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Elements

Chapter: Basic Concepts of Chemistry

Question 1 of 4 NDA MCQ

Which one of the following elements is polyatomic?

Detailed Explanation

Correct Answer

✓ Option (a) is correct. Phosphorus is commonly found as a polyatomic molecule, P4.

📖 Governing Law / Core Concept

Polyatomic elements consist of multiple atoms bonded chemically and exist in specific molecular forms, such as P4 for phosphorus and S8 for sulfur.

🧪 Step-by-Step Breakdown

Phosphorus (P): Exists as tetrahedral P4 molecules, represented as:

\[ \text{P}_4 = \text{P} - \text{P} - \text{P} - \text{P} \]

Sulfur (S): Typically forms octatomic S8 rings, shown as:

\[ \text{S}_8 = \text{S} - \text{S} - \text{S} - \text{S} - \text{S} - \text{S} - \text{S} - \text{S} \]

🔍 Option Analysis

Option (b): Sulfur - Also polyatomic as S8.

Option (c): Chlorine - Exists as Cl2, a diatomic molecule.

Option (d): Aluminium - Present as Al, a monoatomic element.

⚡ Mnemonic / Speed-Run

Remember: "P4 and S8 for Polyatomic Presence!"

📖 Factual Verification & Reference:

Verified against NCERT Class XI Chemistry, Chapter 2 (Structure of Atom).

Question 2 of 4 NDA MCQ

Which one among the following elements is known to be discovered the earliest?

Detailed Explanation
✓

1. Correct Answer

Option A is correct. Copper was the first metal to be widely used by humans.

📖 Core Concept

The question concerns the historical discovery of elements, specifically focusing on which element was known and used by ancient civilizations first.

💡 3. Explanation

Copper (Cu) is one of the first metals discovered and utilized by humans, with archaeological evidence suggesting its use dates back to around 9000 BCE in various regions, including the Middle East. The natural states of metal allowed for easy extraction and shaping, leading to its early prominence in tools and ornaments. In contrast, other elements listed, such as oxygen (O) and uranium (U), were discovered much later in human history, with oxygen being identified in the late 18th century and uranium in the late 18th to early 19th century.

❌ 4. Option Analysis

  • •Option B: Gold (Au) was used as early as 4000 BCE but did not precede copper in terms of widespread utilization.
  • •Option C: Oxygen (O) was discovered in the 18th century and thus cannot be among the earliest discovered elements.
  • •Option D: Uranium (U) was first isolated in 1789, making it the latest of the options presented.
⚡

NDA Speed-Run / Mnemonic Shortcut

Remember: Copper predates gold, oxygen, and uranium in human history. Use the mnemonic 'C’ comes before 'G', 'O', and 'U' to recall this fact.

🎯

5. Key Takeaways

Copper is recognized as one of the first metals utilized by humans, around 9000 BCE, making it the earliest discovered element among the provided options.

Question 3 of 4 NDA MCQ

Match List I with List II and select the answerusing the code given below the Lis

Detailed Explanation

Correct Answer

✓ Option A is correct. The atomicity matches as follows: Phosphorus (P4), Nitrogen (N2), Neon (Ne), and Sulfur (S8).

📖 Governing Law / Core Concept

Atomicity relates to the number of atoms in a molecule of a substance. Elements exhibit varying atomicity based on their molecular structures and bonding.

🧪 Step-by-Step Breakdown

  • Phosphorus exists primarily as P4 (tetraatomic).
  • Nitrogen is diatomic, represented as N2.
  • Neon is a noble gas and exists as monatomic Ne.
  • Sulfur typically forms S8 (octatomic) rings.

🔍 Option Analysis

  • B: Incorrect, as it misattributes nitrogen's atomicity.
  • C: Incorrect; misclassification of Ne and S.
  • D: Incorrect, implies incorrect atomicities.

⚡ Mnemonic / Speed-Run

Mnemonic for Atomicity: "In every contest, Neon is never sulfuring, Phosphorous Tetra!" This helps recall: P4, N2, Ne, S8.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class XI Chemistry, Chapter 4 (Chemical Bonding).

Question 4 of 4 NDA MCQ

Tooth enamel is made up of which one of the following calcium compounds ?

Detailed Explanation

Correct Answer

✓ Option (d) is correct. Tooth enamel is primarily composed of calcium phosphate.

📖 Core Biological Concept

Tooth enamel, the hardest substance in the human body, is mainly made of a mineral called Hydroxyapatite, which is a crystalline structure of calcium phosphate.

🔬 Anatomical & Physiological Mechanism

Hydroxyapatite can be represented as:

\[ Ca_{10}(PO_4)_6(OH)_2 \]

This chemical structure indicates that tooth enamel has a very high mineral content, resulting in its exceptional strength and durability.

🔍 Option Analysis

  • Calcium carbonate (A): Mainly found in rocks and shells but not in tooth enamel.
  • Calcium sulphate (B): Often used in construction, does not form part of tooth structure.
  • Calcium hydroxide (C): Used in dental treatments but not a component of enamel.

⚡ Memory Hook (Mnemonic)

Remember "Calcium Phosphate is for the toughest teeth," associating phosphate with dental strength.

6. Visual Suggestion

6. Visual Suggestion

A detailed diagram showing the structure of tooth enamel, highlighting the Hydroxyapatite crystals and their arrangement, can help visualize the mineral composition.

📖 Factual Verification & Reference

📖 Factual Verification & Reference:

Verified against NCERT Class 12 Biology, Chapter 14 (Biomolecules).

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