Problem 22 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following compounds are covalent? (a) \(\mathrm{H}_{2}\) (b) \(\mathrm{CaO}\) (c) \(\mathrm{KCl}\) (d) \(\mathrm{Na}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

Only \( \mathrm{H}_{2} \) is a covalent compound.

Step 1: Understand Covalent Bonds

Covalent bonds occur between non-metal atoms sharing electrons. It's important to first identify which elements in each compound are non-metals.

Step 2: Analyze Compound (a) \\( \mathrm{H}_{2} \\)

Both atoms in \( \mathrm{H}_{2} \) are hydrogen, which is a non-metal. Therefore, \( \mathrm{H}_{2} \) is covalent because it is a molecule made by two non-metal atoms sharing electrons.

Step 3: Analyze Compound (b) \\( \mathrm{CaO} \\)

Calcium (Ca) is a metal and oxygen (O) is a non-metal. Compounds formed by metals and non-metals typically exhibit ionic bonding. So, \( \mathrm{CaO} \) is not covalent.

Step 4: Analyze Compound (c) \\( \mathrm{KCl} \\)

Potassium (K) is a metal and chlorine (Cl) is a non-metal. \( \mathrm{KCl} \) is formed through ionic bonding as it involves a metal and a non-metal. Thus, \( \mathrm{KCl} \) is not covalent.

Step 5: Analyze Compound (d) \\( \mathrm{Na}_{2} \mathrm{~S} \\)

Sodium (Na) is a metal and sulfur (S) is a non-metal. The bond between them is ionic since it involves a metal and a non-metal. Therefore, \( \mathrm{Na}_{2} \mathrm{~S} \) is not covalent.

Step 6: Conclusion: Identify Covalent Compounds

After analyzing each compound, only \( \mathrm{H}_{2} \) consists entirely of non-metal elements that share electrons, making it covalent.

Key Concepts

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another. This usually occurs between metals and non-metals. A metal, which tends to lose electrons, donates one or more electrons to a non-metal, which tends to gain electrons. This transfer of electrons creates ions: the metal becomes a positively charged cation, and the non-metal becomes a negatively charged anion. These oppositely charged ions attract each other, leading to the formation of an ionic bond.

Key characteristics of ionic bonds include:

• A strong electrostatic force of attraction between ions.
• The formation of crystalline structures in solid forms, such as sodium chloride (\( \mathrm{NaCl} \)).
• High melting and boiling points due to the strong forces binding the ions together.
• The ability to conduct electricity when dissolved in water or molten due to the movement of ions.

It's essential to recognize ionic compounds in chemistry because they have distinct properties and behaviors compared to other types of chemical bonds.

Types of Chemical Bonding

Chemical bonds are the forces holding atoms together in a compound. The three main types of chemical bonding are ionic, covalent, and metallic bonds, each with unique characteristics and formation mechanisms.

• Ionic Bonds: These arise from the transfer of electrons between a metal and a non-metal. As explored earlier, the resulting ions attract each other to form ionic structures.
• Covalent Bonds: Unlike ionic bonds, covalent bonds occur when atoms share electron pairs. They typically form between non-metal atoms, such as in water (\( \mathrm{H}_2\mathrm{O} \)) where oxygen and hydrogen share electrons.
• Metallic Bonds: Found in metals, these bonds involve a 'sea' of delocalized electrons surrounding positive metal ions. This structure accounts for properties like electrical conductivity and malleability.

Understanding these bonds provides clearer insights into how different materials form and why they have specific properties, such as strength, electrical conductivity, and solubility.

Non-Metals in Chemistry

Non-metals play a crucial role in chemical bonding and the formation of various compounds. These elements are typically found on the right side of the periodic table and have distinct physical and chemical properties compared to metals.

Non-metals tend to gain or share electrons when interacting with other elements, leading to the formation of covalent or ionic compounds. Consider hydrogen and oxygen, both non-metals, that share electrons in water molecules (\( \mathrm{H}_2\mathrm{O} \)).

Typically, non-metals:

• Are poor conductors of heat and electricity.
• Have higher electronegativities, making them more likely to attract electrons during bonding.
• May exist in different states at room temperature—solid (like carbon), liquid (like bromine), or gas (like nitrogen).

Their versatility makes non-metals fundamental to organic and inorganic chemistry, contributing to the diverse range of compounds and materials observed in the natural world.