Animated Solution for Chemistry - s and p-Block Elements: The main oxides formed on combustion of Li, Na and K in excess of air respectively are

Burning Alkali Metals

Imagine you are in a chemistry lab, and you have three shiny pieces of metal: Lithium, Sodium, and Potassium. You decide to burn them in an environment with plenty of oxygen. What happens? They don't all react the same way! This simple experiment reveals a profound principle of inorganic chemistry: size compatibility.

Like Stabilizes Like

In the world of ionic compounds, stability is governed by lattice energy. A crystal lattice is most stable when the sizes of the positive ions (cations) and negative ions (anions) are well-matched.

- A small cation has a high charge density and strongly attracts a small anion. - A large cation has a lower charge density and is better suited to stabilize a large, bulky anion.

Let's see how this rule dictates the combustion products of our three metals.

The Tiny Powerhouse

Lithium ($\text{Li}$) is the first element in the alkali metal group. When it loses an electron to become ${\text{Li}}^{+}$, it is incredibly small. Because of its tiny volume, its single positive charge is highly concentrated.

When Lithium burns in oxygen, it seeks out the smallest possible oxygen anion to form a stable lattice. That anion is the normal oxide ion, ${\text{O}}^{2-}$.

Thus, Lithium predominantly forms Lithium Oxide.

The Middle Child

Moving down the group, we find Sodium ($\text{Na}$). The ${\text{Na}}^{+}$ ion is significantly larger than ${\text{Li}}^{+}$. Because it is larger, it cannot pack as tightly with the small ${\text{O}}^{2-}$ ion.

Instead, Sodium finds its perfect match in a slightly larger anion: the peroxide ion, ${\text{O}}_2^{2-}$. The larger size of the peroxide ion perfectly complements the size of the Sodium cation, leading to a highly stable crystal structure.

Therefore, Sodium burns to form Sodium Peroxide.

The Gentle Giant

Finally, we arrive at Potassium ($\text{K}$). The ${\text{K}}^{+}$ ion is quite large. If it tried to bond with the small oxide ion, the lattice would be unstable because the large cations would repel each other before they could get close enough to the small anions.

To achieve maximum stability, Potassium needs a very large anion. It finds this in the superoxide ion, ${\text{O}}_2^{-}$. The bulky superoxide ion is the perfect companion for the large Potassium cation.

Hence, Potassium forms Potassium Superoxide.

By simply looking at the atomic radii, we can predict the chemical behavior of these metals. Lithium forms the oxide (${\text{Li}}_2\text{O}$), Sodium forms the peroxide (${\text{Na}}_2{\text{O}}_2$), and Potassium forms the superoxide (${\text{KO}}_2$). This elegant progression is a testament to the predictive power of periodic trends and lattice energy!