Small Size of Atoms

It is difficult to imagine just how small an individual atom is. An especially small unit of mass, the atomic mass unit, u, is used to express the masses of atoms. This unit is a mass equal to exactly 1/12 that of an atom of the carbon-12 isotope. An atomic mass unit is only 1.66 X 10 g. An average atom of hydrogen, the lightest element, has a mass of only 1.0079 u. The average mass of an atom of uranium, the heaviest naturally occurring element, is 238.03 u. To place these values in perspective, consider that a signature written by ballpoint pen on a piece of paper 

Atoms visualized as spheres have diameters of around 1-3 X 10 ° m (100-300 pm). By way of comparison, a small marble has a diameter of around 1 cm (1 X 10 m), which is about 100 000 000 times that of a typical atom. 

The atomic mass of an element is the average mass of all atoms of the element relative to carbon-12 taken as exactly 12. Since atomic masses are relative quantities, they can be expressed without units. Or atomic masses may be given in atomic mass units. For example, an atomic mass of 14.0067 for nitrogen means that the average mass of all nitrogen atoms is 14.0067/12 as great as the mass of the carbon-12 isotope and is also 14.0067 u. 

---

Likewise, when chemists deal with atoms and molecules, they need a convenient unit that takes into consideration the very small size of atoms and molecules. There is such a unit. It s called a mole. 

When people deal with objects, they often think in terms of a convenient amount. For example, when a woman buys earrings, she normally buys a pair of them. When a man goes to the grocery store, he buys eggs by the dozen. Likewise, when chemists deal with atoms and molecules, they need a convenient unit that takes into consideration the very small size of atoms and molecules. They use a unit called the mole. 

Fluorine is the most electronegative element and thus can oxidize many other elements to their highest oxidation state. The small size of the fluorine atom facihtates the arrangement of a large number of fluorines around an atom of another element. These properties of high oxidation potential and small size allow the formation of many simple and complex fluorides in which the other elements are at their highest oxidation states. 

Table 1 fists many metal borides and their observed melting points. Most metals form mote than one boride phase and borides often form a continuous series of solid solutions with one another at elevated temperatures thus close composition control is necessary to achieve particular properties. The relatively small size of boron atoms facilitates diffusion. 

Because of the speeial atomie arrangement of the earbon atoms in a carbon nanotube, substitutional impurities are inhibited by the small size of the carbon atoms. Furthermore, the serew axis disloeation, the most eommon defeet found in bulk graphite, is inhibited by the monolayer strueture of the Cfj() nanotube. For these reasons, we expeet relatively few substitutional or struetural impurities in single-wall earbon nanotubes. Multi-wall carbon nanotubes frequently show bamboo-like defects associated with the termination of inner shells, and pentagon-heptagon (5 - 7) defects are also found frequently [7]. 

The small size of the hydrogen atom allows the unshared pair of an F, O, or N atom of one molecule to approach the H atom in another very closely. It is significant that hydrogen bonding occurs only with these three nonmetals, all of which have small atomic radii. 

There is, therefore, no critical size requirement for growth, which is solely determined by the a b ratio. This treatment indicates that the role of critical size is also probably dependent on shape, though there are clearly problems in providing a meaningful representation of topography in assemblages which contain a small number of atoms. 

The evidence for the existence of the reported third modification of tin is very weak. The hexagonal crystals which are formed by tin in the presence of mercury26 have a simple structure, with atoms at the points of a hexagonal lattice,26 with lattice constants27 o0 = 3.198 A. arid c0 = 2.980 A. for the alloy with 4.9% mercury. Neglecting the effect of the small number of atoms of mercury (which differ little in size from the atoms of tin), we calculate the value f (l) = 1.401 A. from the coordination 2.980 (2), 3.198 (6) and the valence v = 2.44. Hence in this alloy tin has its lower valence. 

The small size of the proton relative to its charge makes the proton very effective in polarizing the molecules in its immediate vicinity and consequently leads to a very high degree of solvation in a polar solvent. In aqueous solutions, the primary solvation process involves the formation of a covalent bond with the oxygen atom of a water molecule to form a hydronium ion H30 +. Secondary solvation of this species then occurs by additional water molecules. Whenever we use the term hydrogen ion in the future, we are referring to the HsO + species. 

---