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The Development of the Atomic Model

The Development of the Atomic Model
CHEMISTRY • Form 4 • Chapter 2: Matter and the Atomic Structure

2.2The Development of the Atomic Model

Atoms are extremely tiny and cannot be seen with the naked eye or through a microscope. Over centuries, scientists developed models of the atom through experimental evidence and reasoning.

Learning Objectives

  • State the subatomic particles in atoms of various elements.
  • Compare and contrast the relative mass and relative charge of proton, electron, and neutron.
  • Sequence the atomic structure models chronologically based on the Atomic Models of Dalton, Thomson, Rutherford, Bohr, and Chadwick.

Timeline and Evolution of the Atomic Model

From Dalton's solid sphere to Chadwick's complete nuclear model.

1
John Dalton 1766 – 1844

Solid, indivisible sphere.

2
J.J. Thomson 1856 – 1940

Positive sphere with embedded electrons.

3
Ernest Rutherford 1871 – 1937

Central positive nucleus with orbiting electrons.

4
Niels Bohr 1885 – 1962

Electrons move in specific shells around the nucleus.

5
James Chadwick 1891 – 1974

Nucleus contains protons and neutrons, surrounded by electron shells.

Brief Explanation

Subatomic particles and the scientists who shaped the atomic model.

An atom contains a central nucleus made up of protons (charge \(+1\), mass \(1\)) and neutrons (neutral, mass \(1\)). Electrons (charge \(-1\), mass \(\frac{1}{1840}\)) move around the nucleus in shells.
Subatomic particle Symbol Relative charge Relative mass Location
Proton \(p\) \(+1\) \(1\) Inside the nucleus
Neutron \(n\) \(0\) (Neutral) \(1\) Inside the nucleus
Electron \(e\) \(-1\) \(\frac{1}{1840}\) Outside the nucleus (moving in shells)

1. John Dalton (1766 – 1844)

Matter is made up of particles called atoms. An atom is the smallest spherical body that cannot be created, destroyed, nor divided further. Atoms of the same element are identical.

2. J.J. Thomson (1856 – 1940)

Discovered negatively-charged particles called electrons. Proposed that an atom is a positively-charged sphere embedded with several electrons.

3. Ernest Rutherford (1871 – 1937)

Proposed that the nucleus is the centre of the atom. Discovered positively-charged particles called protons in the nucleus. Almost the whole mass of the atom is concentrated in the nucleus. Electrons move outside the nucleus.

4. Niels Bohr (1885 – 1962)

Proposed that electrons in an atom move in specific shells around the nucleus. The nucleus contains protons.

5. James Chadwick (1891 – 1974)

Discovered neutral particles called neutrons located in the nucleus. Proved that neutrons contribute almost half of the mass of an atom. Modern model features a nucleus containing protons and neutrons, surrounded by electrons orbiting in shells.

Try These Questions First

Answer mentally, then check the explanation.

1

What are the three subatomic particles in an atom, and which of them has a relative mass of \(\frac{1}{1840}\)?

Answer: Proton, neutron, and electron. The electron has a relative mass of \(\frac{1}{1840}\).
2

Which scientist discovered the neutron, and what is its relative charge?

Answer: James Chadwick discovered the neutron. Its relative charge is \(0\) (neutral).
3

State the correct chronological order of the five scientists who developed the atomic structure model.

Answer: John Dalton, J.J. Thomson, Ernest Rutherford, Niels Bohr, and James Chadwick.

Common Mistakes

Avoid these errors when describing the atomic model.

!
Writing the relative mass of an electron as 0 instead of \(\frac{1}{1840}\).
!
Confusing Rutherford's model with Bohr's model: Rutherford stated electrons move outside the nucleus, while Bohr specifically introduced electron shells.
!
Attributing the discovery of protons to Thomson instead of Rutherford, or confusing Thomson's electron discovery with Chadwick's neutron discovery.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

An atom cannot be divided into smaller parts.

Correct Fact

Dalton originally proposed that atoms cannot be divided, but subsequent experiments proved that atoms consist of smaller subatomic particles (protons, neutrons, and electrons).

Misconception

The nucleus contains protons and electrons.

Correct Fact

The nucleus contains protons and neutrons (except for ordinary hydrogen). Electrons move outside the nucleus in shells.

Summary

Key facts to remember.

  • Protons (\(p\), \(+1\), mass \(1\)) and neutrons (\(n\), \(0\), mass \(1\)) reside in the nucleus.
  • Electrons (\(e\), \(-1\), mass \(\frac{1}{1840}\)) orbit the nucleus in shells.
  • Dalton → solid indivisible sphere.
  • Thomson → discovered electrons; positive sphere with embedded electrons.
  • Rutherford → discovered nucleus and protons; electrons move outside nucleus.
  • Bohr → electrons orbit in shells.
  • Chadwick → discovered neutrons in the nucleus.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which scientist discovered the positively-charged nucleus containing protons?

2

What are the relative charge and relative mass of a neutron?

B. Fill in the Blanks

3

J.J. Thomson discovered negatively-charged subatomic particles called _____.

4

Niels Bohr proposed that electrons in an atom move in _____ around the nucleus.

C. Match the Scientist with the Contribution

Drag each scientist on desktop. On mobile, tap a scientist and then tap the matching box.

Answer Choices

John Dalton J.J. Thomson Niels Bohr James Chadwick

Contribution / Model Feature

1Described the atom as the smallest indivisible spherical body that cannot be created or destroyed
2Discovered electrons in a positively-charged sphere
3Proposed that electrons move in specific shells around the nucleus
4Discovered neutral particles called neutrons in the nucleus
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Subatomic particle Proton Electron Neutron Nucleus Shell
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

The Development of the Atomic Model

The Development of the Atomic Model
CHEMISTRY • Form 4 • Chapter 2: Matter and the Atomic Structure

2.2The Development of the Atomic Model

Atoms are extremely tiny and cannot be seen with the naked eye or through a microscope. Over centuries, scientists developed models of the atom through experimental evidence and reasoning.

Learning Objectives

  • State the subatomic particles in atoms of various elements.
  • Compare and contrast the relative mass and relative charge of proton, electron, and neutron.
  • Sequence the atomic structure models chronologically based on the Atomic Models of Dalton, Thomson, Rutherford, Bohr, and Chadwick.

Timeline and Evolution of the Atomic Model

From Dalton's solid sphere to Chadwick's complete nuclear model.

1
John Dalton 1766 – 1844

Solid, indivisible sphere.

2
J.J. Thomson 1856 – 1940

Positive sphere with embedded electrons.

3
Ernest Rutherford 1871 – 1937

Central positive nucleus with orbiting electrons.

4
Niels Bohr 1885 – 1962

Electrons move in specific shells around the nucleus.

5
James Chadwick 1891 – 1974

Nucleus contains protons and neutrons, surrounded by electron shells.

Brief Explanation

Subatomic particles and the scientists who shaped the atomic model.

An atom contains a central nucleus made up of protons (charge \(+1\), mass \(1\)) and neutrons (neutral, mass \(1\)). Electrons (charge \(-1\), mass \(\frac{1}{1840}\)) move around the nucleus in shells.
Subatomic particle Symbol Relative charge Relative mass Location
Proton \(p\) \(+1\) \(1\) Inside the nucleus
Neutron \(n\) \(0\) (Neutral) \(1\) Inside the nucleus
Electron \(e\) \(-1\) \(\frac{1}{1840}\) Outside the nucleus (moving in shells)

1. John Dalton (1766 – 1844)

Matter is made up of particles called atoms. An atom is the smallest spherical body that cannot be created, destroyed, nor divided further. Atoms of the same element are identical.

2. J.J. Thomson (1856 – 1940)

Discovered negatively-charged particles called electrons. Proposed that an atom is a positively-charged sphere embedded with several electrons.

3. Ernest Rutherford (1871 – 1937)

Proposed that the nucleus is the centre of the atom. Discovered positively-charged particles called protons in the nucleus. Almost the whole mass of the atom is concentrated in the nucleus. Electrons move outside the nucleus.

4. Niels Bohr (1885 – 1962)

Proposed that electrons in an atom move in specific shells around the nucleus. The nucleus contains protons.

5. James Chadwick (1891 – 1974)

Discovered neutral particles called neutrons located in the nucleus. Proved that neutrons contribute almost half of the mass of an atom. Modern model features a nucleus containing protons and neutrons, surrounded by electrons orbiting in shells.

Try These Questions First

Answer mentally, then check the explanation.

1

What are the three subatomic particles in an atom, and which of them has a relative mass of \(\frac{1}{1840}\)?

Answer: Proton, neutron, and electron. The electron has a relative mass of \(\frac{1}{1840}\).
2

Which scientist discovered the neutron, and what is its relative charge?

Answer: James Chadwick discovered the neutron. Its relative charge is \(0\) (neutral).
3

State the correct chronological order of the five scientists who developed the atomic structure model.

Answer: John Dalton, J.J. Thomson, Ernest Rutherford, Niels Bohr, and James Chadwick.

Common Mistakes

Avoid these errors when describing the atomic model.

!
Writing the relative mass of an electron as 0 instead of \(\frac{1}{1840}\).
!
Confusing Rutherford's model with Bohr's model: Rutherford stated electrons move outside the nucleus, while Bohr specifically introduced electron shells.
!
Attributing the discovery of protons to Thomson instead of Rutherford, or confusing Thomson's electron discovery with Chadwick's neutron discovery.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

An atom cannot be divided into smaller parts.

Correct Fact

Dalton originally proposed that atoms cannot be divided, but subsequent experiments proved that atoms consist of smaller subatomic particles (protons, neutrons, and electrons).

Misconception

The nucleus contains protons and electrons.

Correct Fact

The nucleus contains protons and neutrons (except for ordinary hydrogen). Electrons move outside the nucleus in shells.

Summary

Key facts to remember.

  • Protons (\(p\), \(+1\), mass \(1\)) and neutrons (\(n\), \(0\), mass \(1\)) reside in the nucleus.
  • Electrons (\(e\), \(-1\), mass \(\frac{1}{1840}\)) orbit the nucleus in shells.
  • Dalton → solid indivisible sphere.
  • Thomson → discovered electrons; positive sphere with embedded electrons.
  • Rutherford → discovered nucleus and protons; electrons move outside nucleus.
  • Bohr → electrons orbit in shells.
  • Chadwick → discovered neutrons in the nucleus.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which scientist discovered the positively-charged nucleus containing protons?

2

What are the relative charge and relative mass of a neutron?

B. Fill in the Blanks

3

J.J. Thomson discovered negatively-charged subatomic particles called _____.

4

Niels Bohr proposed that electrons in an atom move in _____ around the nucleus.

C. Match the Scientist with the Contribution

Drag each scientist on desktop. On mobile, tap a scientist and then tap the matching box.

Answer Choices

John Dalton J.J. Thomson Niels Bohr James Chadwick

Contribution / Model Feature

1Described the atom as the smallest indivisible spherical body that cannot be created or destroyed
2Discovered electrons in a positively-charged sphere
3Proposed that electrons move in specific shells around the nucleus
4Discovered neutral particles called neutrons in the nucleus
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Subatomic particle Proton Electron Neutron Nucleus Shell
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Matter and Atomic Structure
Topic : The Development of the Atomic Model
Form 4 Chemistry
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