Question #fbcdc

If you mean which elements can react to form a compounds, basically all of them (except the heavier synthetic elements and lighter noble gases) have been observed to form at least one compound.

If you mean "radioactive" (rough interpretation), then see below:

You can't necessarily determine if an element is radioactive just from its name, symbol, mass, etc.

An element is radioactive if either

1. There are no stable naturally-occurring isotopes

2. There are no naturally-occurring nuclides at all, in which case it is synthetic

A lot of periodic tables will color-code the symbols of each element (or sometimes write a radioactivity symbol in the box). For example, in this periodic table:

Here is the color code for the specific colors here:

• everything purple, orange, and red is plainly "radioactive" (no stable isotopes)

• orange (bismuth) indicates the half-life is large enough that it's practically stable (#t_(1"/"2) ~~ 10^12# #"y"#)

• thorium and protactinium (in red) are mononuclidic (one naturally-occurring radioisotope)

• [uranium is the only radioactive element with more than one naturally-occurring isotope (three, in fact)]

• all elements with a black mark in the upper-left corner is purely synthetic (made in the lab, and do not occur in nature)