Atomic size defined

The size assigned to an atom or ion requires a decision about where an atom stops. From quantum mechanics we learn that an atom has no sharp boundaries or surfaces. Nevertheless, chemists find it convenient to assign sizes to atoms according to the observed distances between atoms. Thus, atomic size is defined operationally—it is determined by measuring the distance between atoms. 

The size of an atom is defined in terms of the interatomic distances that are found in solids and in gaseous molecules containing that atom. For an atom on the left side of the periodic table, gaseous molecules are obtained only at very high temperatures. At normal temperatures, solids are found and there are two important types to consider, metallic solids and ionic solids. Table 21-11 shows the nearest neighbor distances in the... 

The state of an electron in a hydrogen atom is defined by the four quantum numbers n, l, and ms as the value ofn increases, the size of the atom increases. 

The first problem was resolved when it was shown that the Es values for symmetric groups are a linear function of van der Waals radii41. The latter have long been held to be an effective measure of atomic size. The second and third problems were solved by Charton, who proposed the use of the van der Waals radius as a steric parameter42 and developed a method for the calculation of group van der Waals radii for tetracoordinate symmetric top substituents MZ3 such as the methyl and trifluoromethyl groups43. In later work the hydrogen atom was chosen as the reference substituent and the steric parameter v was defined as ... 

Vai der Waals Radii And Related Sterlc Parameters Van der Waals radii have long been considered a valid meas ure of atomic size Taft proposed the first valid set of sterlc parameters for correlation analysis defined from acid hydrolysis of esters caiarton derived equations for the calculation of Van der Waals radii, ry of symmetric top MZs groups( ) These values of the Van der Waals radii were used, together with that for H, to show that Es Is a linear function of i>y ... 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Clearly, Bohr s atom is a long way from Dalton s. No longer is it an indivisible lump it is made from subatomic particles - the electrons and the nucleus - and is mostly just space. The size of the atom is defined not by hard boundaries but by how far the electron orbits reach. 

Isotopes answer a puzzle that troubled chemists ever since Dalton proposed his atomic theory. Dalton said that the key property of an atom is not its size or shape but its weight. Each element is characterized by an atomic weight defined relative to that of hydrogen. The fact that these relative atomic weights were usually... 

The electron cloud around an atomic nucleus makes the concept of atomic size somewhat imprecise, but it is useful to refer to an atomic radius. One can arbitrarily divide the distance between centers of two bonded atoms to arrive at two radii, based on the crude picture that two bonded atoms are spheres in contact. If the bonding is covalent, the radius is called a covalent radius (see Table 8-2) if it is ionic, the radius is an ionic radius (see Table 9-2). The radius for non-bonded atoms may be defined in terms of the distance of closest non-bonding approach such a measure is called the van der Waals radius. These three concepts of size are illustrated in Figure 7-2. 

Apart from detail, reformulation of quantum theory to be consistent with chemical behaviour, requires the recognition of molecular structure. In this spirit, it may be introduced as an essential assumption, or emergent property, without immediate expectation of retrieving the concept from first principles. Medium-sized molecules, especially in condensed phases, are assumed to have a characteristic three-dimensional distribution of atoms, which defines a semi-rigid, flexible molecular frame. The forces between the atoms are of quantum-mechanical origin, but on a macro scale, are best described in terms of classical forces. 

In Investigation 2-A, you will look for a pattern involving the size of atoms. Chemists define, and measure, an atom s size in terms of its radius. The radius of an atom is the distance from its nucleus to the approximate outer boundary of the cloud-like region of its electrons. This boundary is approximate because atoms are not solid spheres. They do not have a fixed outer boundary. 

The observation that different structure maps were required for different electron counts was noted earlier. Thus, the total number of electrons will be important as an index. The concept of atomic size as defined through... 

This rule centers on the much-used concept of ionic size, a concept that has been refined, or rather been better defined in recent years by the use of pseudopotential radii (Zunger and Cohen, 1978). Thus, the use of pseudopotential radii (r,) to define atomic size more closely, and the iden-... 

This is the simplest measure with regard to molecular size, defined as the total number of atoms in a molecule. It is a global, zero-dimensional, highly degenerate descriptor. In several applications for the calculation of - molecular descriptors, the atom number A refers only to non-hydrogen atoms. 

The information index on size is the -> total information content on the atom number, defined as ... 

The electron cloud surrounding a nucleus is based on probability and does not have a clearly defined edge. It is true that the outer limit of an electron cloud is defined as the spherical surface within which there is a 90% probability of finding an electron. However, this surface does not exist in a physical way, as the outer surface of a golf ball does. Atomic size is defined by how closely an atom lies to a neighboring atom. Because the nature of the neighboring atom can vary from one substance to another, the size of the atom itself also tends to vary somewhat from substance to substance. 

Section 5.5, we defined an approximate radius of an atom as the distance at which the electron density had fallen off to a particular value, or as the radius of a sphere containing a certain fraction of the total electron density. A third related measure of atomic size is based on the interatomic separations in a crystal. The radius of a noble-gas or metallic atom can be approximated as half the distance between the center of an atom and the center of its nearest neighbor in the crystal. We picture crystal structures as resulting from packing spheres in which nearest neighbors are in contact. 

This unit is used simply because most atomic sizes and chemical bond lengths fall in the range of one to several angstroms, and the use of either picometers or nanometers is slightly awkward. Next, we use the atmosphere (abbreviated atm) as a unit of pressure. It is not a simple power of 10 times the SI unit of the pascal, but rather is defined as follows ... 

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