Problem 187 - A Complete Resource Book in Chemistry for JEE Main

Question

Select correct statement: (a) when a covalent bond is formed, transfer of electrons takes place (b) pure \(\mathrm{H}_{2} \mathrm{O}\) does not contain any ion (c) a bond is formed when attractive forces overcome repulsive forces (d) HF is less polar than \(\mathrm{HBr}\)

Step-by-Step Solution

Answer

Statements (b) and (c) are correct.

Step 1: Identify the nature of covalent bonds

Covalent bonds involve the sharing of electron pairs between atoms rather than the transfer of electrons. Therefore, statement (a) is incorrect.

Step 2: Analyze the properties of pure water

Pure water, represented as \( \mathrm{H}_2\mathrm{O} \), is a molecular compound and does not contain free ions. Hence, statement (b) is correct.

Step 3: Understand bond formation dynamics

A chemical bond forms when the attractive interactions between atoms overcome any repulsive forces, making statement (c) correct.

Step 4: Compare polarities of HF and HBr

Hydrogen fluoride (HF) is more polar than hydrogen bromide (HBr) due to fluorine's higher electronegativity compared to bromine. Therefore, statement (d) is incorrect.

Key Concepts

Covalent Bonds

Covalent bonds are a fundamental concept in chemistry, representing a specific type of chemical bond where atoms share pairs of electrons. Unlike ionic bonds, which involve the transfer of electrons from one atom to another, covalent bonds focus on the sharing aspect. This sharing allows each atom to achieve a stable electron configuration, often resembling that of the nearest noble gas.
Understanding covalent bonds is crucial because:

• They form when atoms have similar electronegativity, meaning neither atom is strong enough to completely take electrons from the other.
• Usually occur between non-metal elements. For example, in a water molecule (\( \mathrm{H}_2o \)), the oxygen atom shares electrons with two hydrogen atoms, creating covalent bonds.
• The resulting molecule has a more stable electronic structure. This stability is key to the formation of countless molecules necessary for life and chemical reactions.

Recognizing that covalent bonds involve shared electrons helps understand basic molecular structures and predict molecular behavior.

Polarity

Polarity in chemistry refers to the distribution of electrical charge over the atoms joined by the bond. It is an essential concept for determining how molecules interact with each other. When molecules have regions of positive and negative charge, they are termed polar molecules. This happens because of differences in electronegativity between bonded atoms.
A molecule's polarity affects several factors:

• Polarity determines a molecule's solubility in water. Polar molecules tend to dissolve well in water, which is also polar, due to the 'like dissolves like' principle.
• Affects the molecule's boiling and melting points. Polar molecules generally have higher boiling and melting points because of the strong intermolecular forces (like hydrogen bonding) that must be overcome.
• Influences the molecule's reactivity and orientation when interacting with other substances. For instance, HF is more polar than HBr because fluorine is more electronegative than bromine, creating a larger dipole moment.

Knowing how to determine and analyze polarity is key to predicting and understanding the behavior of molecules in diverse chemical environments.

Ions

Ions are charged particles formed when atoms or molecules gain or lose electrons. This conversion usually occurs to satisfy the pursuit of a stable electron configuration, often aligning with the electron arrangements found in noble gases.
Ions play a vital role in many aspects of chemistry:

• Cations are positive ions formed when atoms lose electrons. Common examples include sodium (\( \mathrm{Na}^+\)) and calcium (\( \mathrm{Ca}^{2+}\)).
• Anions are negative ions formed when atoms gain electrons, with examples like chloride (\( \mathrm{Cl}^-\)) and oxide (\( \mathrm{O}^{2-}\)).
• Ionic compounds, like table salt (\( \mathrm{NaCl} \)), consist of a lattice structure where each ion is surrounded by ions of opposite charge. This structure explains the high melting and boiling points of ionic compounds.

In solutions, ions contribute to the conductivity of electricity. For instance, although pure water has very low conductivity, the introduction of salt (an ionic compound) in water can significantly increase its electrical conductivity. Understanding ions helps explain phenomena ranging from the chemistry involved in everyday substances to the principles underlying electrical signals in biological organisms.