Page 57 - Materials Science and Engineering An Introduction

2.4 The Periodic Table • 29

The alkali and the alkaline earth metals (Li, Na, K, Be, Mg, Ca, etc.) are labeled as
Groups IA and IIA, having, respectively, one and two electrons in excess of stable struc-
tures. The elements in the three long periods, Groups IIIB through IIB, are termed the
transition metals, which have partially filled d electron states and in some cases one or
two electrons in the next higher energy shell. Groups IIIA, IVA, and VA (B, Si, Ge, As,
etc.) display characteristics that are intermediate between the metals and nonmetals by
virtue of their valence electron structures.
As may be noted from the periodic table, most of the elements really come un-
electropositive der the metal classification. These are sometimes termed electropositive elements,
indicating that they are capable of giving up their few valence electrons to become
positively charged ions. Furthermore, the elements situated on the right side of the
electronegative table are electronegative—that is, they readily accept electrons to form negatively
charged ions, or sometimes they share electrons with other atoms. Figure 2.9 displays
electronegativity values that have been assigned to the various elements arranged in
the periodic table. As a general rule, electronegativity increases in moving from left to
right and from bottom to top. Atoms are more likely to accept electrons if their outer
shells are almost full and if they are less “shielded” from (i.e., closer to) the nucleus.
In addition to chemical behavior, physical properties of the elements also tend to
vary systematically with position in the periodic table. For example, most metals that
reside in the center of the table (Groups IIIB through IIB) are relatively good conduc-
tors of electricity and heat; nonmetals are typically electrical and thermal insulators.
Mechanically, the metallic elements exhibit varying degrees of ductility—the ability to
be plastically deformed without fracturing (e.g., the ability to be rolled into thin sheets).
Most of the nonmetals are either gases or liquids, or in the solid state are brittle in nature.
Furthermore, for the Group IVA elements [C (diamond), Si, Ge, Sn, and Pb], electrical
conductivity increases as we move down this column. The Group VB metals (V, Nb, and
Ta) have very high melting temperatures, which increase in going down this column.
It should be noted that there is not always this consistency in property variations within
the periodic table. Physical properties change in a more or less regular manner; however,
there are some rather abrupt changes when one moves across a period or down a group.

H
2.1

Be
Li 1.5 O F
1.0 N 3.5 4.1
C 3.1
Mg
Na 2.5
1.3 B
1.0
2.0
K Ca Sc Ti V Cr
0.9 1.1 1.2 1.3 1.5 1.6 Mn Fe
1.6 1.7 Co P S Cl
Rb 1.7 Ni Al Si 2.9
Sr 1.8 Cu 2.1 2.4
0.9 Y Zr Nb 1.5 1.8
1.0 1.1 1.2 1.3 Mo Tc 1.8 Zn Ga Ge Se
As
Br
Cs 1.3 1.4 Ru Rh 1.7 1.8 2.0 2.2 2.5 2.8
0.9 Ba La Hf Ta W 1.4 1.5
0.9 1.1 1.2 1.4 Re Pd
1.4 Os 1.4 Ag Cd
Fr 1.5 Ir Sn
In
0.9 Ra Ac 1.5 1.6 Pt 1.4 1.5 1.5 1.7 Sb Te I
0.9 1.8 2.0
1.0 1.5 Au 2.2
Hg
1.4 TI
1.5 Pb Bi
1.5 Po At
1.6 1.7
1.8 2.0
Lanthanides: 1.0 – 1.2
Actinides: 1.0 – 1.2
Figure 2.9 The electronegativity values for the elements.
(From J. E. Brady and F. Senese, Chemistry: Matter and Its Changes, 4th edition. This material is reproduced with permission of
John Wiley & Sons, Inc.)