If you look at the dates on the scripture chapters, you will see that they go from 130 B.C. in Omni, to A.D. 385 in Words of Mormon, and then back to 124 B.C. for Mosiah 1! Why did it go forward a few hundred years? Did the Nephites invent time travel?! No, Words of Mormon is basically Mormon saying, “Alright, so I abridged everything after Amaleki, then I copied down these other plates.”
But what chapters of Mormon does Words of Mormon take place between? Well, it must be after he gets the records: “And now I, Mormon, seeing that the Lamanites were about to overthrow the land, therefore I did go to the hill Shim, and did take up all the records which Ammaron had hid up unto the Lord.”, So after Mormon 4. The date for Words of Mormon is about A.D. 385, so what chapters have the same datestamps? The only three chapters in the Book of Mormon that do are, Words of Mormon (of course), Mormon 5, and Mormon 6. He probably started abridging as soon as he got the plates. This is probably written about the same time of Mormon 6:6, since that verse and Words of Mormon verse one both talk about giving Moroni some records: “…hid up in the hill Cumorah all the records which had been entrusted to me by the hand of the Lord, save it were these few plates which I gave unto my son Moroni.” and “And now I, Mormon, being about to deliver up the record which I have been making into the hands of my son Moroni, behold I have witnessed almost all the destruction of my people, the Nephites.”, respectively.