Another new answer:
Group 17 in the periodic system of chemical elements, i. e. the halogens (including hydrogen for that purpose).
What am I? I am a group.
Whell, this is a group in the periodic table.
You can't blame anyone for I being in me.
"I" (iodine) is a halogen, and this is due to the atom structure, so nobody (Mendeleev, Courtois etc.) can be blamed for it.
But I might not anyway.
the word "halogens" contains no I's.
It depends on how you see the sixth in the third.
Well, it depends on how you see the "A" (6th word in the riddle) in the whole group (3rd word in the riddle is "I" which refers to it). If "A" is in the group (so this must be the word "halogens", which contains an A), so "I" is not. On the other side, "A" is not in the group itself, but "I" is.
But I wasn't invoking Godwin's Law!
Unsure. Probably a reference to the word "halogen" being invited by a German.
You are not in me.
"U" (uranium) is not a halogen.
She is not, either.
"S" (sulfur) and "He" (helium) are also not halogens.
But the second is, if you cut it.
The 2nd element (helium) is not a halogen, but can be "cut" (not physically of course, and it requires a LOT of energy) into two hydrogen atoms.
But not the sixtieth of a minute!
As pointed by others, not that kind of second.
Now, what am I, specifically?
Hydrogen and the halogens.