Which of the following are the properties of an electron? 1. Electron is a constituent of cathode ray 2. Electron is a negatively charged particle 3. The mass of the electron is equal to the mass of the proton 4. Electron is deflected by the electric field but not by magnetic field Select the correct answer using the code given below:

1. Evolution of the Atomic Model (basic)

For centuries, the atom was thought to be the smallest, indivisible building block of matter. However, the Evolution of the Atomic Model began in earnest when scientists realized that atoms themselves have an internal structure. The first major breakthrough came in 1897 with the discovery of the electron by J.J. Thomson. Through his experiments with cathode rays, Thomson proved that these rays were not just waves, but streams of negatively charged particles that originated from within the atom itself.

Electrons are fundamental subatomic particles with two defining characteristics: charge and mass. Every electron carries a negative charge of approximately 1.6 x 10⁻¹⁹ Coulombs. While they are vital to the atom's identity, they are incredibly light; the mass of an electron is roughly 9.1 x 10⁻³¹ kg, which is about 1/1837th the mass of a hydrogen atom. Because they are charged particles, electrons are highly reactive to their environment. They are deflected by both electric fields (moving toward the positive terminal) and magnetic fields, a principle that is fundamental to understanding electromagnetism Science, Class X, Magnetic Effects of Electric Current, p.207.

In modern chemistry, we focus heavily on where these electrons live. They are distributed in specific "shells" or energy levels around the center of the atom Science, Class X, Carbon and its Compounds, p.59. The electrons in the outermost shell, known as valence electrons, are the "handshakes" of the atomic world—they are transferred or shared between atoms to form chemical bonds, such as the ionic bonds found in magnesium oxide (MgO) Science, Class X, Metals and Non-metals, p.49. Understanding the electron was the first step in moving from a solid "billiard ball" model of the atom to the complex, empty-space models we use today.

Property	Description
Charge	Negative (-1.6 x 10⁻¹⁹ C)
Mass	Negligible (approx. 9.1 x 10⁻³¹ kg)
Field Interaction	Deflected by both Electric and Magnetic fields

Sources: Science, Class X, Magnetic Effects of Electric Current, p.207; Science, Class X, Carbon and its Compounds, p.59; Science, Class X, Metals and Non-metals, p.49

2. Discovery of Electrons & Cathode Rays (basic)

To understand the structure of the atom, we must first look at the groundbreaking experiments of the late 19th century. Before we knew about the nucleus, scientists like J.J. Thomson were exploring electricity using discharge tubes—glass tubes filled with gas at very low pressure. When a high voltage was applied, invisible rays originated from the negative electrode (the cathode) and traveled toward the positive electrode (the anode). These were aptly named cathode rays. Thomson's genius lay in proving that these rays were not just waves, but streams of tiny, negatively charged particles, which we now call electrons. Electrons are the first subatomic particles discovered, and they possess distinct physical properties that define how matter behaves. First, they carry a negative charge of approximately -1.6 x 10⁻¹⁹ C. Second, their mass is incredibly small—roughly 9.1 x 10⁻³¹ kg. To put that in perspective, an electron is about 1/1837th the mass of a hydrogen atom (or a proton). This confirms that atoms, once thought to be indivisible, are actually made of even smaller components. You can see the application of these charged particles in various electrical and magnetic contexts, such as in the study of magnetic effects of currents Science, Class X (NCERT 2025 ed.), Chapter 12, p.207. One of the most important characteristics of electrons is how they react to external forces. Because they are charged, they are deflected by both electric and magnetic fields. In an electric field, they are attracted toward the positive plate. In a magnetic field, they experience a force that pushes them in a direction perpendicular to both the magnetic field and their own motion. This behavior allowed Thomson to calculate the charge-to-mass (e/m) ratio, proving that these particles were universal constituents of all matter, regardless of which gas was used in the tube.

Property	Details
Nature	Negatively charged subatomic particle
Charge	-1.6 x 10⁻¹⁹ Coulombs
Mass	9.1 x 10⁻³¹ kg (Negligible compared to protons)
Deflection	Attracted to positive electric plates; deflected by magnets

Sources: Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.207

3. Mass and Charge of Subatomic Particles (intermediate)

To understand the universe at its most fundamental level, we must look at the building blocks of the atom: protons, neutrons, and electrons. While the atom as a whole is electrically neutral, it is composed of particles with distinct electrical charges and vastly different masses. The electron, discovered by J.J. Thomson through cathode ray experiments, is a negatively charged particle that orbits the nucleus. In contrast, the proton resides in the nucleus and carries a positive charge. Interestingly, the magnitude of their charge is exactly the same—approximately 1.6 × 10⁻¹⁹ Coulombs—but with opposite signs. This balance is what keeps an atom neutral.

However, when we compare their mass, the symmetry disappears. The electron is incredibly light, with a mass of about 9.1 × 10⁻³¹ kg. To put this in perspective, a proton (mass ≈ 1.67 × 10⁻²⁷ kg) is roughly 1,837 times heavier than an electron. Because electrons are so light and reside outside the nucleus, they are the mobile units of electricity. When an atom loses an electron, it becomes a positively charged cation; when it gains one, it becomes a negatively charged anion Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46. For example, a sodium atom becomes a Na⁺ cation by losing its outermost electron, while carbon's difficulty in gaining or losing four electrons highlights the energetic challenges of maintaining charge balance Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.59.

Particle	Relative Charge	Absolute Charge (C)	Mass (kg)
Electron	-1	-1.6 × 10⁻¹⁹	9.1 × 10⁻³¹
Proton	+1	+1.6 × 10⁻¹⁹	1.672 × 10⁻²⁷
Neutron	0	0	1.674 × 10⁻²⁷

Crucially, because electrons and protons are charged particles, they interact dynamically with their environment. They are deflected by both electric and magnetic fields. In an electric field, an electron will accelerate toward the positive plate. In a magnetic field, a moving electron experiences a force perpendicular to its direction of motion, a principle fundamental to the functioning of many electronic devices and the behavior of plasma in space Physical Geography by PMF IAS, Thunderstorm, p.348.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 3: Metals and Non-metals, p.46; Science, Class X (NCERT 2025 ed.), Chapter 4: Carbon and its Compounds, p.59; Physical Geography by PMF IAS, Thunderstorm, p.348

4. Interaction with Electric and Magnetic Fields (intermediate)

To understand how subatomic particles like electrons and protons behave, we must first look at how they respond to external forces. Since these particles possess an intrinsic property called electrical charge, they are inherently sensitive to both electric and magnetic fields. This sensitivity is not just a laboratory curiosity; it is the fundamental principle behind technologies ranging from old cathode-ray tube (CRT) televisions to modern particle accelerators used in cancer treatment.

When a charged particle enters an electric field, it experiences a force proportional to its charge. A negatively charged electron will always be accelerated toward the positive potential (the anode), while a positively charged proton moves toward the negative potential. However, the interaction with a magnetic field is more nuanced. A magnetic field does not exert any force on a stationary charge; the particle must be in motion. Once moving, the magnetic force acts perpendicularly to both the direction of the magnetic field and the direction of the particle's velocity. As noted in Science, Chapter 12: Magnetic Effects of Electric Current, p. 203, the direction of this force is determined by Fleming’s Left-Hand Rule. Here, your thumb represents the force, your forefinger the magnetic field, and your middle finger the conventional current.

It is crucial for your exams to distinguish between "electron flow" and "conventional current." Because electrons are negatively charged, the direction of current is taken as opposite to the direction of electron motion Science, Chapter 12: Magnetic Effects of Electric Current, p. 203. This distinction is a frequent trap in competitive questions. Furthermore, because the magnetic force is always perpendicular to the motion, it can change the velocity and momentum of a particle (by changing its direction), but it does not change the particle's speed or kinetic energy in a uniform field.

Feature	Electric Field Interaction	Magnetic Field Interaction
Condition for Force	Charge can be stationary or moving.	Charge must be moving.
Direction of Force	Parallel or anti-parallel to the field.	Perpendicular to both field and velocity.
Effect on Speed	Can increase or decrease speed.	Speed remains constant (only direction changes).

Sources: Science, Chapter 12: Magnetic Effects of Electric Current, p.203; Science, Chapter 12: Magnetic Effects of Electric Current, p.204

5. Connected Concept: Isotopes and Nuclear Stability (intermediate)

To understand the heart of nuclear physics, we must look at Isotopes. Atoms of the same element always have the same number of protons (atomic number), but they can have different numbers of neutrons. These variations are called isotopes. For example, while most Carbon atoms have 6 neutrons (Carbon-12), a small fraction has 8 (Carbon-14). Because chemistry is driven by electrons, isotopes of an element behave almost identically in chemical reactions. However, in the realm of the nucleus, that extra handful of neutrons changes everything.

Nuclear stability is a delicate balancing act between two opposing forces. On one side, you have the electrostatic force, where positively charged protons try to fly apart from each other. On the other side is the strong nuclear force—a powerful but short-range attraction that acts like "nuclear glue" between protons and neutrons. Neutrons are vital because they contribute to this attractive glue without adding any repulsive electrical charge. In lighter elements, a 1:1 ratio of protons to neutrons usually ensures stability. But as nuclei get heavier and more crowded, the repulsive force of the protons grows faster than the strong force can keep up. To remain stable, heavier atoms require a higher proportion of neutrons to act as buffers.

When this balance is disrupted—either because a nucleus has too many or too few neutrons—it becomes unstable or radioactive. An unstable nucleus will spontaneously undergo radioactive decay to reach a more stable configuration. As noted in Environment, Shankar IAS Academy, Environmental Pollution, p.82, this process involves the emission of alpha particles (protons), beta particles (electrons), or gamma rays. While many isotopes occur naturally, others are used specifically for their energy potential; for instance, Uranium-235 is the primary fuel for nuclear fission in power plants and weaponry Environment, Shankar IAS Academy, Environmental Pollution, p.83.

Isotope Type	Characteristics	Example
Stable Isotopes	Maintain a balanced proton-neutron ratio; do not decay over time.	Carbon-12, Oxygen-16
Radioactive Isotopes (Radioisotopes)	Unstable nuclei that emit radiation to transform into a stable state.	Uranium-235, Iodine-131

Sources: Environment, Shankar IAS Academy, Environmental Pollution, p.82; Environment, Shankar IAS Academy, Environmental Pollution, p.83

6. Connected Concept: Magnetic Effects of Current (exam-level)

For a long time, electricity and magnetism were treated as separate subjects. This changed when it was discovered that a compass needle deflects when placed near a wire carrying current. This simple observation revealed a profound truth: moving electric charges (current) create a magnetic field in the space surrounding them. This phenomenon is known as the magnetic effect of electric current Science, Class VIII, Electricity: Magnetic and Heating Effects, p.48. Unlike a permanent bar magnet, this magnetic field is temporary; it exists only as long as the current flows and disappears the moment the circuit is broken.

The pattern of the magnetic field depends entirely on the geometry of the conductor. For a straight wire, the field lines form concentric circles centered on the wire. If we wind that wire into a coil, known as a solenoid, the magnetic field produced is remarkably similar to that of a bar magnet, with distinct North and South poles Science, Class X, Magnetic Effects of Electric Current, p.206. We can even enhance this effect by placing a soft iron core inside the coil to create an electromagnet, which is the backbone of many modern technologies like electric bells and cranes Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58.

Crucially, because a current-carrying conductor generates its own magnetic field, it will interact with any external magnetic field it is placed in. This interaction results in a mechanical force acting on the conductor. Since an electric current is essentially a stream of moving electrons, individual electrons moving through a magnetic field also experience this force, causing their path to deflect. This principle is fundamental to understanding how electric motors work and how subatomic particles behave in accelerators.

Conductor Shape	Magnetic Field Pattern	Key Characteristic
Straight Wire	Concentric Circles	Direction determined by Right-Hand Thumb Rule.
Solenoid (Coil)	Parallel lines inside, loops outside	Behaves like a Bar Magnet; very uniform internal field.

Sources: Science, Class VIII, Electricity: Magnetic and Heating Effects, p.48; Science, Class X, Magnetic Effects of Electric Current, p.206; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58

7. Deep Dive into Electron Properties (exam-level)

The electron is a fundamental subatomic particle that carries a negative elementary charge. Discovered by J.J. Thomson through his famous cathode ray experiments, electrons are the primary carriers of electricity in solids. In a conductor, a stream of moving electrons constitutes an electric current, though by historical convention, the direction of current is considered opposite to the flow of these negatively charged particles Science, Class X, Electricity, p.192. Beyond electricity, electrons dictate the chemical personality of an atom. Atoms seek stability by gaining, losing, or sharing valence electrons—those in the outermost shell—to achieve a stable noble gas configuration Science, Class X, Carbon and its Compounds, p.59.

Physically, the electron is remarkably light. Its mass is approximately 9.1 x 10⁻³¹ kg, which is roughly 1/1837th the mass of a proton or a hydrogen atom. This extreme lightness allows electrons to be easily manipulated by external forces. Because they are charged particles, they are highly sensitive to electric and magnetic fields. In an electric field, an electron is accelerated toward the positive terminal (anode). In a magnetic field, it experiences a force perpendicular to its direction of motion, causing it to move in a curved path—a principle essential for technologies like old CRT televisions and modern particle accelerators.

Property	Electron	Proton
Charge	Negative (-1.6 x 10⁻¹⁹ C)	Positive (+1.6 x 10⁻¹⁹ C)
Mass	~9.1 x 10⁻³¹ kg (Very Light)	~1.67 x 10⁻²⁷ kg (Heavy)
Location	Orbits the nucleus	Inside the nucleus

In chemical reactions, the loss of an electron creates a net positive charge, resulting in a cation (like Na⁺), while the gain of an electron results in a net negative charge, creating an anion (like Cl⁻) Science, Class X, Metals and Non-metals, p.46. This transfer and sharing of electrons is the foundation of all chemical bonding and molecular structure.

Sources: Science, Class X, Electricity, p.192; Science, Class X, Carbon and its Compounds, p.59; Science, Class X, Metals and Non-metals, p.46

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of atomic structure, this question serves as a perfect synthesis of your learning. You learned that electrons were identified as the primary constituents of cathode rays during J.J. Thomson’s landmark experiments, which directly confirms Statement 1. Furthermore, the very definition of an electron is a negatively charged particle with a charge of 1.6 x 10⁻¹⁹ C, validating Statement 2. In the UPSC Civil Services Examination, these foundational facts act as the essential building blocks for solving more complex multi-statement problems.

To navigate the remaining options, we must evaluate comparative properties and behavior in physical fields. Statement 3 is a common distractor; while both are subatomic particles, the mass of a proton is significantly larger—roughly 1,837 times the mass of an electron. Statement 4 contains a classic UPSC trap—the "half-truth." While it is true that electrons are deflected by electric fields, they are equally susceptible to magnetic fields because any moving charge experiences a force when passing through a magnetic field. By systematically eliminating these two errors, we logically arrive at (A) 1 and 2 only as the only accurate choice.

Developing the habit of critical elimination is as important as content knowledge. As highlighted in Science, Class X (NCERT), understanding the interaction between electricity and magnetism is key to recognizing why Statement 4 must be false. Always be wary of options that suggest a fundamental particle is immune to a basic physical force, as UPSC often tests the limitations and interactions of these particles in various environments.