THE SHAPES OF MOLECULES
To be the central atom in a compound, the atom must be able to simultaneously bond to at least two other atoms.
He, F, and H cannot serve as central atoms in a Lewis structure. Helium is a noble gas, and as such, it does not
need to bond to any other atoms. Hydrogen (1
) and fluorine (1
) only need one electron to complete their
valence shells. Thus, they can only bond to one other atom, and it does not have
orbitals available to expand its
Draw a Lewis structure and then see if additional structures may be drawn.
Resonance must be present any time that a single Lewis structure is inadequate in explaining one or more aspects
of a molecule or ion. The two N-O bonds in NO
are equivalent; no single Lewis structure shows this. The
following Lewis structures may be drawn for NO
The average of all of these structures gives equivalent N-O bonds with a bond length that is between N-O and
For an element to obey the octet rule it must be surrounded by 8 electrons. To determine the number of electrons
present (1) count the individual electrons actually shown adjacent to a particular atom, and (2) add two times the
number of bonds to that atom. Using this method the structures shown give: (a) 0 + 2(4) = 8; (b) 2 + 2(3) = 8;
(c) 0 + 2(5) = 10; (d) 2 + 2(3) = 8; (e) 0 + 2(4) = 8; (f) 2 + 2(3) = 8; (g) 0 + 2(3) = 6; (h) 8 + 2(0) = 8. All the
structures obey the octet rule except: c and g.
For an atom to expand its valence shell, it must have readily available
orbitals do not become
readily available until the third period or below on the periodic table. For the elements in the problem F, S, H, Al,
Se, and Cl, the period numbers are 2, 3, 1, 3, 4, and 3, respectively. All of these elements, except those in the first
two periods (H and F), can expand their valence shells.
Count the valence electrons and draw Lewis structures.
Total valence electrons: SiF
has 32; SeCl
has 20; and COF
has 24. The Si, Se, and the C are the central atoms,
because these are the elements in their respective compounds with the lower group number (in addition, we are
told C is central). Place the other atoms around the central atoms and connect each to the central atom with a
single bond. At this point the Si has an octet, and the remaining electrons are placed around the fluorines (3 pairs
each). The 2 bonds plus 2 lone pairs complete the octet on Se and leave enough electrons to complete the Cl octet
with 3 pairs each. The 3 bonds to the C leave it 2 electrons short of an octet. Forming a double bond to the O
completes the C octet, and leaves sufficient electrons to form 2 lone pairs on the O and 3 lone pairs on each of the
F’s. All atoms in each of the structures end with an octet of electrons.