Animated Solution for Chemistry - Periodicity in Properties: Three elements $X$, $Y$ and $Z$ are in the 3rd period of the periodic table. The oxides of $X$, $Y$ and $Z$, respectively, are basic, amphoteric and acidic. The correct order of the atomic number of $X$, $Y$ and $Z$ is

The Periodic Table is not just a random arrangement of elements; it is a beautifully orchestrated map of chemical behavior. In this problem, we are given three mysterious elements—$X$, $Y$, and $Z$—all residing in the 3rd period. We don't know their exact identities, but we are given a crucial clue: the nature of their oxides.

The problem states that the oxides of $X$, $Y$, and $Z$ are basic, amphoteric, and acidic, respectively. This is the master key to unlocking their identities.

In chemistry, there is a fundamental relationship between the metallic character of an element and the acid-base nature of its oxide. Metals, being electropositive, tend to form basic oxides (like $N{a}_{2}O$ or $MgO$). When these oxides dissolve in water, they form alkaline solutions.

On the other end of the spectrum, non-metals, being electronegative, form acidic oxides (like $S{O}_3$ or $P_4{O}_{10}$). These oxides react with water to form acids.

Caught in the middle are the metalloids (and some weakly metallic elements), which form amphoteric oxides (like $A{l}_2{O}_3$). These unique oxides can act as both acids and bases depending on what they react with.

Armed with this knowledge, we can immediately classify our elements: - Element $X$ forms a basic oxide $\Rightarrow$ $X$ is a Metal. - Element $Y$ forms an amphoteric oxide $\Rightarrow$ $Y$ is a Metalloid (or exhibits intermediate character). - Element $Z$ forms an acidic oxide $\Rightarrow$ $Z$ is a Non-metal.

Now, let's think about the geography of the periodic table. As we traverse any period from left to right, the atomic number ($Z$) steadily increases. Simultaneously, the effective nuclear charge increases, pulling the valence electrons closer. This causes the elements to hold onto their electrons more tightly, meaning the metallic character decreases and the non-metallic character increases.

Because metals are located on the far left of a period, metalloids in the middle, and non-metals on the far right, their atomic numbers must follow the same left-to-right sequence.

Therefore, the metal $X$ must have the lowest atomic number, followed by the intermediate element $Y$, and finally the non-metal $Z$ with the highest atomic number.

This gives us the final, elegant relationship: $X<Y<Z$.

To make this concrete, look at the actual 3rd period: Sodium ($Na$) and Magnesium ($Mg$) are metals forming basic oxides. Aluminum ($Al$) forms an amphoteric oxide. Phosphorus ($P$), Sulfur ($S$), and Chlorine ($Cl$) are non-metals forming acidic oxides. The atomic numbers perfectly align with our derived order!