Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

In which radiation, mass number and atomic number will not change? (a) \(\alpha\) (b) \(\beta\) (c) \(\alpha\) and \(2 \beta\) (d) \(\gamma\)

Step-by-Step Solution

Answer

Option (d) \(\gamma\) radiation.

Step 1: Understanding Radiation Types

Radiation types include alpha (\(\alpha\)), beta (\(\beta\)), and gamma (\(\gamma\)) rays. Each type has specific impacts on atomic and mass numbers. \(\alpha\) particles result in a decrease of 2 in the atomic number and 4 in the mass number. \(\beta\) particles change the atomic number by either increasing or decreasing by 1, depending on the type of \(\beta\) decay. In contrast, \(\gamma\) rays are electromagnetic radiation and do not affect the atomic or mass numbers.

Step 2: Analyzing Each Option

(a) \(\alpha\) radiation: Decreases both atomic and mass numbers.(b) \(\beta\) radiation: Changes the atomic number.(c) \(\alpha\) and \(2 \beta\): Combination still alters atomic and mass numbers.(d) \(\gamma\) radiation: Does not change the atomic or mass numbers.

Step 3: Drawing a Conclusion

Based on the analysis, \(\gamma\) radiation is the only one where neither the atomic nor the mass number changes, as it is a type of energy release without particle emission.

Key Concepts

Gamma Radiation

Gamma radiation, often denoted by the symbol \(\gamma\), is a form of electromagnetic radiation. It is similar to visible light and X-rays but possesses much higher energy. Gamma rays are emitted from the nucleus of an atom, usually following alpha or beta decay. Despite their high energy, gamma rays do not alter the atomic number or the mass number of an element. This is because gamma radiation involves the emission of energy, not particles. As a result, the composition of the nucleus remains unchanged when gamma rays are emitted.

• No change in atomic or mass number.
• Highly penetrating, capable of passing through most materials.
• Requires heavy shielding, such as lead, to be contained.

This makes gamma radiation distinct from other types of radioactive decay, where changes to atomic structure occur.

Alpha Decay

Alpha decay involves the emission of alpha particles from an unstable nucleus. An alpha particle consists of 2 protons and 2 neutrons, equivalent to a helium nucleus. When an atom undergoes alpha decay, its mass number decreases by 4 units, and its atomic number reduces by 2. This results in the formation of a new element positioned two places back on the periodic table.

• Mass number decreases by 4.
• Atomic number decreases by 2.
• Forms a new element.

Due to the relatively large size of alpha particles, they have low penetration power and can be stopped by paper or skin. However, they can cause significant damage if ingested or inhaled.

Beta Decay

In beta decay, a beta particle, which can be either an electron or a positron, is emitted from the nucleus. Beta decay can occur in two forms: beta-minus (\(\beta^-\)) and beta-plus (\(\beta^+\)) decay. In beta-minus decay, a neutron is transformed into a proton, increasing the atomic number by 1, while the mass number remains unchanged. In beta-plus decay, a proton is converted into a neutron, reducing the atomic number by 1.

• Atomic number changes by \(+1\) or \(-1\).
• Mass number remains constant.
• Results in the formation of a new element, one position shifted on the periodic table.

Beta particles have moderate penetration power and can pass through paper but are stopped by materials like aluminum.

Mass Number

The mass number of an atom is the total count of protons and neutrons within its nucleus. Represented by the symbol \(A\), it is essential for identifying isotopes, which are variants of a particular element with differing neutron counts. Although the mass number significantly defines the mass of an atom, it does not change through beta decay but decreases by 4 in alpha decay. In cases like gamma radiation, the mass number remains completely unchanged.

• Total of protons and neutrons in an atom.
• Defines isotopes of the same element.
• Is unaltered by gamma radiation.

Understanding the mass number helps in tracking changes in reactions involving nuclear decay.

Atomic Number

The atomic number, denoted as \(Z\), is the number of protons present in the nucleus of an atom. It determines the identity of an element on the periodic table. Changes in the atomic number result in the formation of new elements. During alpha decay, the atomic number decreases by 2, as two protons are lost. Conversely, during beta-minus decay, the atomic number increases by 1, as a neutron is converted to a proton.

• Defines the element's identity.
• Changes in atomic number lead to different elements.
• No change during gamma radiation.

The atomic number is critical in nuclear decay processes and dictates how an element transforms.