Problem 14 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statements is correct for \(\mathrm{CsBr}_{3}\) ? (a) It is a covalent compound. [1996-1 Mark] (b) It contains \(\mathrm{Cs}^{3+}\) and \(\mathrm{Br}^{-}\)ions. (c) It contains \(\mathrm{Cs}^{+}\)and Br_ions (d) It contains \(\mathrm{Cs}^{+}\), and \(\mathrm{Br}^{-}\)and lattice \(\mathrm{Br}_{2}\) molecule

Step-by-Step Solution

Answer

Option (c) is correct: It contains \( \mathrm{Cs}^{+} \) and \( \mathrm{Br}^{-} \) ions.

Step 1: Identify the Components of CsBr3

The compound given is \( \mathrm{CsBr}_{3} \). Analyze its composition. Cs is the symbol for cesium, which is an alkali metal, and Br is bromine, a halogen. The formula suggests the presence of cesium and bromine atoms.

Step 2: Determine the Possible Ion Formation

Cesium, being in Group 1 of the periodic table, typically forms \( \mathrm{Cs}^{+} \) ions. Bromine usually forms \( \mathrm{Br}^{-} \) ions since it is a halogen with a tendency to gain an electron to achieve a stable electron configuration.

Step 3: Analyze the Formula's Composition

For \( \mathrm{CsBr}_{3} \), one might misinterpret it as \( \mathrm{Cs}^{3+} \) and \( \mathrm{Br}^{-} \). However, \( \mathrm{Cs}^{3+} \) is highly unlikely as cesium commonly forms only \( \mathrm{Cs}^{+} \) due to its low ionization energy as an alkali metal. Two \( \mathrm{Br}^{-} \) ions balance the one \( \mathrm{Cs}^{+} \) charge typically.

Step 4: Evaluate the Structure and Ions

Since \( \mathrm{Cs}^{3+} \) is unlikely, the possibility of a simple combination of \( \mathrm{Cs}^{+} \) ions with multiple bromine structures needs consideration. The term \( \mathrm{Br}_{2} \) in lattice form does not exist. These confusions often arise from misrepresentation or hypothetical compounds in questions.

Step 5: Identify the Correct Statement

Comparing the options with the possible arrangement and properties of the ions forming from alkali metals and halogens, Option (c) is closely aligned with expected electron behavior: \( \mathrm{Cs}^{+} \) ion and \( \mathrm{Br}^{-} \).

Key Concepts

Alkali Metals

Alkali metals are elements found in Group 1 of the periodic table. These include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are known for having a single electron in their outermost shell, which they can easily lose to form cations with a charge of +1. This characteristic makes them very reactive, especially with water and halogens.

Key properties of alkali metals are:

• Softness: They can be cut with a knife.
• Low density: Some of them, like lithium, sodium, and potassium, can float on water.
• Good conductors of electricity.
• Highly reactive: They must be stored under oil to prevent them from reacting with moisture in the air.

Chemically, alkali metals form ionic compounds by transferring their lone outer electron to a more electronegative element, such as a halogen. For example, in the exercise, cesium (Cs) forms the cesium ion ( \(\mathrm{Cs}^{+}\) ) by losing its valence electron, which pairs readily with a \(\mathrm{Br}^{-}\) from bromine in ionic compounds, due to its attraction to negatively charged ions.

Halogens

Halogens are a group of elements located in Group 17 of the periodic table. This group includes fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The name "halogen" originates from Greek, meaning "salt-producing," because when they react with metals, they form a wide range of salts, including common table salt, or sodium chloride (NaCl).

Characteristic properties of halogens include:

• Exist as diatomic molecules, such as F₂ and Br₂, in their elemental forms.
• High electronegativity: They easily attract electrons during chemical reactions.
• Highly reactive, particularly with alkali metals and alkaline earth metals to form ionic compounds.

In ionic compounds, halogens typically become anions with a -1 charge as they gain an electron from another element. For instance, bromine becomes a bromide ion ( \(\mathrm{Br}^{-}\) ) in the presence of metals like cesium. This electron transfer creates a stable ionic bond that forms the basis for many compounds, highlighting the distinguished role that halogens play in chemical combinations.

Ion Formation

Ion formation is a fundamental concept in chemistry involving the transfer or sharing of electrons to achieve a stable electron configuration. This process creates ions, which are atoms or molecules with a net electric charge due to the loss or gain of one or more electrons.

Here's how ion formation works with alkali metals and halogens:

• Alkali metals lose their single valence electron to become positively charged ions ( \(\mathrm{Cs}^{+}\) ), also known as cations.
• Halogens gain an electron to fill their valence shell, resulting in negatively charged ions ( \(\mathrm{Br}^{-}\) ), known as anions.

This electron transfer is crucial in forming stable ionic compounds. The metal donates an electron to the non-metal, leading to the attractive force between positively and negatively charged ions. Such interactions often result in the formation of a crystalline lattice structure characteristic of ionic solids. These structures are responsible for many properties of ionic compounds, such as high melting points and electrical conductivity when dissolved in water or molten.

Understanding the principles of ion formation helps in predicting the formulas and behavior of numerous compounds, like the cesium bromide compound discussed, where cesium and bromine naturally form \(\mathrm{Cs}^{+}\) and \(\mathrm{Br}^{-}\) ions, respectively, leading to a stable ionic compound.