The electronic configuration of an element is 1S$^2$ 2S$^2$ 2P$^6$ 3S$^2$ 3P$^3$. How many unpaired electrons are there in the element?

Analysis

The question asks us to determine the number of unpaired electrons in an element based on its electronic configuration: 1s² 2s² 2p⁶ 3s² 3p³.

Concept: Hund’s Rule of Maximum Multiplicity

To find the number of unpaired electrons, we look at the valence shell (the outermost shell). Electrons in fully filled orbitals are paired, while electrons in partially filled orbitals may be unpaired.

According to Hund’s Rule, when filling orbitals of the same energy (like the three p-orbitals), electrons occupy them singly first before pairing up.

Step-by-Step Solution

1. Identify the valence subshell: Looking at the configuration 1s² 2s² 2p⁶ 3s² 3p³, the outermost subshell is 3p.

2. Analyze the 3p subshell: The p-subshell consists of three orbitals: $p_x$, $p_y$, and $p_z$. Each orbital can hold a maximum of 2 electrons.

3. Distribute the 3 electrons into the 3p orbitals: Following Hund’s Rule, we place one electron into each of the three p-orbitals: • $p_x$: 1 electron • $p_y$: 1 electron • $p_z$: 1 electron

4. Count the unpaired electrons: Since each of the three p-orbitals contains exactly one electron, there are 3 unpaired electrons. The inner shells (1s², 2s², 2p⁶, 3s²) are all completely filled, meaning all electrons in those shells are paired.

Conclusion

The element (which is Phosphorus, atomic number 15) has 3 unpaired electrons in its 3p subshell.

Exam Tip

When asked for unpaired electrons, always write out the subshell boxes. For any p³ configuration, you will always have 3 unpaired electrons. For d⁵, you will have 5 unpaired electrons. This is a common JAMB pattern, so mastering the orbital box diagram will save you time!