Plus One / Class 11 Chemistry
Chapter: Structure of the Atom – Detailed Notes
1. Introduction to Atom
An atom is the smallest unit of an element that retains the chemical properties of that element.
Atoms are extremely small and cannot be seen with the naked eye. Every atom is composed of
three main subatomic particles: electrons, protons and neutrons.
• Electrons – negatively charged particles
• Protons – positively charged particles
• Neutrons – neutral particles
Atoms combine with each other to form molecules and compounds.
2. Discovery of Subatomic Particles
Electron: Discovered by J. J. Thomson through cathode ray experiments.
He showed that cathode rays consist of negatively charged particles called electrons.
Proton: The existence of positively charged particles was confirmed from canal ray experiments.
Neutron: Discovered by James Chadwick in 1932. Neutrons have no charge and are present in the
nucleus of atoms.
3. Atomic Models
Thomson Model:
J. J. Thomson proposed the 'plum pudding model'. According to this model,
electrons were embedded in a positively charged sphere.
Rutherford Model:
Ernest Rutherford conducted the famous gold foil experiment.
Important conclusions:
• Most of the atom is empty space.
• A small dense nucleus is present at the center.
• Electrons revolve around the nucleus.
Limitations:
This model could not explain the stability of atoms.
4. Bohr’s Model of Atom
Niels Bohr proposed that electrons move around the nucleus only in certain allowed circular paths
called energy levels or shells.
Main points:
• Electrons revolve in fixed energy levels.
• Energy is absorbed or emitted when electrons jump between levels.
Chapter: Structure of the Atom – Detailed Notes
1. Introduction to Atom
An atom is the smallest unit of an element that retains the chemical properties of that element.
Atoms are extremely small and cannot be seen with the naked eye. Every atom is composed of
three main subatomic particles: electrons, protons and neutrons.
• Electrons – negatively charged particles
• Protons – positively charged particles
• Neutrons – neutral particles
Atoms combine with each other to form molecules and compounds.
2. Discovery of Subatomic Particles
Electron: Discovered by J. J. Thomson through cathode ray experiments.
He showed that cathode rays consist of negatively charged particles called electrons.
Proton: The existence of positively charged particles was confirmed from canal ray experiments.
Neutron: Discovered by James Chadwick in 1932. Neutrons have no charge and are present in the
nucleus of atoms.
3. Atomic Models
Thomson Model:
J. J. Thomson proposed the 'plum pudding model'. According to this model,
electrons were embedded in a positively charged sphere.
Rutherford Model:
Ernest Rutherford conducted the famous gold foil experiment.
Important conclusions:
• Most of the atom is empty space.
• A small dense nucleus is present at the center.
• Electrons revolve around the nucleus.
Limitations:
This model could not explain the stability of atoms.
4. Bohr’s Model of Atom
Niels Bohr proposed that electrons move around the nucleus only in certain allowed circular paths
called energy levels or shells.
Main points:
• Electrons revolve in fixed energy levels.
• Energy is absorbed or emitted when electrons jump between levels.
