Question

Unshared, or lone, electron pairs play an important role in determining the chemical and physical properties of organic compounds. Thus, it is important to know which atoms carry unshared pairs. Use the structural formulas below to determine the number of unshared pairs at each designated atom. Be sure your answers are consistent with the formal charges on the formulas The number of unshared pairs at atom a is The number of unshared pairs at atom bis The number of unshared pairs at atome is The number of unshared pairs at atom a is -- The number of unshared pairs at atomb is The number of unshared pairs at atom cis

Answer 1

How to find valence electrons ?

Carbon electronic configuration 1s²2s²2p²

Here last shell contain 4 electrons hence carbon has 4valence electrons.Similarly, oxygen has six valence electrons. Lone pairs are electron pairs that are not shared with other atoms.

1.a–2 lone pairs(4 electrons) , oxygen has six valence electrons (group 16) out of that two are shared remaining four ie, two lone pairs .

b–0, since carbon has four valence electrons (group 14) and all are shared.

c–0

2.a–0,b–0,c–2 (4 electrons) lone pairs

3.a–1(2 electrons) ,nitrogen has 5 valence electrons out of that only three are shared.

b–1since 3 valence electrons are shared one remaining   but has a formal  negative charge is present ie, 2 electrons (1electron pair) .

c–1 lone pair, sulphur has six valence electrons (group 16) actually only two bonds must be present so that there are 2 lone pairs but nearby hydrogen ion (H⁺) made a bond with sulphur so that only one lone pair is left.

4.a–2 (4 electrons), b–0,c–1(2 electrons)

5.a–0,b–0

c–2(4 electrons), nitrogen has five valence electrons out of that two are shared remaining three. One electron acquired from another atom hence has formal charge —1.

6.a–0,b–2(4 electrons), c–0 since boron has three valence electrons (group 13) and all are shared .

NOTE:

Bonded electrons are valence electrons  involved in bonding and lone pairs are valence electrons not involved in bonding. Two electrons are required to make a bond. Shortcut if no charge is present on the atom: then count the number of electrons needed to make it octet ie, 8.For eg, in the first part oxygen makes two bonds (4 electrons) so 4 more electrons are required to make a octet ie two electron pairs so answer is two.

But if charge is present do the same as above but verify it by taking into account the valence electrons. Hence,  formal charge is present. Extra electrons acquired are not valence electrons.