Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

There are three isotopes of hydrogen and three naturally occurring isotopes of oxygen \(\left(\mathrm{O}^{16}, \mathrm{O}^{17}\right.\) and \(\mathrm{O}^{18}\) ). How many kinds of water are possible? (a) 9 (b) 16 (c) 8 (d) 18

Step-by-Step Solution

Answer

There are 27 kinds of water possible.

Step 1: Identify the Isotopes

Hydrogen has three isotopes: plain hydrogen (H), deuterium (2H), and tritium (3H). Oxygen has three isotopes: 16O, 17O, and 18O.

Step 2: Determine Water Molecule Composition

A water molecule consists of two hydrogen atoms and one oxygen atom (H2O). Therefore, the composition can vary based on a combination of these isotopes.

Step 3: Calculate Hydrogen Combinations

For the two hydrogen atoms in the water molecule, there are 3 choices for the first hydrogen and 3 choices for the second hydrogen. Thus, there are 3 imes 3 = 9 different combinations of hydrogen isotopes.

Step 4: Consider Different Oxygen Isotopes

There are 3 distinct oxygen isotopes that can be independently selected for each water molecule.

Step 5: Calculate Total Water Variants

Combine the different hydrogen and oxygen isotope possibilities. There are 9 combinations of hydrogen and 3 combinations of oxygen, leading to 9 imes 3 = 27 possible combinations of water isotopes.

Key Concepts

Hydrogen Isotopes

Hydrogen, the simplest element in the periodic table, has three isotopes that naturally occur:

• Protium ([): This is the most common form of hydrogen, containing just one proton and no neutrons in its nucleus.
• Deuterium ([^{2}H[): Known as "heavy hydrogen," this isotope features one proton and one neutron.
• Tritium ([^{3}H[): This is a rare and radioactive isotope, consisting of one proton and two neutrons.

Each isotope of hydrogen differs in mass and nuclear composition, which influences the physical properties of the molecules they form. However, reactivity and chemical behavior remain largely unchanged because they all contain the same single electron and a proton, contributing similarly to bond formation in water (H[^{2}O) molecules.

Oxygen Isotopes

Oxygen, a critical element for life on Earth, naturally occurs in three stable isotopes:

• ^{16}Oxygen ([^{16}O[): This is the most abundant isotope, making up approximately 99.76% of oxygen on Earth.
• ^{17}Oxygen ([^{17}O[): Much less common, this isotope is stable and accounts for about 0.04% of Earth's oxygen.
• ^{18}Oxygen ([^{18}O[): Slightly more prevalent than [^{17}O[, it comprises around 0.20% of oxygen naturally found.

Each oxygen isotope contributes uniquely to studies of paleoclimatology and environmental science because their different masses can influence the behavior of oxygen molecules. In water molecule formation, the basic bonding characteristics of oxygen remain consistent among its isotopes, but the differences in mass can affect physical properties like vapor pressure and boiling point.

Water Molecule Composition

A water molecule, commonly represented as H[_{2}O, is composed of two hydrogen atoms and one oxygen atom. Despite this simple formula, the water's properties can vary significantly with the isotopic composition of these elements. Each water molecule can be constructed using different combinations of hydrogen and oxygen isotopes, resulting in various isotopic forms of water:

• Two potential hydrogen atoms in a single water molecule can each be replaced by [, [^{2}H[, or [^{3}H[. When each substitution is considered, this allows for 9 possible diatomic hydrogen configurations.
• The single oxygen atom in the molecule can be any of [^{16}O[, [^{17}O[, or [^{18}O[. There are thus 3 potential oxygen isotopes for any configuration.

By combining these isotope variations, 27 different water isotopic forms are logically and scientifically possible. Each unique isotope combination can alter some physical properties of the water, such as melting and boiling points, due to differences in atomic mass, but chemically, it remains H[_{2}O.