Electric charge, atomic structure

This description would assign to the caesium atom in the caesium fluoride crystal a resultant charge + and to the fluorine atom a charge — It has seemed to me likely that in general all of the atoms in the complexes that constitute stable chemical substances have resultant electrical charges smaller than those shown by these most electropositive and electronegative atoms in their compounds with one another, and I have accordingly formulated the postulate of the essential electrical neutrality of atoms namely, that the electronic structure of substances is... 

The three structures of type 4 are unstable for two reasons the presence of electric charges of the same sign on adjacent atoms, and the use by the nitrogen atom of only three orbitals. The contribution of these structures to the normal state of the molecule is accordingly small, and we may take it to be zero. Also, structures of type 3, with a double bond and the transfer of the positive charge to a fluorine atom, are stabilized by the formation of an additional covalent bond with use of the fourth orbital and may accordingly make a greater contribution to the normal state moreover, there is an extra factor 2 for the six structures of type 3 over the three of type 1. 

The investigation of methyl azide, methyl nitrate, and fluorine nitrate by electron diffraction is shown to lead to configurations of the molecules corresponding in each case to resonance between two important valence-bond structures. The unimportance of a third otherwise reasonable structure for these molecules as well as for nitrous oxide is ascribed to instability due to the presence of electric charges of the same sign on adjacent atoms. It is shown that the differ-... 

It was apparent to Thomson that if atoms in the metal electrode contained negative particles (electrons), they must also contain positive charges because atoms are electrically neutral. Thomson proposed a model for the atom in which positive and negative particles were embedded in some sort of matrix. The model became known as the plum pudding model because it resembled plums embedded in a pudding. Somehow, an equal number of positive and negative particles were held in this material. Of course we now know that this is an incorrect view of the atom, but the model did account for several features of atomic structure. 

In the early part of the twentieth century, then, a simple model of atomic structure became accepted, now known as the Rutherford nuclear model of the atom, or, subsequently, the Bohr-Rutherford model. This supposed that most of the mass of the atom is concentrated in the nucleus, which consists of protons (positively charged particles) and neutrons (electrically neutral particles, of approximately the same mass). The number of protons in the nucleus is called the atomic number, which essentially defines the nature of... 

Only a few relevant points about the atomic structures are summarized in the following. Table 4.1 collects basic data about the fundamental physical constants of the atomic constituents. Neutrons (Jn) and protons (ip), tightly bound in the nucleus, have nearly equal masses. The number of protons, that is the atomic number (Z), defines the electric charge of the nucleus. The number of neutrons (N), together with that of protons (A = N + Z) represents the atomic mass number of the species (of the nuclide). An element consists of all the atoms having the same value of Z, that is, the same position in the Periodic Table (Moseley 1913). The different isotopes of an element have the same value of Z but differ in the number of neutrons in their nuclei and therefore in their atomic masses. In a neutral atom the electronic envelope contains Z electrons. The charge of an electron (e ) is equal in size but of opposite sign to that of a proton (the mass ratio, mfmp) is about 1/1836.1527). 

As mentioned in the previous section titled A Short History of Chemistry, many scientists identified elements, determined their characteristics, similarities, and differences, and designed symbols for them. Using unique experiments, scientists devised ways to define the structure of atoms and determine atomic weights, sizes, and electrical charges as well as energy levels for atoms. 

Over the centuries, many other concepts were proposed to explain the nature of matter— many of them extensions of the Greek concept of an ultimately indivisible and indestructible elementary bit of matter. But it was not until J. J. Thomson proposed his model of the atom, consisting of a sphere with an agglomeration of particles with negative electric charges somehow positioned randomly inside a very small ball of matter, that the modern structure of the atom began to take shape. 

Structural features such as electrical charge, hydroxyl groups in the side-chains and amino atoms of the amide groups reduce the lipophilicity (increase the hydrophilicity) whereas additional methyl groups sitting on amino moieties or free positions on the benzene ring increase the fipophilicity. Biliary contrast agents with a free position have > 5. 

All 20 of the common amino acids are a-amino acids. They have a carboxyl group and an amino group bonded to the same carbon atom (the a carbon) (Fig. 3-2). They differ from each other in their side chains, or R groups, which vary in structure, size, and electric charge, and which influence the solubility of the amino acids in water. In addition to these 20 amino acids there are many less common ones. Some are residues modified after a protein has been synthesized others are amino acids present in living organisms but not as constituents of proteins. The common amino acids of proteins have been assigned three-letter abbreviations and one-letter... 

---