Right: This model explains all known subatomic particles, SOME OF WHICH WERE only recently discovered.
Right: This model explains all known subatomic particles, SOME OF THEM only recently discovered.
Right: This model explains all known subatomic particles, SOME only recently discovered.

Notice that only the which construction has a working verb {were} in it.
can someone pls explain why the working verb is required only with 'which'

It's actually the 2nd and 3rd examples that are exceptions. I might say "I bought new clothes, some for work, some for school." However, this is just a shorthand way of saying "some of which were for work, and some of which were for school." When we say "some of which" or "some of whom," we are modifying the pronoun "some," which must then be followed by a verb.

Consider these examples:

I went camping with my cousins, three of whom were very old.

I passed several police officers, none of whom noticed my large bags of money.

The inspector took photographs of the mansion, half of which had been destroyed by fire.

Note that we could *almost* break each of these into two sentences, like this:

I went camping with my cousins. Three were very old.
I passed several police officers. None noticed my large bags of money.
The inspector took photographs of the mansion. Half had been destroyed by fire.

However, in the second sentence in each example, we have an ambiguous subject. (Three of what?) We can't start a sentence with "Three of whom" or "Half of which," so we tack the second sentence onto the first as a dependent clause.

_________________
Dmitry Farber
Manhattan GMAT Instructor