Chapter 21, Problem 1PS

Which of the following formulas is incorrect?

1. (a) CaH₂
2. (b) CaI₂
3. (c) CaS
4. (d) Ca₂O₃

Interpretation Introduction

Interpretation: The incorrect formula among the given choices has to be stated.

Concept introduction: The main group elements form chemical compounds in which the central atom has highest oxidation number. The highest oxidation number is always equal to the group number that the element has in the periodic table. Thus,  the oxidation number is determined by the position of the element in the periodic table.

Ionic compounds are formed by the loss of electrons from the metal which are gained by the non-metals. The metal gets positively charge and the non-metal attains a negative charge thus forming cations and anions respectively. They do so to attain a noble gas configuration or to attain stability.

  $\text{M}\to {\text{M}}^{n+}+n{e}^{-}$

Here, by losing electrons metal M achieve the noble gas configuration. These electrons are gained by the non-metals X as shown below.

    $\text{X}+n{e}^{-}\to {\text{X}}^{n-}$

These elements also form compounds in the other lower oxidation numbers apart from the highest oxidation number. These other oxidation numbers are as per the electronic configuration of the element and is determined by the number of electrons in the outer shell of each element.

The correct formula has the valid oxidation number of the central element and the formula which do not satisfy this criteria is incorrect.

Answer to Problem 1PS

The incorrect formula among the given choices is $C{a}_2{O}_3$.

Explanation of Solution

Reason for the correct formulas

Calcium belongs to group $\text{2A}$ of the periodic table. Hence, the highest oxidation number is $+2$. There are two valence electrons in the outermost shell of calcium. Calcium loses these two electrons to attain noble gas configuration. Thus, the oxidation number of calcium is $+2$.

The electronic configuration of calcium is: $\left[\text{Ar}\right]4{\text{s}}^2$

Hydrogen has an oxidation number of $+1$ or $-1$, depending upon the electronegativity of the element to which it is attached. Metals are more electropositive, so the oxidation number of hydrogen in this case is $-1$. Thus, the charge on calcium ion is $+2$ and the charge on hydrogen ion is $-1$. Hence, the given formula ${\text{CaH}}_{\text{2}}$ is correct. Thus, option a is correct.

Iodine belongs to halogen family in the periodic table. Iodine has seven valence electrons in its outermost shell. Iodine gains one electron to complete its octet. So, the oxidation number of iodine is $-1$.

The electronic configuration of Iodine is: $\left[\text{Kr}\right]{\text{4d}}^{\text{10}}{\text{5s}}^{\text{2}}{\text{5p}}^{\text{5}}$

Hence, the charge on iodine is $-1$. Hence, the given formula ${\text{CaI}}_{\text{2}}$ is correct. Thus, option b is correct.

Sulfur belongs to oxygen family in the periodic table. Both sulfur and oxygen have four valence electrons in their outermost shell. Both sulfur and oxygen gain two electrons to achieve noble gas configuration. So, the oxidation number of sulfur is $-2$.

The electronic configuration of Sulfur is: $\left[\text{Ne}\right]{\text{3s}}^{\text{2}}{\text{3p}}^{\text{4}}$

The electronic configuration of Oxygen is: ${\text{1s}}^{\text{2}}{\text{2s}}^{\text{2}}{\text{2p}}^4$

Hence, the charge on sulfur is $-2$. Hence, the given formula $\text{CaS}$ is correct. Thus, option c is correct.

Reason for the incorrect formula

Calcium belongs to group $\text{2A}$ of the periodic table. Hence, the highest oxidation number is $+2$. There are two valence electrons in the outermost shell of calcium. Calcium loses these two electrons to attain noble gas configuration. Thus, the oxidation number of calcium is $+2$.

The electronic configuration of calcium is: $\left[\text{Ar}\right]4{\text{s}}^2$

Oxygen gains two electrons to attain noble gas configuration. Thus, the oxidation number of oxygen is $-2$.

The electronic configuration of Oxygen is: ${\text{1s}}^{\text{2}}{\text{2s}}^{\text{2}}{\text{2p}}^4$

Thus, the charge on calcium ion is $+2$ and the charge on oxygen ion is $-2$. Thus, the formula is $\text{CaO}$.

Hence, the given formula ${\text{Ca}}_{\text{2}}{\text{O}}_{\text{3}}$ is incorrect. So, option d is incorrect.

Conclusion

The incorrect formula among the given choices is $C{a}_2{O}_3$. Thus, option d is incorrect.

The options a, b, c are correct.