Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

The increasing order (lowest first) for the values of \(\mathrm{e} / \mathrm{m}\) (charge/mass) for electron (e), proton (p), neutron (n) and alpha particle (a) is (a) \(\mathrm{n}, \mathrm{p}, \mathrm{a}, \mathrm{e}\) (b) \(\mathrm{n}, \mathrm{p}, \mathrm{e}, \mathrm{a}\) (c) \(\mathrm{n}, \mathrm{a}, \mathrm{p}, \mathrm{e}\) (d) \(\mathrm{e}, \mathrm{p}, \mathrm{n}, \mathrm{a}\)

Step-by-Step Solution

Answer

The correct order is: n, p, a, e (Option a).

Step 1: Understanding the Concept

The charge-to-mass ratio \( \frac{e}{m} \) is given by \( \text{charge} \div \text{mass} \). For particles where charge \( e eq 0 \), this ratio is defined and can be calculated. Particles with no charge (like neutrons) have an undefined or zero \( \frac{e}{m} \). We will compare this ratio for electrons, protons, neutrons, and alpha particles to determine the order.

Step 2: Evaluating Neutron

Neutrons have no charge, which means their charge-to-mass ratio \( \frac{e}{m} \) is zero. This will put them at the lowest position in terms of increasing order as \( \frac{e}{m} = 0 \).

Step 3: Calculating Proton \( \frac{e}{m} \)

For protons, the charge \( e = +1 \) (in elementary charge units) and its mass is relatively high. Thus, the \( \frac{e}{m} \) ratio is small but positive. This comes after the neutron.

Step 4: Determining Alpha Particle \( \frac{e}{m} \)

Alpha particles have a charge of \( +2e \) but are composed of 2 protons and 2 neutrons, making them quite massive. Therefore, their \( \frac{e}{m} \) is smaller than for a single proton but greater than that of a neutron.

Step 5: Calculating Electron \( \frac{e}{m} \)

Electrons have a negative charge \( -1e \) and a very small mass, resulting in a very high \( \frac{e}{m} \). The magnitude of this ratio is greater than that of a proton or alpha particle.

Step 6: Arranging in Increasing Order

Considering the evaluations: neutrons have the lowest \( \frac{e}{m} \) (zero), followed by protons, then alpha particles, and finally electrons with the highest \( \frac{e}{m} \). Therefore, the order by increasing \( \frac{e}{m} \) is: neutron (n), proton (p), alpha particle (a), electron (e).

Key Concepts

Understanding Elementary Particles

Elementary particles are the basic building blocks of matter. They cannot be divided into smaller constituents. These particles include neutrinos, quarks, electrons, and fundamental forces' carriers like photons.
Within an atom, three main types of elementary particles exist: protons, neutrons, and electrons. Using these particles and understanding their properties, like charge and mass, allows scientists to explore atomic and subatomic phenomena.

• Protons and neutrons are found in the atomic nucleus.
• Electrons orbit around the nucleus.

Atoms vary in complexity, but at this fundamental level, understanding these particles is crucial for grasping the concept of the charge-to-mass ratio.

Exploring Protons and Neutrons

Protons and neutrons are core components of an atomic nucleus. They are collectively known as nucleons. These particles are nearly identical in mass, but they differ sharply in terms of charge.

• Protons: Have a positive charge (+1e) and are pivotal in defining the chemical element of the atom. The number of protons in a nucleus is also referred to as the atomic number.
• Neutrons: Carry no charge, making their charge-to-mass ratio zero. They play a crucial role in adding mass and stability to the nucleus.

Together, protons and neutrons make up most of the atom's mass, significantly affecting both the atomic weight and the behavior of the atom in various interactions.

Introduction to Alpha Particles

Alpha particles are a type of ionizing radiation often emitted during radioactive decay. They consist of two protons and two neutrons bound together. This composition makes them analogous to a helium nucleus.
Despite their large mass compared to other particles, alpha particles have a double positive charge (+2e), leading to a moderate charge-to-mass ratio. This property gives them more mass per unit charge than protons, but a smaller ratio than electrons.
Some characteristics of alpha particles include:

• High mass and charge compared to electrons.
• Limited ability to penetrate materials, but capable of causing significant damage to materials they collide with.

Understanding alpha particles' nature and characteristics helps in the analysis of nuclear reactions and decay processes.

Examining Electron Properties

Electrons are one of the primary carriers of electricity in conductive materials. These negatively charged particles, with a charge of -1e, have a mass significantly less than protons or neutrons. This light mass leads to electrons having a very high charge-to-mass ratio, which surpasses that of protons and alpha particles.
Despite their small size and mass, electrons hold vast importance in chemistry and physics:

• Responsible for forming chemical bonds.
• Dictate the chemical behavior and reactivity of atoms.

The high charge-to-mass ratio of electrons makes them highly reactive and easy to accelerate, playing a key role in electricity and electronic engineering.