About the Notes
The notes about Atomic theory from Vedantu are necessary for every student who has an interest in science or is studying any branch of science. The notes are easy to understand and made in a simple manner to educate the students about the atomic theory and how it came into existence. The notes first begin with the introduction which includes the meaning of atoms and famous scientist John Dalton who is known for being the first scientist who talked about the atomic theory with credibility. Then follows the various other scientists who helped in the development of the Atomic theory. The students can also find the mentioning of Aristotle’s theory of elements which completely disagrees with Dalton’s Atomic theory. The notes talk about how the Atom is divided into three parts namely protons, electrons and neutrons. The electron is the negatively charged part of the atom, the proton being the positive charge and the neutron with no charge at all. Students also need to learn the basic formula such as how to calculate the mass number of an atom and other details such as the mass number of a neutron, proton, electron, etc.
Atomic Theory - Dalton's Theory and Thomson's Theory
The word Atom comes from the pristine Greek word “invisible”. In the 19th-century, chemists began using the term in connection with irreducible chemical elements.
According to the current theoretical model of the atom, it is a dense nucleus covered by a probabilistic "cloud" of electrons. The smallest piece of an element that maintains the identity of that element is known as an atom.
John Dalton (1766-1844) was an English chemist, physicist, and meteorologist. He is known for introducing the atomic theory into physics. He is also called the creator of proposing the atomic theory. This theory explains some concept that is relevant in the observable world.
John Dalton used a basic concept for his theory: the law of conservation of mass and the law of constant composition.
According to the law of conversion of mass, the total mass available before a chemical reaction is the same as the total mass available after the chemical reaction. In other words, we can explain this theory as “mass is conserved”. The law of conservation of mass was formulated by Antoine Lavoisier in 1743 - 1794.
Atomic theory is a theory that classifies many elementary states, facts, and properties including both postulate and axiom about atoms. According to Dalton, the atomic model consists of an atom in hard spheres. According to Rutherford, the atomic model includes atoms to view in the shape of solar systems.
The Concept that Atoms Play a Major Role in Getting Formalized by the Modern Atomic Theory Consists of the Following Three Parts:-
All matter is made from atoms.
Atoms of the different elements are different and atoms of the same elements are the same.
Atoms are the combination of whole numbers and ratios that are combined to form the compound.
These concepts comprise the basic building blocks of chemistry. We all know that Atom comes from the Greek word “invisible” and that atoms are composed of smaller parts known as subatomic particles. The first part to be explored was the electron. It is also known as a small subatomic particle with a negative charge. The proton is still a tiny subatomic particle with a positive charge. It is represented as P+. The Neutron is a subatomic particle with about the same mass as that of the proton but has no charge. It is represented as n or n0. Atoms have protons and neutrons in the centre and electrons in the orbit in the nucleus.
All hydrogen atoms have only one proton in their nucleus; all iron atoms have 26 protons in their nucleus. With the advancement of quantum mechanics and Schrodinger equation, the atomic theory has become an accurate mathematical science. Atomic radii are 10-8 cm. It contains three subatomic particles named electron, proton, and neutrons. The electron was discovered in the theory of cathode rays by JJ Thomson. It carried negative charges (-1.6*10-19c). Rutherford discovered protons on the basis of anode rays experiments. It carried positive charges (+1.6*10-19c). Neutrons are neutral particles and it was discovered by Chadwick. The total mass of the neutron is 1.675*10-24g or 1.008665 amu.
94 Be + 42He+ → 12 6C + 10n
An atomic number of the elements corresponds to the total number of protons available in the nucleus and the total number of electrons available in the neutron atom.
The mass number of elements = Number of protons + Number of neutrons
The Story of Atomic Theory
Here we state the whole specification about the atomic theory. The Atomic theory is the scientific theory that defines the nature and behaviour of the Atom. The matter is composed of separate units known as Atoms. There are multiple theories given by the scientists named below:-
Dalton's Theory
J.J. Thomson's Theory
Rutherford's Hypothesis
Bohr's Theory
Einstein, Heisenberg and Quantum Mechanics
Quark Theory
According to this theory, everything in the world is made up of tiny particles and it is surrounded by empty space. It varies in size and shape. These particles are known as atoms. The first theory about atoms was invented by Dalton and his theory is named Dalton’s Theory. He was an English physicist, born in 1766 at Eaglesfield, Cumbria, United Kingdom.
According to Dalton’s atomic theory, every element in space is composed of small particles that are indivisible and cannot be seen by the naked eye. These particles are known as atoms. Atoms are imperishable as “Atoms cannot be made nor be destroyed by the human being”. All the elements of the atoms are identical in mass and the behaviour of the atom of one element is different from the behaviour of the atoms of all the other elements.
There were some issues in Dalton’s theory so his idea was opposed by the prominent philosophers of that time. Aristotle in that instance disagreed completely stating instead that matter involved four elements which are earth, wind, water and fire, and after that scientists followed this suit. Atomic theory was now accepted by the scientific community, but the next advancement was through the discovery by J. J. Thomson’s in the year 1897. He defined in his theory that the atoms as uniformly packed spheres of positive charges matter filled with the negatively charged electron.
Thomson won Nobel Prize for this research. But his model of the atom did not go around for long. Ernest Rutherford invented a new theory. It is known as the Rutherford model and is generally compared to Thomson’s model. According to Thomson’s model, the atom thinly dispersed positive charges and there would not be enough space to recurve the particle in any one place. The effect would have been like a wad of tennis balls punching via a thin paper screen. But while most of the particles did cross some bounced right back, suggesting that the foil was more like a thick net with a very volumetric mesh. According to Rutherford, the atoms dwell largely in empty space with just a few electrons, while mass was concentrated in the centre, which he termed as the nucleus. The Alpha particle crossed through the gaps but bounced back from the intensive, positively charged nucleus.
But the atomic theory was not complete just yet. After some time another scientist named Niels Bohr expanded on Rutherford’s nuclear model. He confirmed that electrons orbit the nucleus at fixed energies and distance and had the ability to jump from one level to other levels in their orbits. However, they cannot exist in space. Bohr’s planetary model took centre stage, but soon, it too gave rise to some difficulties. His experiment has shown that rather than simply being separate particles, electrons cumulatively behave like a wave. They are not confined to a particular point in space.
After some time, again new research came in by Werner Heisenberg. He formulated his famous uncertainty principle. According to this, it was impossible to specify both the exact position and speed of electrons as they moved around an atom. The idea was that the electrons cannot be pinpointed but they exist within a range of eventual locations giving rise to the same quantum model of the atom, a fascinating theory whose absolute implications have yet to be fully grasped.
FAQs on Atomic Theory
1. What is the fundamental concept of atomic theory as per the CBSE 2025–26 syllabus?
The fundamental concept of atomic theory is that all matter is composed of tiny, indivisible particles called atoms. These atoms are the basic building blocks of matter and retain the identity of the element, as established in both chemistry and physics frameworks.
2. How did Dalton's atomic theory differ from previous ideas about matter?
Dalton's atomic theory proposed that all elements are made of indivisible atoms and that atoms of a given element are identical in mass and properties. This differed from earlier beliefs, such as Aristotle’s four-element theory, by introducing the scientific basis for individual, indestructible particles forming matter.
3. What are the main differences between Thomson's and Rutherford's models of the atom?
- Thomson's model described the atom as a sphere of positive charge with embedded electrons, known as the 'plum pudding model.'
- Rutherford's model showed that the atom has a tiny, dense, positively charged nucleus with electrons revolving around it, leaving most of the atom as empty space.
4. Why was the discovery of subatomic particles important for the advancement of atomic theory?
The discovery of electrons, protons, and neutrons showed that atoms are not indivisible, but are composed of smaller particles. This led to modern atomic models and deepened our understanding of chemical reactions, atomic structure, and material properties.
5. How does modern atomic theory incorporate quantum mechanics in explaining atomic structure?
Modern atomic theory states that electrons occupy a probabilistic 'cloud' around the nucleus, as described by quantum mechanics. Instead of fixed orbits, the position of electrons is given by probability distributions, reflecting principles like the Heisenberg Uncertainty Principle and Schrödinger’s equations.
6. What key laws and experiments laid the foundation for the development of atomic theory?
- Law of Conservation of Mass (Lavoisier): Mass remains constant during chemical reactions.
- Law of Definite Proportions (Proust): Compounds have fixed, definite ratios of elements.
- Cathode ray experiment (J.J. Thomson): Discovered the electron.
- Gold foil experiment (Rutherford): Identified the atomic nucleus.
7. What is the significance of the atomic number and mass number in atomic theory, as per CBSE syllabus requirements?
The atomic number is the number of protons in an atom's nucleus, which defines the element. The mass number is the sum of protons and neutrons. Both are essential for identifying elements and understanding isotopes, crucial topics in the CBSE 2025–26 curriculum.
8. How do misconceptions about the indivisibility of atoms impact understanding of atomic theory?
A common misconception is that atoms are indivisible. Modern discoveries of subatomic particles (electrons, protons, neutrons) show that atoms can be further divided. Addressing this helps students grasp the evolving nature of scientific theories and appreciate the depth of atomic models.
9. In what ways has atomic theory influenced the study and advancement of physics and chemistry?
Atomic theory provides a foundation for understanding chemical bonding, material science, nuclear physics, and quantum mechanics. It guides explanations for material properties, reaction mechanisms, and the behavior of matter at microscopic levels, central to both physics and chemistry.
10. How does the evolution of atomic theory illustrate the scientific method?
The evolution of atomic theory—from Dalton, Thomson, Rutherford, Bohr, to quantum models—shows how scientific knowledge builds on experiments, evidence, and correction of prior mistakes. It is a prime example of how scientific theories are refined as new data and better models emerge.
