Frequency of a single sample in a digital signal and aliasing
A single sample being non-zero is known as an impulse.  The effect in the discrete frequency domain depends upon where it is within a the frame.  The DFT will give you the coefficients required to reproduce whatever finite signal is withing the sample frame.  It is misleading, even erroneous, to say the signal is composed of those frequencies.  This can only be said about periodic signal for which the sampling frame is a whole number of periods.  Recall that the DFT and inverse DFT are basically the same, except for a sign change in the exponent.  When you have a pure tone with a whole integer frequency in the sample frame, the result is a single spike in the DFT at the bin corresponding to that frequency.  So when you have a single spike, or impulse, in the signal domain, the result will be a DFT with the bin values being the same as a pure complex tone.

"Alias" is a different concept entirely.  It has to do with the sampling of tones higher than the Nyquist frequency appearing to be tones of a lower frequency.  The easiest way to envision what alias frequency will look like is to imagine the bins of the DFT arranged along the unit circle.  The Nyquist bin will occur at $\pi$ radians.  Any alias frequency has to appear somewhere on the circle.  It may be beyond $\pi$, or even beyond multiple times around the circle.  The frequency it will appear to be (for real valued signals) will be the frequency associated with the bin number of the first half of the circle that has the same projection onto the horizontal axis.

I would recommend that you read my blog articles (link on my profile page) from the start to get a perspective on this material.

Hope this helps.

Ced