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Unformatted text preview: Exceptions to the Octet Rule 3 Most main-group elements tend to form covalent bonds according to the
octet rule. However. there are exceptions. As you have seen, hydrogen
forms bonds in which it is surrounded by only two electrons. Boron, B,
has just three valence electrons ([He]2522p‘). Because electron pairs are ‘ 4 "‘ " ‘“ " "'alent bonds, boron tends to form bonds in which it is 1 O o f 41 six electrons. In boron trifluoride, BF3. for example, the iurrounded by its own three valence electrons plus one
.Ie three fluorine atoms bonded to it. Other elements can be
. surrounded by more than eight electrons when they combine with the
highly electronegao've elements fluorine. oxygen, and chlorine. In these
1 cases of expanded valence, bonding involves electrons in d orbitals as well
3 as in s and p orbitals. Examples of compounds that have an expanded
l valence include PC]5 and SFG, as shown in Figuro 5.4 (in Section 5). Chemical Bonding 173 0 MAIN IDEA Writing Electron-Dot Notations To write an Dots placed around an element’s symbol element's electron-dot notation. determine the elements can rep resent vaIence e|ectronsl
nu other of valence electrons, Tnen place a corresponding no other at dots around the elem en t's symbol, as shown. j Covalent bond formation usually involves only the
m of E t electrons in an atom's outermost energy levels. or the
val-ten m m mi. : atom’s valence electrons. To keep track of these electrons,
— ' it is helpful to use electron-dot notation. Elam-dot
notation is at electron-comiguration notation in which only
the valence electrons of an atom of a particular element are
shown, indicated by dots placed around the element's
symbol. The inner<shell electrons are not shown. For
example, the electron-dot notation for a fluorine atom
(electron configuration [Hel2522p5) may be written
as follows. 1 :IIF': In general, an element's number of valence electrons can
be determined by adding the superscripts of the ele-
ment’s noble-gas notation. In this book, the electron-dot
notations for elements with 1—8 valence electrons are
written as shown in Figure 2.8. PREMIUM CONrENr Electro Dot Notation Sample Problem I I. Write the electron-dot notation for hydrogen.
I). Write the electron-dot notation [or nitrogen. . sow: a. A hydrogen atom has only one occupied energy level, the n = 1 level,
which contains a single electron. Therefore, the electron-dot notation for
hydrogen is written as follows. H. . The group notation for nitrogen’s family of elements is nsznp3, which
indicates that nitrogen has five valence electrons. Therefore, the
electron-dot notation for nitrogen is written as follows. .N: . MAIN IDEA
Electron-dot notations can represent compounds. 3 Electron-dot notation can also be used to represent molecules. For
‘ example. a hydrogen molecule, Hz, is rep resented by combining the
notations of two individual hydrogen atoms. as follows. HlH Chapter 6 , The pair of dots represents the shared electron pair of the hydrogen-hydrogen
covalent hand. For a molecule of fluorine, F2, the electron— dot notations of
, two fluorine atoms are combined, Here. too, the pair of dots between the two symbols represents the shared
pair of a covalent bond. In addition, each fluorine atom is surrounded by 3 three pairs of electrons that are not shared in bonds. An unshared pair,
also called a lone pair, is a pair of electrons that is not involved in bonding
3 and that belongs exclusively to one atom. ‘ The pair of dots representinga shared pair of electrons in a covalent
3 bond is often replaced by a long dash. According to this convention,
hydrogen and fluorine molecules are represented as follows. ...
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