To draw the Lewis structure for C2H2 (ethyne), we start by determining the total number of valence electrons available.
a) Total number of valence electrons:
Carbon (C) has 4 valence electrons, and hydrogen (H) has 1 valence electron. In C2H2, there are 2 carbon atoms and 2 hydrogen atoms:
- 2 Carbon atoms: 2 x 4 = 8 valence electrons
- 2 Hydrogen atoms: 2 x 1 = 2 valence electrons
Thus, the total number of valence electrons is 8 + 2 = 10 valence electrons.
b) Number of shared electron pairs around the central atom:
In the Lewis structure of C2H2, each carbon atom forms a triple bond with the other carbon atom and a single bond with a hydrogen atom. Therefore, there are three pairs of shared electrons in the C≡C bond and one pair in each C–H bond.
- Shared electron pairs in C≡C bond: 3 pairs
- Shared electron pairs in C–H bonds (two bonds, one for each H): 1 pair each
So, around each carbon atom, we have 3 pairs (C≡C) + 1 pair (C–H) = 4 shared pairs
c) Number of unshared electron pairs:
In the Lewis structure of C2H2, each carbon atom is bonded to hydrogen atoms and to another carbon atom, so there are no unshared (lone) electron pairs on the carbon atoms. Similarly, hydrogen does not have any lone pairs. Therefore, there are:
- Unshared electron pairs: 0
In summary, for the Lewis structure of C2H2:
- Total number of valence electrons: 10
- Number of shared electron pairs around the central atom: 4
- Number of unshared electron pairs: 0