Problem 92 - A Complete Resource Book in Chemistry for JEE Main

Question

The radiations from a naturally occurring radioactive substance, as seen after deflection by a magnetic field in one direction, are (a) definitely beta rays (b) either alpha or beta rays (c) both alpha and beta rays (d) definitely alpha rays

Step-by-Step Solution

Answer

(a) definitely beta rays

Step 1: Understanding Beta and Alpha Rays

Beta rays are composed of electrons or positrons and are negatively or positively charged. Alpha rays consist of two protons and two neutrons and carry a positive charge. In a magnetic field, beta particles are deflected much more than alpha particles because they are lighter and have a different charge.

Step 2: Deflection Behavior in a Magnetic Field

When charged particles, like alpha and beta rays, pass through a magnetic field, they experience a force that causes them to deflect. Due to their lighter mass, beta particles are deflected more significantly than alpha particles, which are heavier and have a greater charge-to-mass ratio.

Step 3: Analyze the Given Situation

The problem states that radiation from a radioactive substance is deflected by a magnetic field in one direction. Since beta particles experience greater deflection due to their lower mass and specific charge, deflection in one clear direction indicates the presence of beta rays specifically.

Step 4: Conclusion

Since the deflection is noted in one direction, which is characteristic of beta rays, the radiation is more likely to be beta rays rather than alpha rays, which would show less deflection and could result in less distinct movement in the field.

Key Concepts

Alpha Rays

Alpha rays are a type of radiation consisting of positively charged particles. Each alpha particle comprises two protons and two neutrons—a configuration identical to the helium nucleus.
Because they carry a large mass and a +2 charge, alpha particles travel relatively slowly compared to other radiation types. They tend to lose energy quickly as they travel through material, making them much less penetrating compared to beta particles or gamma rays.

• Composition: Two protons and two neutrons
• Charge: Positively charged (+2)
• Penetration Power: Low; can be stopped by a sheet of paper or skin

In a magnetic field, their deflection is minimal due to their relatively high mass and positive charge, making them less fraught to influence than lighter particles like beta particles.

Beta Rays

Beta rays are essentially electrons or positrons that are ejected from a radioactive nucleus during decay. Being much lighter than alpha particles, they carry only a single negative charge in the case of electrons, or a positive charge in the case of positrons.
Due to their much smaller mass, beta particles can achieve higher speeds, giving them greater penetration power compared to alpha particles. They can pass through paper and require materials like aluminum to be stopped.

• Composition: Electrons or positrons
• Charge: Negative (-1) for electrons; positive (+1) for positrons
• Penetration Power: Moderate; stopped by a few millimeters of aluminum

In the presence of a magnetic field, beta particles are significantly deflected because of their lighter mass and electric charge. This distinct behavior is crucial for determining the type of radiation in scenarios like the one discussed.

Magnetic Field Deflection

Deflection in a magnetic field is an essential concept in understanding how different types of radiation behave. When charged particles move through a magnetic field, they experience a force perpendicular to both the field and their velocity. This phenomenon is known as the magnetic Lorentz force.

• Alpha particles: Experience minimal deflection due to larger mass and lower charge-to-mass ratio.
• Beta particles: Exhibit greater deflection as they are much lighter and have a larger charge-to-mass ratio.

This difference in deflection allows scientists to use magnetic fields as a tool for identifying the type of radiation present. In the given problem, the significant deflection in one direction implies the presence of beta particles due to their characteristic behavior.

Radioactive Substance

A radioactive substance is a material that spontaneously emits radiation as it decays to a more stable form. This decay process occurs because the nucleus of an atom in the substance is unstable. Over time, the unstable nucleus loses energy by emitting particles or electromagnetic waves.
Radioactive substances can release different types of radiation, including alpha rays, beta rays, and gamma rays, each with distinct characteristics and levels of penetration.

• Alpha decay: Releases alpha particles, decreasing the atomic number by 2 and the mass number by 4.
• Beta decay: Involves the conversion of a neutron into a proton or vice versa, emitting electrons or positrons.
• Gamma decay: Occurs when the nucleus emits energy in the form of gamma rays without changing the number of protons or neutrons.

Each type of radiation emitted by radioactive substances can be distinguished and analyzed due to their unique interactions with materials, such as magnetic fields, which can help in determining their identity as illustrated in the problem.