The number of unpaired valence electrons in an atom of phosphorus is : (a) 0 (b) 2 (c) 3 (d) 4

Understanding the periodic table is crucial when determining the electron configuration of an element. For phosphorus, located in group 15, determining the number of valence electrons is straightforward when you know its group number. The periodic table is arranged in such a way that elements in the same group typically have the same valence shell electron configuration. This regularity makes it easier to predict the chemical properties of an element. For instance, all elements in group 15 have five valence electrons.

When looking at the period to which phosphorus belongs, it's located in the third period, indicating that these valence electrons occupy the third shell. As an 'A' group element, its valence electrons are placed in the s and p subshells. Therefore, with this understanding of the periodic table, simply by knowing an element's group and period, you can gain valuable information about its electron configuration and potential reactivity.

With the electron configuration of phosphorus partially understood, applying Hund's rule gives a complete picture of how its valence electrons are distributed. Hund's rule states that electrons must fill all degenerate orbitals singly before any orbital gets a second electron. Translated into simpler terms, within the same subshell, electrons prefer to have their own orbital (room) with parallel spins (like having all clocks' hands pointing in the same direction), before they have to share an orbital.

For phosphorus, with its five valence electrons, this rule means that after filling up the 3s orbital with two electrons, the remaining three will each take up one of the three available 3p orbitals. These electrons stay unpaired as long as there are empty orbitals available in that subshell. This distribution minimizes electron-electron repulsions and results in a lower energy and more stable arrangement for the atom.

Delving into the concept of p-orbitals can help to clarify why phosphorus has three unpaired electrons. A p-orbital is a dumbbell-shaped region around an atom where an electron is likely to be found. Each principal energy level above the first has three p-orbitals that are degenerate, meaning they have the same energy level. These are denoted as px, py, and pz.

Looking at phosphorus again, it's in the third period, so it uses the 3p orbitals for its valence electrons beyond the first two, which occupy the 3s orbital. With three electrons remaining and explaining through Hund's rule, each of these electrons occupies a separate p-orbital to minimize repulsion: one in 3px, one in 3py, and one in 3pz. This arrangement provides a visual insight into how these unpaired valence electrons are spatially distributed in the atom, giving the atom its chemical and magnetic properties.