Melting points of the elements

Many other properties have been found to show periodic variations and these can be displayed graphically or by circles of varying size on a periodic table, e.g. melting points of the elements, boiling points, heats of fusion, heats of vaporization, energies of atomization, etc. Similarly, the properties of simple binary... 

A. The phenomenon of periodicity is particularly clear in the melting points of the elements. It is however remarkable, because this is a purely physical property. The melting point is not an atomic property, but is determined by the relationships in the crystal lattice. Therefore the maxima and minima do not coincide with the beginning or end of a period as is the case with the atomic radii and ionization energies. 

An interesting effect of the half-filled and filled 4/ shell is shown when a graph is made of the melting point of the elements. Such a graph is shown in Figure 11.6. Although it is not shown, a plot of atomic radii for the metals shows a large increase in size for Eu and Yb. For example, the radii of Sm and Gd are approximately 180 pm, but Eu, situated between them, has a radius of 204 pm. The difference in size between Yb and the atoms before and after it also amounts to about 20 pm. Europium and ytterbium... 

Figure 6.1 The melting points of the elements, arranged in lines by their periods, plotted against the group number...

A closer look at the periodic table points out some interesting trends. These trends not only help us predict how one element might perform relative to another, but also give us some insight into the important properties of atoms and ions that determine their performance. For example, examination of the melting points of the elements in Table 1.3 shows that there is a general trend to decrease melting point as we go down... 

Table 1.3 Electronic Structure and Melting Points of the Elements... 

One physical property that shows a periodic change linked to the Periodic Table is the melting point of an element. Below is a chart of the melting points of the elements in the second and third Periods plotted against the proton number of the element. 

Consult a handbook of chemistry, and look up melting points of the elements of Periods 2 and 3. Show that melting point is a property that varies periodically for these elements. 

As you move down Group 5A(15), the melting points of the elements increase and then decrease. Explain. 

An important use of DTA is for the generation of phase diagrams and the study of phase transitions. An example is shown in Figure 31-9, which is a differential thermogram of sulfur, in which the peak at IB C corresponds to the solid-phase change from the rhombic to the monoclinic form. The peak at 124°C corresponds to the melting point of the element. Liquid sulfur is known to exist in at least three forms, and the peak at 179°C apparently involves these transitions, whereas the peak at 446 C corresponds to the boiling point of sulfur. 

This would work for the other calcium halides as well, but the product CaF2 is easier to handle under the reaction conditions (heated to 50°C above the melting point of the element in an argon atmosphere). Excess calcium is removed from the reaction mixture under vacuum [42]. 

Fig. 14.74. The melting points of the elements as a function of atomic number. Shown also are the magnetic ordering temperatures of the 3d and 4f elements (open circles).

Both gallium (Ga) and mercury (Hg) can be eliminated from consideration as base materials just based on their low melting points. The melting point of the elements Ag, Cu, Ni, and Au are too high to serve as base materials and some are also too expensive, only leaving In and Sn. Indium is too expensive to use in quantities required of a base alloy. That leaves Sn, whose solder alloy melt points are typically between about 120°C and 280°C, providing a baseline for lead-free solder alternatives to eutectic Sn-Pb. Eutectic alloys with either Cu or Ag have melt points close to pure Sn, so only a few percent of these elements can be added to Sn before the melting point level is considered unacceptable [31]. 

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