Final answer:
HCN consists of three bonded atoms (H, C, N) with a triple bond between C and N, and no unbonded electrons. C₂H₂ has four bonded atoms with a triple bond between the two C atoms and single bonds to each H, also with no unbonded electrons.
Step-by-step explanation:
Understanding Bonded Atoms and Unbonded Electrons in HCN and C₂H₂
For the molecule HCN, after calculating the valence electrons, we identify that HCN has a total of 10 valence electrons. Using these to form a structure, we end up with a triple bond between carbon and nitrogen, and a single bond between hydrogen and carbon. This results in no unbonded electrons remaining on HCN. Thus, HCN has three bonded atoms and no unbonded electrons.
Looking at C₂H₂ (ethyne or acetylene), we calculate a total of 10 valence electrons. A Lewis structure for C₂H₂ shows a triple bond between the two carbons and one bond to each hydrogen. Again, this structure uses all valence electrons, so there are no unbonded electrons. Hence, C₂H₂ has four bonded atoms and no unbonded electrons.
To further clarify, C₂H₂ is connected through a triple bond between the carbons, denoted as a σ bond and two pi (π) bonds, which fulfill the octet rule for each carbon atom without leaving any unbonded electrons.