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LEWIS STRUCTURES OF NEGATIVE IONS For writing the Lewis structures of negative ions like SO 42-, NO 3-, CO 32- etc. we shall have to first write the Lewis structures of the corresponding acids and then remove (wipe out) the H atoms from that structure. Note that while rubbing of the H atoms, leave the electron of H atom on the O atom, so that a new lone pair is formed. The presence of additional electron coming from H atom forms a negative charge on O atom. CO 32- : First let us draw Lewis structure of H 2CO3. C is the central atom and has four unpaired electrons for bonding. First we have to make a double bond between C and O by using two unpaired electrons each. Then we make single bonds between C and the remaining two O atoms (to be connected with H atoms) by using the remaining unpaired electrons of C and one unpaired electrons of each of the two O atoms. These two O atoms also make single bonds with H atoms by using each of its remaining unpaired electrons with unpaired electrons of H atoms. Note that we cannot make a dative bond in place of a double bond in between C atom and O atom. That will violate the octet rule as in that case C will share only six electrons. X X
OXX
OXX
X X
XX
XX
H
X
XX
C
OX
O
X
X
XX
XX
H or
XX
H
C
O
XX
XX
O
H
XX
(oval marks not shown)
Now the two H atoms have to be removed with the creation of a new lone pair and consequently -1 charge on each oxygen. In stead of writing -1 on each O, we shall write the total charge of 2- above the bracket as shown below. 2X X
X X
XX XX X
O
X
C
2-
X
OX
O
X X
XX X
O
XX X
X
or
XX
XX
O XX
XX
C
OX XX
(oval marks not shown)
Note that the electron of H indicated by a symbol of triangle(D) is present on each O atom. This is responsible for the negative charge of -2 for the two additional electrons left by H atoms. Similarly we can write the structure of SO 42- and NO3- ion first writing the structures of H 2SO4 and HNO3 respectively and then rubbing off the H atoms as explained before. Refer the structures of these two acids given earlier. The structures of their ions would be as follows. We give only the dash structures. X X
XX
X
SO42-:
X
XX
O XX
XX
S
O XX
-
XX X X X X
2-
OX
O
X
XX
NO3- :
O XX
XX
N
X
O XX
X X O XX
X X
Likewise the reader is supposed to practise writing Lewis structures of some more ions.
LEWIS STRUCTURES OF MOLECULES VIOLATING THE OCTET RULE: There are many molecules in which the central atom cannot, by any means, share eight electrons. It shares either less than eight electrons, in such case we call it to be an electron deficient molecule or in other cases it shares more than eight electrons. See the following cases. BeCl2, BCl3, PCl5, SF6 BeCl2 : Be has two electrons in the valence shell for bonding (see table). It forms two single bonds with two chlorine atoms. Each Cl atom uses its one unpaired electron for the purpose. Thus Be shares only four electrons. This is a violation of octet rule. Note that when there is no other way, octet rule is violated. So you should not remain under the impression that octet rule can never be violated. Cl
Be
Cl
Cl
or
Be
Cl
BCl 3: B has three electrons in the valence shell for bonding (see table). Each Cl has one unpaired electron for bonding. So there will be three single bonds and B will share only six electrons. This is violation of octet rule. Cl
