Chemistry Reference
In-Depth Information
by a subscript 2, the words “taken twice” are used; for a subscript 3, the words
“taken three times” are used, and so on. The centered dot is read “dot.” Here
are some examples to illustrate these conventions:
Hg
2
Br
2
“H g two B r two”
Ba
3
(PO
4
)
2
“B a three P O four taken twice”
CH
3
(CH
2
)
4
CH
3
“C H three, C H two taken four times, C H three”
CuSO
4
·5H
2
O
“C u S O four dot five H two O”
Snapshot Review
❒
The chemical formula for a substance tells us (a) the relative numbers
of atoms of each element and (b) that the elements in it are all
bonded in some way.
A. How many atoms of hydrogen are present per atom of phosphorus in
(NH
4
)
2
HPO
4
?
B. Which element in SeCl
2
is more electronegative?
C. What is the formula for elemental oxygen?
5.2
Ionic Bonding
The electrons in atoms are arranged in groups, those in each group having nearly
the same energies. These energy levels are often referred to as shells. The first
shell of any atom can hold a maximum of 2 electrons; the second shell can hold
a maximum of 8 electrons; and the other shells can hold a maximum of 8 elec-
trons when they are the outermost shell, but a greater number when they are
not (Table 5.1). The
outermost shell
is the last shell that contains electrons.
Before the next-to-outermost shell starts to expand past 8 electrons, the (new)
outermost shell must hold 2 electrons.
Electrons can be transferred from metal atoms to nonmetal atoms to achieve
a more stable, lower energy state. The noble gases are composed of stable
atoms; no reactions of the first three (He, Ne, Ar) have been discovered, and
the others (Kr, Xe, Rn) are almost completely unreactive. The stability of the
Table 5.1
Maximum Electron Occupancy of Shells
Maximum Occupancy
Maximum Occupancy
Shell Number
as the Outermost Shell
as an Inner Shell
1
2
2
2
8
8
3
8
18
4
8
32
5
8
50*
6
8
72*
7
8*
98*
*More than the number of electrons available in any atom
