Introduction

In 1803, John Dalton proposed atoms to be indivisible in his famous ‘Dalton’s atomic theory’. However, in the late 19th century, discharge tube experiments revealed that atoms are composed of even smaller particles – electrons and protons.

Interesting Information!

Although the nucleus is less than one hundred-thousandth (1/100,000) of the size of the atom, it contains more than 99.9% of the mass of the atom.

Discovery of Electrons

J.J. Thomson’s Experiment (1897)

Experimental Setup

Apparatus: A discharge tube (hard glass tube) with two metal electrodes (cathode and anode) connected to a high voltage source (thousands of volts). The tube is evacuated to low pressure using a vacuum pump.

Procedure: When high voltage is applied to the evacuated tube, rays were observed emanating from the cathode towards the anode. These were called cathode rays.

Observations
  1. Cathode rays traveled in straight lines from cathode to anode.
  2. They caused fluorescence when they struck the glass wall opposite the cathode.
  3. When an electric field was applied, cathode rays deflected towards the positive plate, indicating they carry negative charge.
  4. When a magnetic field was applied, cathode rays were deflected perpendicular to the field direction.
  5. The nature of cathode rays was independent of the gas in the tube or the material of the cathode.
Thomson’s Calculations

Thomson measured the deflection of cathode rays in electric and magnetic fields to calculate the charge-to-mass ratio (e/m) of cathode ray particles.

Result: e/m ratio was constant (1.76 × 10¹¹ C/kg) regardless of the gas used, proving cathode rays consist of identical negatively charged particles.

Conclusion: Cathode rays are streams of negatively charged particles, which Thomson named “corpuscles” (later renamed electrons by Stoney).

Key Points: Discovery of Electron

Discovered by J.J. Thomson in 1897 through cathode ray experiments
Cathode rays deflect toward positive plate in electric field (negative charge)
e/m ratio constant for all cathode rays (1.76 × 10¹¹ C/kg)
Electrons are fundamental particles present in all atoms
Mass of electron = 9.109 × 10⁻³¹ kg, Charge = -1.602 × 10⁻¹⁹ C

Discovery of Protons

Goldstein’s Experiment (1886)

Experimental Setup

Apparatus: Modified discharge tube with a perforated cathode (cathode with holes).

Procedure: When high voltage was applied to the gas at low pressure, Goldstein observed new rays traveling in the opposite direction to cathode rays – from anode to cathode through the holes in the cathode.

Observations
  1. These new rays traveled from anode toward cathode, opposite to cathode rays.
  2. They passed through holes in the cathode, so they were called canal rays or anode rays.
  3. In an electric field, canal rays deflected toward the negative plate, indicating they carry positive charge.
  4. The e/m ratio of canal rays depended on the gas in the tube.
  5. For hydrogen gas, canal rays had the highest e/m ratio.
Goldstein’s Findings

Interpretation: Canal rays are positively charged ions of the gas present in the discharge tube.

When cathode rays (electrons) collide with gas atoms, they knock out electrons, creating positive ions. These positive ions are attracted to the cathode and pass through its holes as canal rays.

For hydrogen gas (lightest element), the positive particle had the highest e/m ratio and was the lightest positive particle – later named the proton.

Rutherford’s Contribution (1917)

Experiment: Rutherford bombarded nitrogen gas with alpha particles (helium nuclei).

Observation: He detected hydrogen nuclei (protons) being emitted from nitrogen atoms.

Conclusion: Hydrogen nucleus (proton) is present in other nuclei. Rutherford proposed that protons are fundamental building blocks of all atomic nuclei.

Key Points: Discovery of Proton

First observed by E. Goldstein in 1886 as canal rays
Canal rays deflect toward negative plate in electric field (positive charge)
Rutherford (1917) proved protons are present in all nuclei
Proton identified as hydrogen nucleus (H⁺)
Mass of proton = 1.673 × 10⁻²⁷ kg, Charge = +1.602 × 10⁻¹⁹ C
A proton is 1836 times heavier than an electron

Discovery of Neutrons

Chadwick’s Experiment (1932)

Background Problem

Before 1932, scientists knew atomic mass ≠ (protons × proton mass). Example: Helium has mass 4 but charge +2. Where was the extra mass?

Rutherford predicted (1920) a neutral particle with proton-like mass might exist in nucleus.

Experimental Setup

Apparatus: Chadwick bombarded beryllium with alpha particles from polonium decay.

Procedure: When alpha particles hit beryllium, a highly penetrating radiation was produced. This radiation could knock protons out of paraffin wax.

Observations
  1. The radiation from beryllium was highly penetrating (could pass through several cm of lead).
  2. When this radiation hit paraffin wax (rich in hydrogen), it knocked out protons.
  3. The knocked-out protons were detected and their energy measured.
  4. The radiation was not deflected by electric or magnetic fields (no charge).
  5. Energy calculations showed the radiation particles had mass similar to protons.
Chadwick’s Interpretation

Nuclear Reaction: ⁴He (alpha) + ⁹Be → ¹²C + ¹n

Chadwick concluded the radiation consisted of neutral particles with mass approximately equal to protons. He named them neutrons.

Proof: Conservation of energy and momentum calculations showed only a neutral particle with proton-like mass could explain the observed proton energies from paraffin.

Key Points: Discovery of Neutron

Discovered by James Chadwick in 1932
Neutral particle (no charge) with mass similar to proton
Mass of neutron = 1.675 × 10⁻²⁷ kg (slightly heavier than proton)
Found in nucleus of all atoms except ordinary hydrogen
Explained difference between atomic mass and atomic number
Neutrons stabilize nucleus by countering proton-proton repulsion

Electron

Discoverer: J.J. Thomson

Year: 1897

Experiment: Cathode ray tube

Proton

Discoverer: E. Goldstein

Year: 1886

Experiment: Canal ray tube

Neutron

Discoverer: J. Chadwick

Year: 1932

Experiment: Beryllium bombardment

Multiple Choice Questions

MCQ 1

How many electrons can be accommodated at the most in the third shell of an atom?

(a) 8
(b) 18
(c) 10
(d) 32
Solution:

Maximum electrons in a shell = 2n², where n is shell number.

For third shell (n=3): 2 × (3)² = 2 × 9 = 18 electrons.

Correct answer: (b) 18

MCQ 2

What information was obtained from discharge tube experiments?

(a) Structure of atom was discovered
(b) Neutrons and protons were discovered
(c) Electrons and protons were discovered
(d) Presence of nucleus in an atom was discovered
Solution:

Discharge tube experiments led to the discovery of cathode rays (electrons) by J.J. Thomson and canal rays (protons) by E. Goldstein.

Neutrons were discovered later by Chadwick in 1932 through different experiments.

Correct answer: (c) Electrons and protons were discovered

MCQ 3

What does keep the particles present in the nucleus intact?

(a) Particles are held together by strong nuclear force
(b) Particles are held together by weak nuclear force
(c) Particles are held together by electrostatic force
(d) Particles are held together by dipolar force
Solution:

The strong nuclear force (also called strong interaction) binds protons and neutrons together in the nucleus.

This force is stronger than the electrostatic repulsion between positively charged protons but acts only over very short distances (≈ 10⁻¹⁵ m).

Correct answer: (a) Particles are held together by strong nuclear force

Short Answer Questions

Question 1

Why is it said that almost all the mass of an atom is concentrated in its nucleus?

Answer: Almost all the mass of an atom is concentrated in the nucleus because:

  1. The nucleus contains protons and neutrons, which are heavy particles.
  2. Proton mass = 1.673 × 10⁻²⁷ kg, Neutron mass = 1.675 × 10⁻²⁷ kg.
  3. Electron mass = 9.109 × 10⁻³¹ kg, which is about 1/1836 of proton mass.
  4. Thus, electrons contribute negligible mass compared to protons and neutrons.
  5. Over 99.9% of atomic mass comes from the nucleus.
Question 2

Why are elements different from one another?

Answer: Elements differ from one another because:

  1. Each element has a unique number of protons (atomic number).
  2. The number of protons determines the element’s identity.
  3. Different elements have different numbers of electrons, which govern chemical properties.
  4. The arrangement of electrons in shells differs, leading to different chemical behaviors.
  5. Isotopes of the same element have different numbers of neutrons but same number of protons.
Question 3

How many neutrons are present in ²⁰⁹₈₃Bi?

Answer: For Bismuth-209 (²⁰⁹₈₃Bi):

Mass number (A) = 209

Atomic number (Z) = 83 (number of protons)

Number of neutrons (N) = A – Z = 209 – 83 = 126

Therefore, Bismuth-209 has 126 neutrons.

Investigative Questions

Question 1

During discharge tube experiments, how did scientists conclude that the same type of electrons and protons are present in all elements?

Answer: Scientists concluded that the same type of electrons and protons are present in all elements based on these observations:

  1. For electrons: J.J. Thomson found that the e/m ratio for cathode rays was constant (1.76 × 10¹¹ C/kg) regardless of:
    • The gas used in the discharge tube (air, hydrogen, helium, etc.)
    • The material of the cathode (copper, platinum, aluminum, etc.)
  2. For protons: When different gases were used in canal ray experiments:
    • The positive particles had different e/m ratios depending on the gas
    • But the particle with the highest e/m ratio was always the same – the hydrogen ion (proton)
    • This indicated protons are fundamental particles present in all atoms
  3. Consistency: The charge of electrons and protons was found to be equal in magnitude but opposite in sign, and this was true for all elements tested.
Question 2

How can scientists synthesize elements in the laboratory?

Answer: Scientists synthesize elements in the laboratory through nuclear reactions:

  1. Nuclear Fusion: Combining lighter nuclei to form heavier ones.
    • Example: Creating helium by fusing hydrogen isotopes
    • Used to create elements up to iron in stars
  2. Particle Acceleration: Using particle accelerators to bombard target atoms with high-speed particles:
    • Light ions (protons, alpha particles) are accelerated to high energies
    • They collide with target nuclei, forming new elements
    • Example: Creating technetium (first artificially produced element)
  3. Transmutation: Converting one element into another by changing the number of protons:
    • Bombarding elements with neutrons can create isotopes
    • Beta decay can change neutrons to protons, creating new elements
  4. Recent Examples:
    • Copernicium (Cn, element 112) synthesized in 1996
    • Oganesson (Og, element 118) synthesized in 2002
    • These superheavy elements are highly unstable and exist only for milliseconds

Atomic Number and Mass Number

Atomic Number (Z): Number of protons in an atom. Since atoms are neutral, it also equals number of electrons.

Mass Number (A): Total number of protons and neutrons in an atom.

Neutron Number (N): Calculated as N = A – Z

Solved Examples

Example 1: Barium (¹³⁷₅₆Ba)

Calculate number of neutrons, protons and electrons in barium atom.

Solution:

Atomic number Z = 56, so protons = 56, electrons = 56

Mass number A = 137

Neutrons N = A – Z = 137 – 56 = 81

Answer: 81 neutrons, 56 protons, 56 electrons