Division of Scripture into chapters and verses was not a Jewish invention. (These seem to have been the invention of a sixteenth century French publisher.) The Rabbis divided the Torah into portions corresponding to the weekly cycle of public readings. Each portion encompassed a unifying theme or related to a specific event or set of related events. Given this fact, it is difficult to understand why the Rabbis divided the narrative of the plagues into two portions: seven outlined in the first and three in the following. In his masterful commentary, Don Isaac Abarbanel, a refugee of the expulsion from Spain in 1492, gives three reasons for the serialization of the plague narrative.
First, with the eighth plague (locusts), Pharaoh and his servants express for the first time a sense of fear in anticipation of the coming plague. This marks a turning point in the narrative. Pharaoh’s advisors now counsel Pharaoh to release the Hebrews and avoid further suffering (Exodus 10:7). To underline the dramatic shift, the eighth plague leads off the second part of the plague narrative.
Second, as a result of the advice he received, Pharaoh is now willing to make an accommodation he refused previously. His stubbornness has given way to resignation. Pharaoh now tries to minimize his losses.
And third, the final three plagues (locusts, darkness, and the death of Egyptian firstborn) all affected the air, submerging Egypt into darkness. The locusts blotted out the sun (Exodus 10:15). The darkness was total and palpable (Exodus 10:22). And the death of the firstborn occurred in the middle of the night (Exodus 12:29). Together, the three last plagues symbolized the pall of doom that engulfed Egypt.
Each of these three last plagues shares the common factor of darkness. Each of these three last plagues also signals a radical change in Pharaoh’s heart. Hence, their linkage in one portion of the Torah, though distinct from their precursors, is both logical and dramatically effective, conveying the imminent collapse of a once-invulnerable kingdom and the beginning of Israelite redemption.