120
Sizes and Shapes of Molecules
This is equivalent to treating the one odd electron as a lone pair,
though it is less effective in repulsion than a true lone pair See p
140
3 Deteimme the electron-pair geometry of the molecule or ion, using the
guidelines in Table 9-2
4 Deteimme the molecular geometry taking into consideration the bond
pairs and lone pairs involved in the electron-pair geometry See pp
121-132.
Electron-Dot Formulas
The first thing you must be able to do in order to predict molecular shapes is to
draw an electron-dot formula, so we'll tackle that subject first Including H,
there are 16 active nonmetals for which you should know the numbers of
valence electrons in the uncombmed atoms Except for H (which has only one v
electron), these elements are all found to the right of the diagonal in the;? block
of the periodic table (see inside front cover) Each atom has two 9 electrons in
its valence shell, the number of p electrons is different for different atoms
(Basically, we are uninterested in metals here, metals rarely form predominantly covalent bonds, but tend to form ionic bonds Except for Xe, we also can
ignore the noble gases, with an already filled s
2 p
h configuration, they are
unreactive )
It will pay you to know (without having to look in the periodic table or tables
of electron configurations) that the halogens (F, Cl, Br, I, At) all have seven
valence electrons, that the oxygen family (O, S, Se, Te) all have six, that the
nitrogen family (N, P, As) have five, that the carbon family (C, Si) have four,
and that the boron family (B) have three It will also pay you to know that
electronegativities decrease from right to left in a row, or from top to bottom in
a column, in the periodic table
Figure 9-1 shows electron-dot formulas in which each hgand possesses eight
electrons as a result of sharing the needed number from the central atom (except H, which needs only two electrons to fill its valence shell) In each case,
the least electronegative atom is central, except for NH1; where H cannot be
central In the case of the NHJ and SOi~ ions, the number of dots reflects the
loss or gain of electrons as required by the charge on the ions In three cases
(BC1 3 , C1F 3 , and PC1 D ) the octet rule has been violated for the central atom, but
it is not possible to rectify this because the hgands (all halogens in this case) are
not able to form multiple bonds If both the hgands and the central atom never
violated the octet rule, there would always be just four pairs of electrons
around the central atom, and all molecular structures would be tetrahedral It's
because there are so many exceptions to the octet rule for the central atom that
there are so many different shapes of molecules
The halogens and boron can form only single bonds The oxygen-family
Sizes and Shapes of Molecules
This is equivalent to treating the one odd electron as a lone pair,
though it is less effective in repulsion than a true lone pair See p
140
3 Deteimme the electron-pair geometry of the molecule or ion, using the
guidelines in Table 9-2
4 Deteimme the molecular geometry taking into consideration the bond
pairs and lone pairs involved in the electron-pair geometry See pp
121-132.
Electron-Dot Formulas
The first thing you must be able to do in order to predict molecular shapes is to
draw an electron-dot formula, so we'll tackle that subject first Including H,
there are 16 active nonmetals for which you should know the numbers of
valence electrons in the uncombmed atoms Except for H (which has only one v
electron), these elements are all found to the right of the diagonal in the;? block
of the periodic table (see inside front cover) Each atom has two 9 electrons in
its valence shell, the number of p electrons is different for different atoms
(Basically, we are uninterested in metals here, metals rarely form predominantly covalent bonds, but tend to form ionic bonds Except for Xe, we also can
ignore the noble gases, with an already filled s
2 p
h configuration, they are
unreactive )
It will pay you to know (without having to look in the periodic table or tables
of electron configurations) that the halogens (F, Cl, Br, I, At) all have seven
valence electrons, that the oxygen family (O, S, Se, Te) all have six, that the
nitrogen family (N, P, As) have five, that the carbon family (C, Si) have four,
and that the boron family (B) have three It will also pay you to know that
electronegativities decrease from right to left in a row, or from top to bottom in
a column, in the periodic table
Figure 9-1 shows electron-dot formulas in which each hgand possesses eight
electrons as a result of sharing the needed number from the central atom (except H, which needs only two electrons to fill its valence shell) In each case,
the least electronegative atom is central, except for NH1; where H cannot be
central In the case of the NHJ and SOi~ ions, the number of dots reflects the
loss or gain of electrons as required by the charge on the ions In three cases
(BC1 3 , C1F 3 , and PC1 D ) the octet rule has been violated for the central atom, but
it is not possible to rectify this because the hgands (all halogens in this case) are
not able to form multiple bonds If both the hgands and the central atom never
violated the octet rule, there would always be just four pairs of electrons
around the central atom, and all molecular structures would be tetrahedral It's
because there are so many exceptions to the octet rule for the central atom that
there are so many different shapes of molecules
The halogens and boron can form only single bonds The oxygen-family