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Chapter 12 – Atoms

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Atom is the smallest unit of matter consisting of nucleus and electrons. The nucleus consists of protons (positive charges) and neutrons (neutral charges). This article goes in-depth on these concepts for understanding. You can refer to these notes for exam revision too.  

Electrons are electromagnetically bound to the nucleus. The answer to What is an Atom? Has undergone a lot of variation in history. It started at the time of Democritus, later presented by J.J Thomson explained the atom as a plum pudding model where the atom has uniformly distributed positive charge, and negative charges are embedded into it. Along came Rutherford’s explanation, Bohr Postulates, and de Broglie duality explanation to explain the atomic structure and nature. With the development of physics and specifically in the quantum mechanics part, better explanations of the atoms were given.



Table of Contents:

  • Atomic Structure
  • Different atomic models
  • Alpha- Particle Experiment
  • Electron Orbits
  • Atomic Spectra
  • Spectral Series
  • Bohr Model of Hydrogen atom
  • Energy Levels
  • Line Spectra of the Hydrogen atom
  • De Broglie duality concept

Atom structures have protons, electrons, and neutrons, with the arrangement of the nucleus made up of neutrons and protons and electrons surrounding it with the atom. Every atom has a varied atomic structure due to their different number of electrons and neutrons present in it.

Different atomic models

  • Dalton’s Atomic Theory

All matter differ, have their own characteristic size and mass, and are made of atoms that can not be divided or destroyed and are responsible for all chemical reactions.

  • Thomson Atomic Model

Thompson explained atoms as “Plum pudding” as the structure is electrically neutral and where the negative charges are embedded into the positively charged nucleus.

The model does not explain the stability concept of the atom.


  • Rutherford Nuclear Model

Rutherford later presented a planetary model/ nuclear model of atom where the inside atom, positive charge, and most of the mass of the atom is concentrated in a small volume called the nucleus, and electrons revolve around the nucleus.

The experiment conducted by H. Geiger and E. Marsden’s known Alpha particle scattering experiment was conducted in 1911, where a beam of alpha-particles was directed to a thin gold metal sheet. Depending on the impact parameter (a perpendicular distance of initial velocity vector from the center of the nucleus) the beam is directed into various directions with different probabilities called the alpha particle trajectory.

Alpha particle close to the nucleus has large scattering, and for a large impact parameter, the alpha-particle goes undefeated. This is in conformity with the Rutherford nuclear model, as the mass of the atom is concentrated in a small volume, and this experiment gives a way to determine the size of the nucleus.

In classical theory, an atom was considered as a neutral sphere consisting of a positively charged nucleus at the center and electrons surrounding the nucleus in a stable orbit. The Electromagnetic force between the positive nucleus and negative electrons provides the necessary centripetal force to keep the electrons in their orbit in which they revolve, known as electron orbits. Based on these electron orbits, further, the atom stability, energy levels, and atomic spectra were explained.

Atomic spectra could be compared to fingerprints of the gas. Every element has its own characteristic spectrum radiation, and the spectrum consists of specific wavelengths corresponding to that element.

When the white light is passed through the gas, and the bands are observed under the spectrometer, dark lines appear in the spectrum. These dark lines relate to the wavelengths which were observed in the emission line spectrum of the gas.

Each element emits light of a particular frequency showing a similar pattern. These contain dark lines and are found in the emission spectrum of the gas called the absorption spectrum.

Spacing between the set of lines decreases in a regular pattern, which is termed as spectral series. The first such series was found by Jahan Jakob in 1885 in the visible region of the Hydrogen spectrum and named the Balmer Series.

These lines appear closer, and intensity decreases as the wavelength decreases. Frequency of any line of the series can be expressed –

Lyman series: v = Rc(\frac{1}{1^2} - \frac{1}{n^2}) ; where n=2, 3, 4..

Balmer series: v = Rc(\frac{1}{2^2} - \frac{1}{n^2}) ; where n=3, 4, 5..

Paschen series: v = Rc(\frac{1}{3^2} - \frac{1}{n^2}) ; where n=4, 5, 6..

Brackett series: v = Rc(\frac{1}{4^2} - \frac{1}{n^2}) ; where n=5, 6, 7..

Where R is Rydberg constant with value 1.097107m-1 and c is the speed of light.

After Rutherford nuclear model, a clear picture of an atom but few fundamental differences were present in his model related to the atom being unstable because as the charged particles would emit radiation in the form of electromagnetic waves, energy would continuously decrease, and the electron will eventually fall into the nucleus. Niels Bhor presented his postulates, where he combined concepts of classical theory and new quantum concepts to explain the phenomena. These are

  • For electrons in an atom to revolve without energy emission, it is required to orbit in certain stable orbits.
  • For an electron to revolve, orbits with the angular momentum are some integral multiple of h/2 allowed. Angular momentum is given by L=nh/2, where n is the quantum number and his Planck constant.
  • The third postulates state that for an electron to move from the non-radiating orbit to an orbit having lower energy, a photon of equal energy required for this transition from initial to the final state is emitted. The frequency (v) is given by hv=Ei-Ef.

These explanations were termed as the Bohr model of the hydrogen atom, which explained the atom behavior and their stability.

Energy levels vary from least to high as we go from the electrons revolving in orbit closest to the nucleus to in the outer orbits. The lowest energy level or ground state is the lowest energy where the electron revolves in the orbit of the smallest radius. When processes help gain energy, atoms receive sufficient energy to raise electrons from lower state to higher state, which is said as an excited energy state.

The energy required to excite electrons to upper energy levels is given by En-Ek, where En is excited-state energy, and Ek is the energy of revolving electron orbit.

Transitions between different atomic levels, light is radiated in various frequencies. The line spectra of hydrogen atoms were observed due to the transition of the energy levels, which were known as Lyman, Balmer, Paschen, Brackett, and Pfund series. These spectral lines, also called emission lines, are produced when electrons jump from higher energy states to the lower energy state, and photons are emitted.

De Broglie presented the hypothesis that electrons have a wavelength =h/mv and explained the behavior using the wave-particle duality concept. He stated that the orbits which correspond to the circular standing waves, in which the circumference of the orbit equals a whole number of wavelengths. This was known as De Broglie’s explanation of Bohr’s second postulate of Quantisation.

The Bohr model explained the phenomena of the atom given by Rutherford and offered a much clear picture of an atom and was explained by the duality concept given by De Broglie’s explanation. Still, the applicability of the Bohr was limited to hydrogenic atoms, which are single-electron atoms. Model, when tried with atoms of helium having two electrons, was not a success. Even though the Bohr model correctly predicted the frequencies of the light of emission of a Hydrogen atom, the model could not explain the relative intensities of the frequencies emitted by hydrogenic atoms.

New radical theories based on Quantum Mechanics provide a complete picture of the atomic structure.

For a better understanding of the concepts behind them in detail, check out videos on MSVgo. It has an array of interesting and informative topics and chapters from the syllabus and helps understand every concept in-depth. Through video learning, you will be able to grasp knowledge much better. Download the MSVgo application or visit the website and explore the video library that explains concepts with examples and explanatory visualizations, animations.

High School Physics

  • Alternating Current
  • Atoms
  • Communication Systems
  • Current Electricity
  • Dual nature of Radiation and Matter
  • Electric Charges and Fields
  • Electricity
  • Electromagnetic Induction
  • Electromagnetic Waves
  • Electron Beams and Radioactivity
  • Electrons and Photons
  • Electrostatic Potential and Capacitance
  • Fluid Pressure
  • Force and Acceleration
  • Force And Laws Of Motion
  • Gravitation
  • Internal Energy
  • Kinetic Theory
  • Law of motion
  • Light – Reflection And Refraction
  • Magnetic Effects Of Electric Current
  • Magnetism and Matter
  • Management Of Natural Resources
  • Mechanical properties of Fluids
  • Mechanical properties of Solids
  • Motion
  • Motion in a plane
  • Motion in a straight line
  • Moving Charges and Magnetism
  • Nuclear Energy
  • Nuclei
  • Oscillations
  • Our Environment
  • Paths of Heat
  • Physical world
  • Ray optics and optical instruments
  • Semiconductor Devices
  • Semiconductor Electronics: Materials, Devices and Simple Circuits
  • Simple Machines
  • Sound
  • Sources Of Energy
  • Specific and Latent Heats
  • Spherical Mirrors
  • Static Electricity
  • Systems of Particles and Rotational motion
  • Thermal properties of matter
  • Thermodynamics
  • Units and Measurement
  • Vectors, Scalar Quantities and Elementary Calculus
  • Wave Optics
  • Waves
  • Work, Power and Energy

High School Chemistry

  • Acids, Bases and Salts
  • Alcohols, Phenols and Ethers
  • Aldehydes, Ketones and Carboxylic Acids
  • Aliphatic and Aromatic Hydrocarbons
  • Alkyl and Aryl Halides
  • Amines
  • Analytical Chemistry 
  • Atomic Structure
  • Atoms And Molecules
  • Basic concepts of Chemistry
  • Biomolecules
  • Carbon And Its Compounds
  • Carboxylic acids and Acid Derivatives
  • Chemical Bonding and Molecular Structures
  • Chemical Energetics
  • Chemical Equilibria
  • Chemical Kinetics
  • Chemical Reactions And Equations
  • Chemical Reactions and Their Mechanisms
  • Chemistry in Everyday Life
  • Chemistry of p-Block elements
  • Chemistry of Transition and Inner Transition
  • Classification of Elements
  • Coordination Compounds
  • Cyanide, Isocyanide, Nitro compounds and Amines
  • Electrochemistry
  • Electrolysis
  • Elements, Compounds and Mixtures
  • Environmental Chemistry
  • Equilibrium
  • Ethers and Carbonyl compounds
  • Haloalkanes and Haloarenes
  • Hydrocarbons
  • Hydrogen
  • Ideal solutions
  • Introduction to Organic Chemistry
  • Ionic equilibria
  • Matter
  • Matter Around Us
  • Matter In Our Surroundings
  • Metallurgy
  • Metals And Non-Metals
  • Mole Concept and Stoichiometry
  • Natural Resources
  • Organic Chemistry – Basic Principles
  • Periodic Classification of Elements
  • Physical and Chemical Changes
  • Physical and Chemical Properties of Water
  • Polymers
  • Preparation, Properties and Uses of Compounds
  • Principles and Processes of Isolation of Elements
  • Redox Reactions
  • Relative Molecular Mass and Mole
  • States of Matter
  • Structure Of The Atom
  • Study of Compounds
  • Study of Gas Laws
  • Study of Representative Elements
  • Surface Chemistry
  • The d-block and f-block elements
  • The Gaseous State
  • The p-Block Elements
  • The Periodic Table
  • The s-Block Elements
  • The Solid State
  • Thermodynamics

High School Biology

  • Absorption and Movement of Water in Plants
  • Adolescent Issues
  • Anatomy of Flowering Plants
  • Animal Kingdom
  • Bacteria and Fungi-Friends and Foe
  • Biodiversity and Conservation
  • Biofertilizers
  • Biological Classification
  • Biomedical Engineering
  • Biomolecules
  • Biotechnology and its Applications
  • Biotic Community
  • Body Fluids and Circulation
  • Breathing and Exchange of Gases
  • Cell – Unit of Life
  • Cell Cycle and Cell Division
  • Cell Division and Structure of Chromosomes
  • Cell Reproduction
  • Cellular Respiration
  • Chemical Coordination and Integration
  • Circulation
  • Control And Coordination
  • Crop Improvement
  • Digestion and Absorption
  • Diversity In Living Organisms
  • Ecosystem
  • Environmental Issues
  • Excretory Products and their Elimination
  • Flowering Plants
  • Genes and Chromosomes
  • Health and Diseases
  • Health and Its Significance
  • Heredity And Evolution
  • Heredity and Variation
  • How Do Organisms Reproduce?
  • Human Diseases
  • Human Eye And Colourful World
  • Human Health and Disease
  • Human Population
  • Human Reproduction
  • Hygiene
  • Improvement In Food Resources
  • Integumentary System- Skin
  • Kingdom Fungi
  • Kingdom Monera
  • Kingdom Protista
  • Life Processes
  • Locomotion and Movement
  • Microbes in Human Welfare
  • Mineral Nutrition
  • Molecular Basis of Inheritance
  • Morphology of Flowering Plants
  • Neural Control And Coordination
  • Nutrition in Human Beings
  • Organism and Population
  • Photosynthesis
  • Photosynthesis in Higher Plants
  • Plant Growth and Development
  • Plant Kingdom
  • Pollination and Fertilization
  • Pollution; Sources and its effects
  • Principles of Inheritance and Variation
  • Reproduction and Development in Angiosperms
  • Reproduction in Organisms
  • Reproductive Health
  • Respiration in Human Beings
  • Respiration in Plants
  • Respiratory System
  • Sexual Reproduction in Flowering Plants
  • Strategies for Enhancement in Food Production
  • Structural Organisation in Animals
  • Structural Organisation of the Cell
  • The Endocrine System
  • The Fundamental Unit Of Life
  • The Living World
  • The Nervous System and Sense Organs
  • Tissues
  • Transpiration
  • Transport in Plants

High School Math

  • Algebra – Arithmatic Progressions
  • Algebra – Complex Numbers and Quadratic Equations
  • Algebra – Linear Inequalities
  • Algebra – Pair of Linear Equations in Two Variables
  • Algebra – Polynomials
  • Algebra – Principle of Mathematical Induction
  • Algebra – Quadratic Equations
  • Binomial Theorem
  • Calculus – Applications of Derivatives
  • Calculus – Applications of the Integrals
  • Calculus – Continuity and Differentiability
  • Calculus – Differential Equations
  • Calculus – Integrals
  • Geometry – Area
  • Geometry – Circles
  • Geometry – Conic Sections
  • Geometry – Constructions
  • Geometry – Introduction to Euclid’s Geometry
  • Geometry – Three-dimensional Geometry
  • Geometry – Lines and Angles
  • Geometry – Quadrilaterals
  • Geometry – Straight Lines
  • Geometry – Triangles
  • Linear Programming
  • Matrices and Determinants
  • Mensuration – Areas
  • Mensuration – Surface Areas and Volumes
  • Number Systems
  • Number Systems – Real Numbers
  • Permutations and Combinations
  • Probability
  • Sequence and Series
  • Sets and Functions
  • Statistics 
  • Trignometry – Height and Distance
  • Trignometry – Identities
  • Trignometry – Introduction

Middle School Science

  • Acids, Bases And Salts
  • Air and Its Constituents
  • Basic Biology
  • Body Movements
  • Carbon and Its Compounds
  • Cell – Structure And Functions
  • Changes Around Us
  • Chemical Effects Of Electric Current
  • Chemistry in Your Life
  • Coal And Petroleum
  • Combustion And Flame
  • Components Of Food
  • Conservation Of Plants And Animals
  • Crop Production And Management
  • Electric Current And Its Effects
  • Electricity And Circuits
  • Elements and Compounds
  • Fibre To Fabric
  • Food production and management
  • Force And Pressure
  • Forests: Our Lifeline
  • Friction
  • Fun With Magnets
  • Garbage In, Garbage Out
  • Getting To Know Plants
  • Health and Hygiene
  • Heat
  • Hydrogen
  • Life Processes: Nutrition in Animals and Plants
  • Light, Shadows And Reflections
  • Materials: Metals And Non-Metals
  • Matter and Its States
  • Metals and Non-metals
  • Micro Organisms: Friend And Foe
  • Motion And Measurement Of Distances
  • Motion And Time
  • Nutrition In Animals
  • Nutrition In Plants
  • Organization in Living Things
  • Our Environment
  • Physical And Chemical Changes
  • Pollution and conservation
  • Pollution Of Air And Water
  • Reaching The Age Of Adolescence
  • Reproduction In Animals
  • Reproduction In Plants
  • Respiration In Organisms
  • Rocks and Minerals
  • Separation Of Substances
  • Simple Machines
  • Soil
  • Some Natural Phenomena
  • Sorting Materials Into Groups
  • Sound
  • Stars And The Solar System
  • Structure of Atom
  • Synthetic Fibers And Plastics
  • The Living Organisms And Their Surroundings
  • Transfer of Heat
  • Transformation of Substances
  • Transportation In Animals And Plants
  • Universe
  • Waste-water Story
  • Water: A Precious Resource
  • Weather, Climate And Adaptations Of Animals To Climate
  • Winds, Storms And Cyclones

Middle School Math

  • Addition
  • Area and Its Boundary
  • Boxes and Sketches
  • Data Handling
  • Fun With Numbers
  • Heavy and Light
  • How Many
  • Long And Short
  • Mapping
  • Measurement
  • Money
  • Multiplication and Factors
  • Multiply and Divide
  • Numbers
  • Parts and Wholes
  • Pattern Recognition
  • Patterns
  • Play With Patterns
  • Rupees And Paise
  • Shapes And Angles
  • Shapes And Designs
  • Shapes and Space
  • Similarity
  • Smart Charts
  • Squares
  • Subtraction
  • Tables And Shares
  • Tenths and Hundredths
  • Time
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