   Chapter 21, Problem 1PS

Chapter
Section
Textbook Problem

Which of the following formulas is incorrect? (a) CaH2 (b) CaI2 (c) CaS (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.

MMn++ne

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

X+neXn

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.

The incorrect formula among the given choices is Ca2O3.

Explanation

Reason for the correct formulas

Calcium belongs to group 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: [Ar]4s2

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 CaH2 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: [Kr]4d105s25p5

Hence, the charge on iodine is 1. Hence, the given formula CaI2 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: [Ne]3s23p4

The electronic configuration of Oxygen is: 1s22s22p4

Hence, the charge on sulfur is 2. Hence, the given formula CaS is correct. Thus, option c is correct.

Reason for the incorrect formula

Calcium belongs to group 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: [Ar]4s2

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

The electronic configuration of Oxygen is: 1s22s22p4

Thus, the charge on calcium ion is +2 and the charge on oxygen ion is 2. Thus, the formula is CaO.

Hence, the given formula Ca2O3 is incorrect. So, option d is incorrect.

Conclusion

The incorrect formula among the given choices is Ca2O3. Thus, option d is incorrect.

The options a, b, c are correct.

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