Question

An element in its ground state has the following electron configuration, 1s^2 2s^2 2p^6 3s^2. How many energy levels of the atom are completely filled?

A) 1
B) 2
C) 3
D) 4

Answer 1

The atom with electron configuration 1s² 2s² 2p¶ 3s² has two completely filled energy levels, the first (n = 1) and the second (n = 2).

Step-by-step explanation:

The electron configuration given is 1s² 2s² 2p¶ 3s². This represents an element in its ground state. We need to count the number of electrons and determine how many energy levels are completely filled.

To begin with, we fill the 1s orbital with two electrons, giving us 1s². As such, the n = 1 level or first energy level is completely filled. Next, we fill the 2s orbital with another two electrons, resulting in 1s² 2s². We then proceed to fill the 2p sublevel with six electrons, and now the n = 2 level or second energy level is also completely filled. Finally, the 3s sublevel is filled with two electrons, leading to 1s² 2s² 2p¶ 3s².

The n = 3 level, however, is not completely filled as it is missing electrons in its 3p sublevel. Hence, we conclude that two energy levels are completely filled - the first and the second.

The correct answer to the question is B) 2.