What is the difference in atomic structure between Lead-205 and Lead-208?

  the number of neutrons in the nucleus

Explanation:

The mass number 205 is the total number of protons and neutrons in the nucleus. Lead has 82 protons, so a mass number of 205 indicates a nucleus with 123 neutrons.

Similarly, the mass number 208 indicates a lead nucleus with 126 neutrons.

The difference in atomic structure is the difference in the number of neutrons in the nucleus.

c is having 4 valence electrons and is bonded to three hydrogens and one sulphur atom, therefore, the number of bond pair is equal to four. as all the valence electrons are used in forming the bonds so, there is no lone pair left on carbon.

s is having 6 valence electrons and is bonded to one other sulfur atom and one carbon atom, therefore, the number of bond pair is equal to 2. as only two valence electrons are used in forming the bonds so, four electrons are left. therefore, two lone pairs are present on s.