Inspection of 10 individual spectra taken over the 24 hr monitoring of 21 Com with International Ultraviolet Explorer on 1991 April 17 reveals variations of low excitation lines of Si II between rotational phases φ = 0.18 and φ = 0.65 of the 2 days period, suggesting a inhomogeneous repartition of silicon over the surface of 21 Com. The silicon lines are weaker at rotational phase φ = 0.18. More recent optical high resolution high signal-to-noise ELODIE spectra taken in 2004 April confirm the variations of silicon lines from phase φ = 0.99 to φ = 0.47. The synthesis of the optical Si II lines yields estimates of the abundances of silicon: 8 times solar at phase 0.99 and 10 times solar at phase 0.47. The large FUV variations of 21 Com appear then to be partly due to changes in the silicon line and continuous opacity as the star rotates. In contrast, the Mg II lines do not show convincing variations between the two rotational phases observed in the optical.