Which of the following goes against the Aufbau principle?
1s^22s^2
1s^22s^12p^5
1s^22s^22p^2
1s^1
To determine which of the given electron configurations goes against the Aufbau principle, we first need to understand what the Aufbau principle states.
The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy. Specifically, electrons fill the lowest energy orbitals first before moving to higher energy orbitals.
Now let's analyze each option:
1. 1s^22s^2: This configuration follows the Aufbau principle since the 1s orbital is the lowest energy orbital and is filled first, followed by the 2s orbital.
2. 1s^22s^12p^5: This configuration violates the Aufbau principle because it fills the 2p orbital before completely filling the 2s orbital. According to the Aufbau principle, the 2s orbital should be filled before moving to higher energy orbitals such as 2p.
3. 1s^22s^22p^2: This configuration follows the Aufbau principle since it fills the 2s orbital completely before moving to the 2p orbital.
4. 1s^1: This configuration follows the Aufbau principle since it fills the 1s orbital completely.
Therefore, the configuration that goes against the Aufbau principle is option 2: 1s^22s^12p^5.