Chemistry Reference
In-Depth Information
noble gases is due to the 8 electrons in the outermost shell of each atom (2 elec-
trons in the case of helium). In fact, 8 electrons in the outermost shell is a stable
configuration for most main group atoms. Atoms other than those of the noble
gases tend to form ionic or covalent bonds (or both) with other atoms to achieve
this electronic configuration. The 8 electrons in the outermost shell are called
an
octet.
The tendency of atoms to be stable with 8 electrons in the outermost
shell is called the
octet rule.
In some compounds, one (or more) of the atoms
does not obey the octet rule. Some exceptions to the octet rule will be men-
tioned briefly in Section 5.5.
Because the maximum number of electrons in the first shell of an atom is 2,
helium is stable with 2 electrons in its only occupied shell. The other very light
elements—hydrogen, lithium, and beryllium—tend to form stable states by
achieving the 2-electron configuration of helium. Having 2 electrons in the first
shell,
when that is the only shell and therefore the outermost shell,
is a stable
state, and the 2 electrons are sometimes called a
duet.
When there is only one
shell, 2 electrons in that shell act like 8 electrons in any other outermost shell.
Therefore, an atom with 2 electrons in its outermost first shell is often said to
obey the octet rule, although “duet rule” would be more precise.
The
valence shell
of electrons in an atom is the outermost shell of elec-
trons of the
uncombined
atom. The electrons in that shell are called
valence
electrons.
If all the electrons are removed from that shell, the next inner shell
becomes the new outermost shell. For example, the sodium atom has 2 elec-
trons in its first shell, 8 electrons in its second shell (the maximum), and its last
electron in its third shell. The valence shell is the third shell. If the 1 electron
is removed from the third shell, the second shell becomes the outermost shell,
containing 8 electrons. The valence shell is still the (now empty) third shell.
The number of electrons in the valence shell of an uncombined main group atom
is equal to the classical periodic group number of the element (Figure 5.7). The
exceptions to this rule are that helium has 2 valence electrons and the other
noble gases have 8 valence electrons.
All metals react with nonmetals to form ionic compounds
. Main group met-
als tend to transfer their valence electrons to nonmetals, and nonmetals tend
to accept enough electrons from these metals to achieve their octets. For exam-
ple, a sodium atom has 1 electron in its valence shell, and a chlorine atom has
7 electrons in its valence shell. When they react, the sodium atom transfers that
1 electron to the chlorine atom, forming two charged species called
ions.
Both
of these ions have 8 electrons in their outermost shells. (The sodium ion has
8 electrons in its second shell, now its outermost shell.)
The electronic config-
urations of both ions are those of noble gas atoms
(the sodium ion has that of
a neon atom, and the chlorine ion that of argon). The atoms have not been
changed into noble gas atoms, however, because their nuclei have not changed.
Atoms tend to accept, donate,
or share electrons to achieve
the electronic structure of the
nearest noble gas.
The valence shell is the shell of
highest principal quantum
number.
1s
2
2s
2
2p
6
3s
1
Na:
Na
: 1s
2
2s
2
2p
6
3s
0
The number of electrons in the
valence shell of an uncombined
main group atom is equal to
the classical periodic group
number of the element.
Classical group number:
Modern group number:
IA
IIA
IIIA
IVA
VA
VIA
VIIA
0
Figure 5.7
Numbers of
Valence Electrons for Atoms of
Main Group Elements
1
2
13
14
15
16
17
18
Period 1
1
2
All other periods
1
2
3
4
5
6
7
8
