Re: Why are most elements of Periodic Table represented on earth?

The quick answers: yes, yes, and no.

Yes, all of the elements can be found on Earth - although some, such as
Technetium, are radioactive and may be found in only minute amounts. (There
is still some dispute about whether Technetium occurs "naturally"!)

And this is what you would expect of most "solid planets". This goes back
to the way that the elements are formed in the first place. The process
of nucleosynthesis (the building of all of the elements from protons or
hydrogen nuclei) is probably best described in Steven Weinberg's "The 
First Three Minutes". 

In essence, the atoms are built by combining protons to form helium nuclei
and then combining helium nuclei to get all of the other elements. There
are some stumbling blocks along the way - like the beryllium problem - but
it all culminates in "iron" where the binding energy for the nucleons is
at a maximum. The heavier elements come about because of rare reactions and
hence, are more scarce in the Universe. But as all of these elements are
formed in the nuclei of stars and then released during a Super Nova 
explosion, the relative distribution of the non-hydrogen and helium 
elements in the Universe should be pretty much the "same". Not exactly,
but to a first approximation.

The result means that any star that is capable of generating "earth-type"
planets will be doing so from the same set of starting materials and result
in roughly the same sort of planets. Maybe a little more of one element 
than another, but roughly the same. (By the way, if you look at the whole
earth and not just the lithosphere, then the most abundant elements are 
iron and nickel which make up much of the core.)

As to the moon, overall, you would expect roughly the same composition 
with similar sorts of elemental distribution. Not exactly the same, but
similar. However, because of subtleties in the exposure to radiation and
extra-planetary forces, and the absense of an active tectonic system, the
surface rocks of the moon tend to show differences in their isotope 
distribution relative to the same elements on earth. (Similar analysis of 
the isotopic distribution of the elements resulted in the identification
of meteorites found in the Antarctic as being of Martian origin.)

So, is the Earth unique? Well, I would answer that it is likely that it
is not. The same sorts of elements should be present elsewhere. On the 
other hand, we only know of one planet that has earth-like properties, so
may be it is!

Hope this answers your questions and my apologies for the late reply!