How many unpaired electrons are in the ground state electron configuration of selenium?
Enter your answer as a number symbol (i.e. 1 or 3 or 5) not as a word (one or three or five).
The Correct Answer and Explanation is:
Answer: 2
Explanation:
Selenium (Se) has an atomic number of 34, meaning it has 34 electrons in its neutral ground state. To determine how many unpaired electrons it has, we need to examine its electron configuration and apply Hund’s Rule and the Pauli Exclusion Principle.
Step 1: Determine the Electron Configuration
The ground-state electron configuration of selenium is:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
Breaking it down:
- The core electrons (1s² to 3p⁶) are all in filled subshells with paired electrons.
- The 4s² and 3d¹⁰ subshells are also completely filled.
- The 4p⁴ subshell is where the unpaired electrons occur.
Step 2: Focus on the 4p Subshell
The p-subshell can hold a maximum of 6 electrons in three orbitals (pₓ, pᵧ, p_z), with each orbital capable of holding 2 electrons of opposite spin.
In selenium, the 4p⁴ subshell has 4 electrons. According to Hund’s Rule, electrons occupy orbitals singly first before pairing. So:
- The first 3 electrons go into 3 separate p orbitals: ↑ ↑ ↑
- The 4th electron must pair with one of them: ↑↓ ↑ ↑
So, there are 2 unpaired electrons in the 4p subshell.
Step 3: Total Unpaired Electrons
All other subshells (1s, 2s, 2p, 3s, 3p, 3d, 4s) are fully filled and have no unpaired electrons. Only the 4p⁴ contributes unpaired electrons.
Therefore, selenium has 2 unpaired electrons in its ground-state electron configuration.
Conclusion
Understanding the electron configuration and using Hund’s Rule helps us conclude that the number of unpaired electrons in selenium is 2, which influences its chemical reactivity, magnetic properties, and bonding behavior.
