Find the number of unpaired electrons in (a) P, (b) Si, (c) Cr, (d) Fe and (e) Kr.

To determine the number of unpaired electrons in various elements, we need to look at their electron configurations. Unpaired electrons are those that are alone in an orbital, which can influence an atom's magnetic properties and reactivity. Let’s break down each element one by one.

Phosphorus (P)

Phosphorus has an atomic number of 15, which means it has 15 electrons. Its electron configuration is:

• 1s² 2s² 2p⁶ 3s² 3p³

In the 3p subshell, there are three electrons. According to Hund's rule, these electrons will occupy separate orbitals before pairing up. Therefore, all three electrons in the 3p subshell are unpaired. Thus, phosphorus has 3 unpaired electrons.

Silicon (Si)

Silicon has an atomic number of 14, giving it 14 electrons. Its electron configuration is:

• 1s² 2s² 2p⁶ 3s² 3p²

In the 3p subshell, there are two electrons. Following Hund's rule, both electrons will occupy separate orbitals, resulting in 2 unpaired electrons for silicon.

Chromium (Cr)

Chromium has an atomic number of 24. Its electron configuration is a bit unique due to stability preferences:

• [Ar] 4s¹ 3d⁵

Here, the 4s subshell has one electron, and the 3d subshell has five electrons. All five electrons in the 3d subshell are unpaired because they occupy separate orbitals. Therefore, chromium has 6 unpaired electrons (1 from 4s and 5 from 3d).

Iron (Fe)

Iron has an atomic number of 26, leading to the following electron configuration:

• [Ar] 4s² 3d⁶

In the 3d subshell, there are six electrons. According to Hund's rule, the first five electrons will occupy separate orbitals, and the sixth will pair with one of them. Thus, iron has 4 unpaired electrons (5 in separate orbitals minus 1 paired).

Krypton (Kr)

Krypton, with an atomic number of 36, has a complete electron configuration:

• [Ar] 4s² 3d¹⁰ 4p⁶

All the orbitals in krypton are fully filled, meaning there are no unpaired electrons. Therefore, krypton has 0 unpaired electrons.

Summary

To summarize the number of unpaired electrons in each element:

• Phosphorus (P): 3 unpaired electrons
• Silicon (Si): 2 unpaired electrons
• Chromium (Cr): 6 unpaired electrons
• Iron (Fe): 4 unpaired electrons
• Krypton (Kr): 0 unpaired electrons

Understanding the distribution of electrons helps us predict the chemical behavior of these elements, including their bonding and magnetic properties.