Final answer:
The error in the electron configuration 1s2 2s2 2p6 3s2 3p5 4s2 is that it places electrons in the 4s sublevel before filling the 3d sublevel. The correct order should fill the 3d sublevel before the 4s according to the Aufbau principle.
Step-by-step explanation:
The student's question is about identifying the error in the given electron configuration, which is 1s2 2s2 2p6 3s2 3p5 4s2. The error in this configuration is the arrangement of electrons in the 4s sublevel before the 3d sublevel is filled. According to the Aufbau principle, which dictates the order in which sublevels are filled based on increasing energy, after the 3p sublevel is filled, electrons should start filling the 3d sublevel before the 4s. However, in the given configuration, it incorrectly shows two electrons in the 4s sublevel before the 3d sublevel has any electrons. The correct electron configuration after 3p6 should be 3d1 before moving to 4s1 or 4s2 depending on the element in question. For example, the correct configuration for potassium (Z=19) is 1s²2s²2p63s²3p64s¹, and for calcium (Z=20), it is 1s²2s²2p63s²3p64s².