Structure of the Atom: Discovery of Electrons, Protons, and Neutrons

 

Structure of the Atom

Introduction

Matter is made up of tiny particles called atoms. An atom is the smallest unit of an element that retains its chemical properties. Earlier, scientists believed atoms were indivisible, but later discoveries revealed that atoms are made up of even smaller particles. These particles are called subatomic particles, which include:

• Electrons – negatively charged particles
• Protons – positively charged particles
• Neutrons – neutral particles (no charge)

Understanding the structure of an atom helps explain how elements react and form compounds.

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Discovery of Subatomic Particles

Discovery of Electrons

The first subatomic particle discovered was the electron. It was identified by J.J. Thomson in 1897 during his cathode ray tube experiment.

J.J. Thomson’s Cathode Ray Experiment

Setup:

• A glass tube was filled with a gas at low pressure.
• Two metal plates (electrodes) were placed at both ends of the tube.
• A high voltage was applied, creating a glowing beam.

Observations:

• The beam moved from the negative electrode (cathode) to the positive electrode (anode).
• When an electric or magnetic field was applied, the beam bent towards the positive side.

Conclusion:

• The beam was made up of negatively charged particles, later named electrons.
• Electrons are present in all atoms.

Properties of Electrons

• Charge: Negative (-1)
• Mass: Very small (1/1836 of a hydrogen atom)
• Location: Move around the nucleus in energy levels

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Discovery of Protons

After the discovery of electrons, scientists realized atoms must also contain positive charge to balance the negative charge. In 1886, E. Goldstein discovered protons through his canal ray experiment.

Goldstein’s Canal Ray Experiment

Setup:

• A glass tube was used, similar to Thomson’s experiment.
• A gas was placed inside, and a high voltage was applied.
• A second set of rays, called canal rays, was observed moving in the opposite direction of cathode rays.

Observations:

• The rays were positively charged.
• The mass of these particles was greater than that of electrons.

Conclusion:

• The positive particles were called protons.
• Protons are present in the nucleus of an atom.

Properties of Protons

• Charge: Positive (+1)
• Mass: Equal to a hydrogen atom
• Location: Inside the nucleus

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Discovery of Neutrons

Scientists found that the atomic mass was greater than the total mass of protons and electrons. This suggested the presence of another particle. In 1932, James Chadwick discovered the neutron.

Chadwick’s Neutron Experiment

Setup:

• A beryllium metal sheet was bombarded with fast-moving alpha particles.
• A new type of radiation was observed.

Observations:

• The radiation was not affected by electric or magnetic fields.
• The particles had mass similar to protons but no charge.

Conclusion:

• The radiation consisted of neutral particles, later called neutrons.
• Neutrons are located in the nucleus of an atom.

Properties of Neutrons

• Charge: No charge (neutral)
• Mass: Almost equal to a proton
• Location: Inside the nucleus

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Structure of the Atom

An atom consists of two main parts:

1. Nucleus

   • Located at the center of the atom.
   • Contains protons and neutrons.
   • Holds most of the atom’s mass.

2. Electron Shells (Energy Levels)

   • Electrons move around the nucleus in fixed orbits.
   • These orbits are called energy levels or shells.
   • Shells are named as K, L, M, N, etc.

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Key Points

• Atoms are not indivisible; they consist of electrons, protons, and neutrons.
• Electrons were discovered by J.J. Thomson using the cathode-ray tube experiment.
• Protons were discovered by E. Goldstein using the canal ray experiment.
• Neutrons were discovered by James Chadwick using the beryllium experiment.
• The nucleus contains protons and neutrons, while electrons revolve around it in shells.

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Multiple Choice Questions (MCQs)

1. What is the smallest unit of matter?
   a) Molecule
   b) Atom
   c) Electron
   d) Proton
   (Answer: b) Atom

2. Which scientist discovered the electron?
   a) Rutherford
   b) Dalton
   c) J.J. Thomson
   d) Chadwick
   (Answer: c) J.J. Thomson

3. What charge does an electron carry?
   a) Positive
   b) Negative
   c) Neutral
   d) Double positive
   (Answer: b) Negative

4. Protons were discovered by:
   a) Bohr
   b) Goldstein
   c) J.J. Thomson
   d) Rutherford
   (Answer: b) Goldstein

5. Neutrons are found in the:
   a) Nucleus
   b) Electron shells
   c) Outside the atom
   d) Proton cloud
   (Answer: a) Nucleus

6. Which experiment helped in the discovery of electrons?
   a) Gold foil experiment
   b) Cathode ray experiment
   c) Oil drop experiment
   d) Rutherford experiment
   (Answer: b) Cathode ray experiment

7. The charge of a proton is:
   a) +1
   b) -1
   c) 0
   d) +2
   (Answer: a) +1

8. The discovery of neutrons was made by:
   a) Thomson
   b) Rutherford
   c) Chadwick
   d) Bohr
   (Answer: c) Chadwick

9. Electrons are present in:
   a) Nucleus
   b) Outside the nucleus
   c) Inside protons
   d) Inside neutrons
   (Answer: b) Outside the nucleus

10. Which subatomic particle has no charge?
    a) Proton
    b) Electron
    c) Neutron
    d) Nucleus
    (Answer: c) Neutron

11. The mass of an electron is approximately:
    a) 1 atomic mass unit
    b) 1/1836 of a proton
    c) Equal to a proton
    d) 10 times a neutron
    (Answer: b) 1/1836 of a proton

12. Who proposed the nuclear model of the atom?
    a) J.J. Thomson
    b) Dalton
    c) Rutherford
    d) Chadwick
    (Answer: c) Rutherford

13. Neutrons help in:
    a) Providing charge to an atom
    b) Holding the nucleus together
    c) Producing light
    d) Controlling electrons
    (Answer: b) Holding the nucleus together

14. What is the main component of an atom's mass?
    a) Electrons
    b) Neutrons and protons
    c) Proton and electron
    d) Neutron and electron
    (Answer: b) Neutrons and protons

15. The charge of an atom with equal protons and electrons is:
    a) Positive
    b) Negative
    c) Neutral
    d) Double positive
    (Answer: c) Neutral

16. Electrons are arranged in:
    a) Nucleus
    b) Electron clouds
    c) Proton shells
    d) Inside neutrons
    (Answer: b) Electron clouds

17. The atomic number of an element represents the number of:
    a) Neutrons
    b) Electrons
    c) Protons
    d) Nuclei
    (Answer: c) Protons

18. What is the relative mass of a neutron?
    a) Equal to a proton
    b) Half of a proton
    c) Double of an electron
    d) 1/1836 of a proton
    (Answer: a) Equal to a proton

19. Which particle is responsible for chemical bonding?
    a) Proton
    b) Neutron
    c) Electron
    d) Nucleus
    (Answer: c) Electron

20. The space in an atom is mostly:
    a) Filled with neutrons
    b) Filled with electrons
    c) Empty
    d) Filled with protons
    (Answer: c) Empty

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Short Questions

1. What is an atom?
2. Name the three fundamental subatomic particles.
3. Who discovered the electron and how?
4. Write two properties of an electron.
5. Where are protons found in an atom?
6. What is the role of neutrons in an atom?
7. How does an electron differ from a proton in charge and mass?
8. What experiment led to the discovery of the nucleus?
9. What is the significance of Rutherford's atomic model?
10. How did J.J. Thomson prove the presence of negatively charged particles?
11. What is the atomic number of an element?
12. Define nucleus in terms of atomic structure.
13. Why is the neutron important in the stability of the nucleus?
14. Compare the relative masses of protons, neutrons, and electrons.
15. What is the difference between atomic number and mass number?
16. How are electrons arranged around the nucleus?
17. Why do electrons not fall into the nucleus?
18. What determines the chemical properties of an atom?
19. Why is most of an atom empty space?
20. How does the discovery of subatomic particles change our understanding of matter?