Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The numbers of protons, electrons and neutrons in a molecule of heavy water are respectively: [Main Online April 23, 2013] (a) \(8,10,11\) (b) \(10,10,10\) (c) \(10,11,10\) (d) \(11,10,10\)

Step-by-Step Solution

Answer

The numbers of protons, electrons, and neutrons in heavy water are 10, 10, 10 (option b).

Step 1: Understanding Heavy Water

Heavy water is a form of water where the usual hydrogen atoms (protium) in the water molecule are replaced by deuterium, which is an isotope of hydrogen. Deuterium has one proton and one neutron, so it differs from regular hydrogen, which has only one proton.

Step 2: Determine the Composition of Heavy Water

A molecule of heavy water consists of two deuterium atoms and one oxygen atom. Deuterium (D) has 1 proton, 1 electron, and 1 neutron, while oxygen (O) usually has 8 protons, 8 electrons, and typically 8 neutrons.

Step 3: Calculating the Number of Protons

Each deuterium contributes 1 proton, so 2 deuterium atoms contribute a total of 2 protons. The oxygen atom contributes 8 protons. Therefore, the total number of protons is 2 (from deuterium) + 8 (from oxygen) = 10.

Step 4: Calculating the Number of Electrons

In a neutral heavy water molecule, the number of electrons equals the sum of the electrons of the constituent atoms. Hence, 2 deuterium atoms supply 2 electrons and 1 oxygen atom supplies 8 electrons, making the total 10 electrons.

Step 5: Calculating the Number of Neutrons

Similarly, each deuterium atom has 1 neutron, so 2 deuterium atoms contribute 2 neutrons. Oxygen has typically 8 neutrons, contributing to a total of 8. Therefore, the total number of neutrons is 2 (from deuterium) + 8 (from oxygen) = 10.

Step 6: Comparing with Given Options

Based on the calculations done, the numbers of protons, electrons, and neutrons are 10, 10, and 10, respectively. The correct option is (b) 10, 10, 10.

Key Concepts

Deuterium

Deuterium is an isotope of hydrogen, distinctly different due to its atomic composition. Most hydrogen atoms have just a single proton in their nucleus, lacking neutrons. However, deuterium atoms contain one proton and one neutron in the nucleus.
This additional neutron essentially doubles the atomic mass of hydrogen, hence the term 'heavy hydrogen'.
When water is formed using deuterium instead of regular hydrogen, we get heavy water. This small change in mass can lead to variations in physical and chemical behaviors, which is why deuterium and heavy water are significant in research and industry.

Neutron Calculation

To determine the number of neutrons in a molecule of heavy water, it's essential to sum up the neutrons contributed by its components. Heavy water consists of two deuterium atoms and one oxygen atom.

• Each deuterium atom has one neutron, contributing 2 neutrons in total (1 neutron per deuterium x 2 deuterium atoms).
• Oxygen commonly has 8 neutrons.

So, the heavy water molecule has a total of 10 neutrons: 2 from deuterium and 8 from oxygen.

Oxygen Atom Composition

The oxygen atom in a heavy water molecule doesn't differ from that in regular water. It holds the atomic number 8, which means it has 8 protons in its nucleus. This consistency is vital for understanding how the oxygen atom behaves in compounds.
Additionally, it usually has 8 neutrons and 8 electrons, contributing to the overall stability of the oxygen atom. The balanced composition of oxygen helps maintain the molecular structure and reactivity in water and heavy water alike.

Proton Calculation

Calculating the number of protons in heavy water involves considering each constituent atom. Here's a breakdown:

• Deuterium: 1 proton per atom. With 2 deuterium atoms, this contributes 2 protons.
• Oxygen: Always contributes 8 protons, regardless of the water type.

When combined, the total number of protons in heavy water is 10: 2 from deuterium and 8 from oxygen. This is crucial in identifying the chemical identity and stability of heavy water.

Electron Calculation

Electrons are negatively charged particles that balance the positive charge of protons in an atom. In a neutral heavy water molecule, the number of electrons must equal the total number of protons.

• Each deuterium atom provides 1 electron. With two deuterium atoms, we have 2 electrons.
• The oxygen atom contributes 8 electrons, as its atomic number is 8.

Thus, with 2 electrons from deuterium and 8 from oxygen, a molecule of heavy water has a total of 10 electrons. This balance ensures the overall non-charged state of the molecule.