The geometry of the thiocyanate ion (SCN–) can be predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory. In this ion, we have a sulfur atom (S) bonded to a carbon atom (C), which in turn is bonded to a nitrogen atom (N).
First, let’s determine the total number of valence electrons. Sulfur has 6 valence electrons, carbon has 4, and nitrogen has 5. Since SCN is a negatively charged ion, we add one more electron, giving us a total of:
- 6 (S) + 4 (C) + 5 (N) + 1 (charge) = 16 valence electrons.
Next, we draw the Lewis structure. The central atom will be carbon, as it is bonded to both sulfur and nitrogen. We will distribute the 16 electrons to satisfy the octets of the surrounding atoms:
- Carbon will form a single bond with sulfur and a triple bond with nitrogen.
This arrangement uses 6 electrons for the three bonds (3 bonds = 6 electrons) and leaves us with 10 electrons. Since nitrogen is bonded triply, it satisfies its octet. Sulfur, forming a single bond, can hold more than 8 electrons, and thus can also accommodate the additional lone pairs.
When we consider the electron domain geometry around the carbon atom, it has one single bond and one triple bond resulting in two domains. According to VSEPR theory, two electron domains will arrange themselves in a linear geometry to minimize repulsion. Thus, the predicted shape of SCN– is linear.
In summary, using the VSEPR method, we can conclude that the geometry of the SCN– ion is linear, with a bond angle of approximately 180°.