Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

Decrease in atomic number is observed during. (a) \(\alpha\) emission (b) \(\beta\) emission (c) positron emission (d) electron capture Select the correct answer. (a) \(1,2,3\) (b) \(2,3,4\) (c) \(1,3,4\) (d) \(1,2,3,4\)

Step-by-Step Solution

Answer

Option (c) \(1,3,4\): Decrease in atomic number is observed in \(\alpha\) emission, positron emission, and electron capture.

Step 1: Understanding Atomic Number Changes

First, let's consider what determines the atomic number of an element. The atomic number is the number of protons in an atom's nucleus.

Step 2: Examining Alpha Emission

During \(\alpha\) (alpha) emission, an atom emits an \(\alpha\) particle, which consists of 2 protons and 2 neutrons. This emission reduces the atomic number by 2.

Step 3: Checking Beta Emission

In \(\beta\) (beta) emission, a neutron is converted to a proton and an electron is emitted. This increases the atomic number by 1, not decrease it.

Step 4: Studying Positron Emission

During positron emission, a proton is converted to a neutron and a positron is emitted, leading to a decrease in atomic number by 1.

Step 5: Exploring Electron Capture

Electron capture occurs when an atomic nucleus captures an inner electron, converting a proton into a neutron. This decreases the atomic number by 1.

Key Concepts

Alpha Emission

Alpha emission is a type of nuclear reaction where an atomic nucleus emits an alpha particle. An alpha particle is essentially a helium nucleus consisting of 2 protons and 2 neutrons. This emission is characterized by a significant drop in both the mass number and the atomic number of the original atom.

• Reduction in atomic number: The emission leads to the loss of 2 protons, thereby reducing the atomic number by 2.
• Mass number reduction: In addition to the atomic number, the mass number also decreases by 4 since an alpha particle carries away 2 neutrons along with the 2 protons.

Alpha particles are relatively heavy and positively charged. Due to their size, they don't penetrate materials deeply, which makes them easier to shield against in various applications.

Beta Emission

Beta emission involves the conversion of a neutron into a proton within the nucleus, accompanied by the ejection of a beta particle, which is an electron. Despite involving a change inside the nucleus, it often results in an increase in the atomic number rather than a decrease.

• Change in atomic number: As the neutron is changed into a proton, the atomic number of the element increases by 1.
• Conservation laws: While the beta particle (electron) is emitted externally, the process respects the conservation of charge by converting the neutron into a positively influenced proton.

Because beta particles are lighter than alpha particles, they have a greater ability to penetrate materials, requiring more substantial shielding.

Positron Emission

Positron emission is an intriguing process where a proton transmutes into a neutron. Consequently, a positron, which is a particle similar to an electron but with a positive charge, is emitted

• Decrease in atomic number: The conversion from proton to neutron results in one fewer proton, thus decreasing the atomic number by 1.
• Positron properties: Like electrons, positrons are light, carrying a positive charge. When they encounter electrons, they can annihilate each other, releasing energy in the form of gamma rays.

This form of emission plays a significant role in applications such as Positron Emission Tomography (PET) scans in medical imaging.

Electron Capture

Electron capture is a process where the nucleus of an atom captures one of the innermost electrons, typically from the K-shell. This electron combines with a proton to form a neutron.

• Descending atomic number: By converting a proton into a neutron, the atomic number decreases by 1.
• Energy release: The captured electron leads to the emission of X-rays and other particles as the atom transitions to a lower energy state.

Electron capture often occurs in heavier elements that are in need of achieving a more stable state. This process is equally essential in understanding radioactive decay sequences.