For the second-row elements, BeO is an amphoteric oxide on the
borderline between bases and acids, and boron oxide is acidic. For
the third row, MgO is still basic, and the amphoteric borderline
does not occur until aluminum oxide, Al203.
Atoms become more electronegative with increasing atomic number
within a shell, but less electronegative from one shell to the next.In
the diagram with which this chapter opened, each element resembles
the one below it and to the right, more than it does the one directly
below.
Second
Row
|
Li
|
Be
|
B
|
C
|
N
|
O
|
F
|
1.0
|
1.5
|
2.0
|
2.5
|
30.
|
3.5
|
4.0
|
Third
Row
|
Na
|
Mg
|
Al
|
Si
|
P
|
S
|
Cl
|
0.9
|
1.2
|
1.5
|
1.8
|
2.1
|
2.5
|
3.0
|
The elements therefore show a diagonal trend in many chemical properties.
Lithium is a moderately reactive metal like magnesium, but not as
reactive as sodium.
Beryllium has an amphoteric oxide like aluminum, rather than a
basic oxide like magnesium.
|
|
Boron is a nonmetal like silicon, diagonally below it to the right,
whereas aluminum, directly below it, is a metal. This diagonal trend
of chemical properties is important, and we will encounter it often.
Among the nonmetals, the increase in size of atoms accounts for
the larger number of oxygen atoms that can be accommodated around
the central atom.
Only three oxygens can be fitted around carbon in the carbonate
ion, CO32-,
or nitrogen in the nitrate ion, NO3-.
The larger third-row atoms have room for four oxygens in the silicate
ion, SiO44-,
phosphate ion, PO43-,
and sulfate ion, SO42-.
As we mentioned in the preceding chapter, fluorine is so electronegative
that it tears electrons away from oxygen and forms F- ions, instead
of sharing electrons with oxygen in an oxyacid ion of the type we
have been considering.Chlorine is not as strongly electronegative,
and is content to share bonds with oxygen, as in the perchlorate
ion, ClO4-.
|