Journey Inside the Atom
Explore the World Inside the Atom
Take a closer look at the tiny building blocks that make up everything around us. Learn how scientists discovered that atoms are not indivisible, explore the roles of protons, neutrons and electrons, and understand how these particles are arranged inside an atom. From atomic models to atomic number, mass number and electronic configuration, discover how the structure of an atom helps explain the behaviour of elements.
1.0Learning Outcomes
After completing this lesson, you will be able to:
- Explain why an atom is considered the basic building block of matter.
- Trace how different scientists contributed to our understanding of the structure of an atom.
- Identify the three main subatomic particles: electrons, protons and neutrons.
- Describe the basic structure of an atom and the arrangement of its particles.
- Compare the properties and charges of electrons, protons and neutrons.
- Understand the main ideas behind different atomic models.
- Explain atomic number and mass number.
- Understand how electrons are arranged in different shells of an atom.
- Relate electronic configuration to the structure of different elements.
- Use simple examples to understand how atomic structure helps us identify elements.
2.0Introduction to the Lesson
Everything around us is made of matter, and matter is built from atoms. For a long time, scientists thought an atom was the smallest particle that could not be divided further. Later discoveries showed that an atom has an internal structure and contains even smaller particles.
In this lesson, you will look inside the atom and learn how our understanding of its structure developed over time. You will meet electrons, protons and neutrons, explore the atomic models proposed by different scientists, and understand how these particles are arranged within an atom. You will also learn about atomic number, mass number and electronic configuration, which help us describe the structure of different elements.
Important Topics
3.0Discovery of Subatomic Particles
An atom was once considered to be indivisible. However, experiments with electricity and gases showed that atoms contain smaller particles called subatomic particles. The three main subatomic particles are electrons, protons and neutrons.
- Electron: Negatively charged particle found outside the nucleus. Its relative mass is about .
- Proton: Positively charged particle present in the nucleus. Its relative mass is about 1.
- Neutron: Particle with no charge present in the nucleus. Its relative mass is about 1.
The electron was discovered by J. J. Thomson in 1897 through cathode ray experiments. Eugen Goldstein's canal ray experiments provided evidence for positively charged particles, while James Chadwick discovered the neutron in 1932. These discoveries showed that an atom has an internal structure.
4.0Thomson Model of the Atom
After discovering the electron, J. J. Thomson proposed a model to explain the structure of an atom. According to this model, an atom is a sphere of positive charge with electrons embedded in it.
The main features of Thomson's model are:
- Positive charge is spread throughout the atom.
- Electrons are embedded within the positive charge.
- The total positive charge balances the total negative charge.
- Therefore, the atom is electrically neutral.
- The model does not contain a separate central nucleus.
Thomson's model helped explain the presence of electrons, but later experiments showed that the positive charge is actually concentrated in a small central region called the nucleus.
5.0Rutherford Model of the Atom
Rutherford's alpha-particle scattering experiment provided important evidence about the internal structure of an atom. In the experiment, alpha particles were directed towards a very thin gold foil.
The observations were:
- Most alpha particles passed straight through the foil.
- Some particles were deflected through small angles.
- A very small number were deflected through large angles or bounced back.
From these observations, Rutherford concluded that an atom has a small, dense and positively charged nucleus at its centre. Most of the atom is empty space, and electrons are present outside the nucleus.
However, Rutherford's model could not explain why electrons moving around the nucleus did not continuously lose energy and fall into the nucleus. This limitation led to the development of Bohr's model.
6.0Bohr Model of the Atom
To explain the stability of the atom, Niels Bohr proposed that electrons occupy fixed energy levels or shells around the nucleus.
These shells are represented as:
- K shell – first shell
- L shell – second shell
- M shell – third shell
- N shell – fourth shell
According to the Bohr model, an electron does not continuously lose energy while it remains in its permitted shell. An electron can move from one energy level to another only by gaining or losing the required amount of energy.
The maximum number of electrons that can be accommodated in a shell is given by:
where represents the shell number. Thus, the K shell can hold up to 2 electrons, the L shell up to 8, and the M shell up to 18 electrons.
7.0Atomic Structure
An atom mainly consists of a nucleus and electrons. The nucleus contains protons and neutrons, while electrons are arranged in shells around it.
The nucleus contains most of the mass of an atom because protons and neutrons are much heavier than electrons. At the same time, it occupies only a very small part of the atom, so most of an atom is empty space.
Electrons are arranged in shells around the nucleus. The arrangement of electrons in these shells is called electronic configuration. For example, carbon has 6 electrons arranged as 2, 4, while oxygen has 8 electrons arranged as 2, 6.
8.0Atomic Number
The atomic number of an element is the number of protons present in the nucleus. It is represented by .
The number of protons determines the identity of an element. For example, an atom with 6 protons is carbon, while an atom with 8 protons is oxygen.
In a neutral atom, the number of protons is equal to the number of electrons.
Therefore, if an atom has atomic number 8, it has 8 protons and 8 electrons when it is neutral.
9.0Mass Number
The mass number is the total number of protons and neutrons present in the nucleus. It is represented by .
The number of neutrons can be found using:
For example, an atom with mass number 23 and atomic number 11 has 11 protons and:
So, it has 12 neutrons. If the atom is neutral, it will also have 11 electrons.
10.0Atomic Number and Mass Number
Atomic number and mass number give different information about an atom.
In the standard representation:
the upper number represents the mass number, while the lower number represents the atomic number.
For example, in , 23 is the mass number and 11 is the atomic number. This information can be used to find the number of protons, neutrons and electrons in the atom.
11.0Supporting Study Materials
This study material for Class 9 Science, Journey Inside the Atom, is aligned with the latest NCERT syllabus. Along with these concepts, students can refer to NCERT Solutions for step-by-step answers and CBSE Notes for quick revision of important topics such as atomic structure, subatomic particles, atomic models, atomic number, and mass number.
12.030-Second Quick Revision
- Subatomic particles: An atom contains electrons, protons and neutrons. Electrons are negative, protons are positive, and neutrons have no charge.
- Thomson model: The atom was considered a positively charged sphere with electrons embedded in it.
- Rutherford model: The atom has a small, dense, positively charged nucleus, and most of the atom is empty space.
- Bohr model: Electrons occupy fixed energy levels or shells around the nucleus.
- Atomic structure: Protons and neutrons are present in the nucleus, while electrons are arranged in shells around it.
- Atomic number : Number of protons in the nucleus. In a neutral atom, it is also equal to the number of electrons.
- Mass number : Total number of protons and neutrons.
Quick memory tip:
Z → Protons → Element identity
A → Protons + Neutrons → Nucleus
13.0Sample Question and Answers
Q1. What are the three main subatomic particles of an atom?
The three main subatomic particles are electrons, protons and neutrons. Electrons are negatively charged, protons are positively charged, and neutrons have no charge.
Q2. What did Rutherford's alpha-particle scattering experiment show?
It showed that an atom has a small, dense and positively charged nucleus at its centre and that most of the atom is empty space.
Q3. What is the main idea of Bohr's model of the atom?
Bohr proposed that electrons move around the nucleus in fixed energy levels or shells. Electrons can move between these levels by gaining or losing energy.
Q4. What is atomic number?
Atomic number is the number of protons present in the nucleus of an atom. It is represented by . In a neutral atom, it is also equal to the number of electrons.
Q5. What is mass number?
Mass number is the total number of protons and neutrons present in the nucleus.
Q6. An atom has atomic number 8 and mass number 16. Find the number of protons, neutrons and electrons.
Protons = 8 and electrons = 8 in a neutral atom. Neutrons = . Therefore, the atom has 8 protons, 8 neutrons and 8 electrons.
Q7. How did the discovery of subatomic particles change the idea of an atom?
The discovery of electrons, protons and neutrons showed that an atom is not indivisible. It has an internal structure made up of smaller particles.
14.0Recommended Next Topics