Final answer:
To achieve an octet, an atom with the electron configuration 1s2s2p1 must gain 3 electrons.
Step-by-step explanation:
The student's question asks how many electrons an atom with the electron configuration 1s22s22p1 must gain to achieve a full valence shell, also known as an octet. To determine this, first recognize that the highest energy level is the second level (2s and 2p orbitals). In a stable octet, the second energy level can hold a maximum of 8 electrons. The atom in question currently has a total of five electrons in the second level (2 in the 2s orbital and 3 in the 2p orbitals). Therefore, it needs to gain 3 more electrons to achieve an octet (1s22s22p6).