, as in water. This specificity is observed for elements in the top-right corner of the periodic table. For compounds to contain only single covalent bonds, and thus follow the even-odd rule, further explanations are necessary. The present paper proposes that those specific elements experience a transfer of electrons from the valence shell into the inner shell, making them unavailable for further bonding. These elements will be described as organic, hereby providing a clear and hopefully unifying definition of the term. In opposition, inorganic elements have a constant inner shell no matter their electrical state or the number of bonds they maintain. More than 70 compounds involving 11 elements of the main group are studied, revealing a progression from fully inorganic elements at the left of the periodic table to fully organic elements. The transition between inorganic or organic elements is made of few elements that take an organic form when negatively charged; they are labelled semi-organic. The article concludes that the fully organic elements of the main group are Oxygen and Fluorine, whereas semi-organic elements are more numerous: C, N, S, Cl, Se, Br and I. Thus, the even-odd rule becomes fully compatible with scientific knowledge of compounds in liquid or gaseous phase.
Highlights
The first rule to describe the organization of electrons in atoms belonging to a compound was the octet rule in the 1920s [1] [2] [3]
Three States for Organic Elements Unlike inorganic elements, the numbers of electrons in the inner shell of organic elements vary depending on their state: - In a neutral state, some electrons pairs have moved into the inner shell and the organic valence number has the expected parity, albeit with a lower number of electrons pairs than calculated from the periodic table. - In a positive state, an electrons pair moves out of the inner shell and the organic valence number counts an extra pair compared to the neutral state. - In negative state, one electrons pair moves into the inner shell and the organic valence number counts one pair less than in the neutral state
The even-odd rule is meant to explain how electrons dispatch into the electrons shells, when an atom is bonded to other atoms in a compound
Summary
The first rule to describe the organization of electrons in atoms belonging to a compound was the octet rule in the 1920s [1] [2] [3] This rule imposes that chemically bonded elements should at all time be surrounded by eight electrons. It perfectly suited molecules like methane, ammonia, water and hydrogen fluoride. Four important constrains should be remembered throughout the following: 1) Studied elements belong to the main group of the periodic table; 2) Chemical compounds are ions and molecules in gaseous or liquid phases; 3) Two neighbor elements in a compound are never connected through more than one single covalent bond [10]; 4) Elements cannot bear more than a single charge [17] [18]. The software used to draw ions and molecules is Chemsketch [19]
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