If plutonium-238 (Pu-238) absorbs a neutron, does it become Pu-239? I am asking this simple question because I am always hearing about how thorium reactors are less perilous to the world because, unlike uranium reactors, they produce some Pu-238, which is not suitable for a bomb, rather than Pu-239, which is.
But can't Pu-238 be just as easily turned into Pu-239 by absorption of a single neutron, just like U-238 can?
 A: Yes, when Pu-238 absorbs a neutron it becomes Pu-239. However, it's extremely difficult to separate Pu-238 from Pu-239, much harder than separating U-235 from U-238.
Pu-239 is normally produced by subjecting U-238 to neutrons. U-239 emits a beta, converting to Np-239, which also emits a beta, converting to Pu-239. Both those steps have relatively short half-lives, and chemical techniques can be used to separate the elements from one another.
Isotopes of the same element cannot be separated chemically, only physically, and the separation rate is a function of the mass ratio. Isotope separation is necessary in the enrichment of uranium, and that process is very slow because U-238 is only slightly more massive than U-235. The separation processes almost always use uranium hexafluoride gas. Fluorine has an atomic mass of 19, so the mass ratio is around $(238 + 6×19)/(235 + 6×19)$.
Uranium hexafluoride is nasty stuff which requires very careful handling. It reacts with water (even traces of water in the atmosphere or equipment) to produce hydrofluoric acid (which is highly toxic), and of course it's radioactive.
The prime reason that Pu-239 is used is because it's a lot easier to produce (using the process mentioned earlier) than it is to enrich natural uranium.
Similar techniques to uranium enrichment can be used to separate Pu-238 and Pu-239, but the mass ratio is much worse. Here are the mass ratios, using 7 digit atomic mass numbers for the hexafluorides.




Element
Mass ratio




Uranium
$1.008615$


Plutonium
$1.002858$




In addition, plutonium hexafluoride is even nastier than uranium hexafluoride. It's quite corrosive, and it's prone to auto-radiolysis, which means that the energy released in the radioactive decay of the plutonium has a tendency to chemically decompose it. This is especially the case with Pu-238.
Plutonium-238 is an alpha emitter with a half-life of ~87.7 years. It's a preferred fuel for radiothermal generators (RTGs) because it emits a lot of heat, for decades. Here's a photo (courtesy of the US DOE, via Wikipedia) of a small pellet of Pu-238 dioxide, glowing from its own heat.

The alpha particles produced inside the pellet lose most of their kinetic energy creating heat and doing general damage. Similarly, any Pu-238 present in plutonium destined for weapons use causes damage to the crystal structure (as well as being a neutron absorber).
Of course, plutonium hexafluoride in the gaseous state doesn't trap alpha particles like a solid compound does, so it doesn't get quite as hot, but the energy from all those alphas is still a problem.

Your question is about plutonium in the context of a thorium reactor. It takes (at least) 7 neutrons to get from Th-232 to Pu-239. You aren't going to get much of it produced in a simple thorium reactor cycle, although some plutonium is likely to be produced in a reactor that uses recycling. However, thorium reprocessing is difficult. Some of the waste isotopes are heavy gamma emitters which can only be handled remotely. This has been a significant barrier in the commercial development of thorium-based fission power.
A: According to [1],

Neutron absorption in Pu-238 that does not cause fission instead
generates Pu-239.

Let us, however. make it clear what specifically we are trying to compare. If it is Pu-238 and Pu-239, then one can use the latter in weapons directly, and the former needs to be used as a fuel in a nuclear reactor first. Not everyone uses the word "easily" for this process:-)
