Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following properties of the noble gases 1\. they readily form compounds which are colourless 2\. they generally do not form ionic compounds. 3\. they have variable oxidation states in their compounds 4\. they generally do not form covalent compounds Select the correct properties. (a) (b) 1,3 (c) \(2,3,4\) (d) \(1,2,3\)

Step-by-Step Solution

Answer

Option (d) 2,3 is correct.

Step 1: Understand the Nature of Noble Gases

Noble gases are a group of elements that are known for their lack of reactivity due to having full valence electron shells. This inert nature means they are typically unreactive under normal conditions.

Step 2: Analyze Property 1

Consider the first property: 'they readily form compounds which are colourless'. Noble gases do not readily form compounds because of their stable electronic configuration. However, when they do, such compounds (like xenon hexafluoride) can be colorless. This property can be conditionally true but is not a defining characteristic.

Step 3: Analyze Property 2

Assess the statement: 'they generally do not form ionic compounds'. This is true because noble gases have no tendency to form ions due to their full valence shells, making it energetically unfavourable to gain or lose electrons.

Step 4: Analyze Property 3

Examine the claim: 'they have variable oxidation states in their compounds'. While noble gases are typically inert, under certain conditions, heavier noble gases like xenon form compounds where they exhibit multiple oxidation states, such as +2, +4, +6, and +8.

Step 5: Analyze Property 4

Consider the statement: 'they generally do not form covalent compounds'. This statement is partially true; while noble gases typically do not form covalent compounds, some noble gas elements (especially xenon) can form certain covalent compounds under specific conditions.

Step 6: Evaluate the Options

Given the analysis, (b) 1,3 and (d) 1,2,3 align best with the noble gas properties discussed. However, statement 1 is often not generally true as noble gases do not "readily" form compounds. Thus, the most accurate depiction is (d) 2,3.

Key Concepts

Chemical Reactivity

The chemical reactivity of noble gases is quite low compared to most other elements. This is due to their full valence electron shells, which provide them with a very stable electronic configuration. Typically, atoms are reactive because they aim to achieve a stable electron configuration similar to that of noble gases, often by gaining, losing, or sharing electrons. However, noble gases already have this stable arrangement, making them less likely to react with other elements.

There are exceptions under specific conditions. Heavy noble gases like xenon can be coerced into reactions, particularly with very electronegative elements like fluorine and oxygen, forming compounds such as xenon hexafluoride (XeF_6). These reactions often require high pressures or temperatures, which are conditions not normally encountered in everyday surroundings.

• Their nonreactivity also means they do not readily form compounds, which is why they are often used in conditions that require non-reactive atmospheres, like in lighting technologies and in gas chromatography.

Oxidation States

Oxidation states indicate the degree of oxidation (loss of electrons) that an atom can undergo. While noble gases are known for their general inertness, the heavier noble gases can exhibit variable oxidation states in their compounds under certain conditions.

Xenon is the most notable noble gas that forms compounds with distinct oxidation states, such as +2 in xenon difluoride (XeF_2), +4 in xenon tetrafluoride (XeF_4), and +6 in xenon hexafluoride (XeF_6). This versatility in oxidation states allows xenon to form a range of fluorides and oxides when under specific laboratory conditions.

• This behavior is unusual for noble gases and is largely facilitated by xenon's relatively large atomic size and lower ionization energy compared to lighter noble gases.
• The ability to form compounds with different oxidation states expands the chemical realm of noble gases, debunking the myth of their complete inertness.

Ionic Compounds

Ionic compounds are chemical compounds composed of ions held together by electrostatic forces termed ionic bonding. These typically involve a metal ion donating electrons to a non-metal. However, noble gases do not normally form ionic compounds due to their complete valence electron shells.

For an atom to form an ion, it must either lose or gain electrons to achieve a stable noble gas configuration. Since noble gases are already in the most stable electron configuration, they have no tendency to form ions. Therefore, forming ionic compounds is energetically unfavorable for noble gases.

• This behavior exemplifies their reputation for being chemically inert.
• As they naturally resist forming ions, noble gases maintain their neutral, electrically balanced state, making ionic compound formation practically impossible.

Covalent Compounds

Covalent compounds are characterized by the sharing of electron pairs between atoms. Although noble gases typically do not form covalent compounds due to their complete valence shells, there are exceptions.

Xenon, for example, can form covalent bonds with fluorine and oxygen, resulting in compounds like XeF_2 and XeO_3. These occur under specific conditions such as high pressure or in the presence of strong fluorine sources, which provide the energy and environment needed for covalent bond formation.

• This demonstrates that, while uncommon, noble gas involvement in covalent bonds is possible, primarily with elements whose atoms have high electronegativity.
• Such compounds are rare and typically only synthesized for study in labs, highlighting the unique conditions required for noble gases to engage in covalent bonding.