Atom radius of an element

The atom radius of an element is the shortest distance between like atoms. It is the distance of the centers of the atoms from one another in metallic crystals and for these materials the atom radius is often called the metal radius. Except for the lanthanides (CN = 6), CN = 12 for the elements. The atom radii listed in Table 4.6 are taken mostly from A. Kelly and G. W. Groves, Crystallography and Crystal Defects, Addison-Wesley, Reading, Mass., 1970. 

The atomic radius of an element is considered to be half the interatomic distance between identical (singly bonded) atoms. This may apply to iron, say, in its metallic state, in which case the quantity may be regarded as the metallic radius of the iron atom, or to a molecule such as Cl2. The difference between the two examples is sufficient to demonstrate that some degree of caution is necessary when comparing the atomic radii of different elements. It is best to limit such comparisons to elements with similar types of bonding, metals for example. Even that restriction is subject to the drawback that the metallic elements have at least three different crystalline arrangements with possibly different coordination numbers (the number of nearest neighbours for any one atom). 

Electron clouds do not have sharp boundaries, so we cannot really speak of the radius of an atom. However, when atoms pack together in solids and molecules, their centers are found at definite distances from one another. The atomic radius of an element is defined as half the distance between the nuclei of neighboring atoms (11). If the element is a metal or a noble gas, we use the distance between the centers of neighboring atoms in a solid sample. For instance, because the distance between... 

Describe the three types of cubic unit cells and explain how to find the number of particles in each and how packing of spheres gives rise to each calculate the atomic radius of an element from its density and crystal structure distinguish the types of crystalline solids explain how the electron-sea model and band theory account for the properties of metals and how the size of the energy gap explains the conductivity of substances ( 12.6) (SP 12.4) (EPs 12.57-12.75)... 

The atomic radius of an element is half the distance between the centers of neighboring atoms in a solid (such as Cu) or, for nonmetals, in a homonuclear molecule (such as Hj or Sg). If there is one single attribute of an element that determines its chemical properties (either directly, or indirectly through the variation of other properties), then it is atomic radius. 

The physical properties of the elements, such as melting point, boiling point and density are related to the atomic radius of the elements. Also, the atomic radius directly affects the ability of an atom to gain and lose electrons. The atomic radius is practically defined by assuming the shape of the atom as a sphere. The atomic radius is the distance between the nucleus and the outermost electron. But it is impossible to measure the atomic radius by separating the atoms from each other. Atomic Radius within a Group... 

Atomic radius increases as you move down a group, as Figure 20 shows. As you proceed from one element down to the next in a group, another principal energy level is filled. The addition of another level of electrons increases the size, or atomic radius, of an atom. 

Although these simple considerations help to frame in a general logic the behavior of these bimetallic surface, there are at present no such simple models to explain the more complex mesoscopic reconstructions, such as the pyramids observed on Pt3Sn(100) or the hill and valley structure observed on PtsSnCl 10). These phenomena are obviously related to the tendency of the system to relax in-plane stress, in turn resulting from the different atomic radius of the elements involved in the presence of concentration gradients. This relaxation appears to take place on the (111) oriented plane simply by an outward relaxation of the tin atoms. On the other two low index surfaces, instead, it takes a more complex route leading to reconstruction phenomena (pyramids on the (100) and hill and valley on the (110)) which are so far unique to the Pt-Sn system. 

The atomic radius of an atom is defined as half of the distance between two atoms of the same element held together in a chemical bond. Not surprisingly, these are very small distances For hydrogen, the smallest atom, the atomic radius is 0.37 Angstroms, or 3.7 X 10" meters. 

The second factor controlling carbide formation is the atomic radius of the constituent elements. The radii of elements forming carbides are listed in Table 2.2. A certain caution is in order when considering the radius of an element since the size of an atom is related to a wave function and it follows that no atom has a precise radius. Thus, the values given in Table... 

The covalent radius of an element may be considered to be one half of the covalent bond distance of a molecule such as Cl, (equal to its atomic radius in this case), where the atoms concerned are participating in single bonding. Covalent radii for participation in multiple bonding are also quoted in data books. In the case of a single bond between two different atoms, the bond distance is divided up between the participants by subtracting from it the covalent radius of one of the atoms, whose radius is known. A set of mutually consistent values is now generally accepted and, since the vast majority of the elements take part in some... 

The van der Waals radius of an element is half the distance between two atoms of an element which are as close to each other as is possible without being formally bonded by anything except van der Waals inter-molecular forces. Such a quantity is used for the representation of the size of an atom with no chemical bonding tendencies the Group 18 elements. That for krypton, for instance, is half of the distance between nearest neighbours in the solid crystalline state, and is equal to the atomic radius. Van der Waals radii of atoms and molecules are of importance in discussions of the liquid and solid states of molecular systems, and in the details of some molecular structures where two or more groups attached to the same atom may approach each other. 

The electron configuration or orbital diagram of an atom of an element can be deduced from its position in the periodic table. Beyond that, position in the table can be used to predict (Section 6.8) the relative sizes of atoms and ions (atomic radius, ionic radius) and the relative tendencies of atoms to give up or acquire electrons (ionization energy, electronegativity). 

Atomic radii. The radii are determined by assuming that atoms in closest contact in an element touch one another. The atomic radius is taken to be one half of the closest internuclear distance, (a) Arrangement of copper atoms in metallic copper, giving an atomic radius of 0.128 nm for copper, (b) Chlorine atoms in a chlorine (Cl2) molecule, giving an atomic radius of 0.099 nm for chlorine. 

Steel is an alloy of about 2% or less carbon in iron. Carbon atoms are much smaller than iron atoms, and so they cannot substitute for iron in the crystal lattice. Indeed, they are so small that they can fit into the interstices (the holes) in the iron lattice. The resulting material is called an interstitial alloy (Fig. 5.48). For two elements to form an interstitial alloy, the atomic radius of the solute element must be less than about 60% of the atomic radius of the host metal. The interstitial atoms interfere with electrical conductivity and with the movement of the atoms forming the lattice. This restricted motion makes the alloy harder and stronger than the pure host metal would be. 

The radius of an atom helps to determine how many other atoms can bond to it. The small radii of Period 2 atoms, for instance, are largely responsible for the differences between their properties and those of their congeners. As described in Section 2.10, one reason that small atoms typically have low valences is that so few other atoms can pack around them. Nitrogen, for instance, never forms penta-halides, but phosphorus does. With few exceptions, only Period 2 elements form multiple bonds with themselves or other elements in the same period, because only they are small enough for their p-orbitals to have substantial tt overlap (Fig. 14.6). 

Information about internuclear distances in organic compounds has led to the view that the effective radius of an atom varies directly with bond order. This is understandable for elements like carbon, with a limited range of hybridized states, but less so for metallic (cluster) systems. The problem is threefold ... 

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