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Atomic Models

Atomic Models

Content Standards

In this lesson, learners will demonstrate an understanding of atomic structure, including subatomic particles, Thomson’s model, Rutherford’s scattering experiment, Millikan’s oil-drop method, isotopes, isobars, atomic number, and mass number. Students will recognise, represent, and differentiate these using diagrams, symbols, and numerical data.

Performance Standards

By the end of the session, students will be able to:

  • Identify and describe sub-atomic particles and their properties.
  • Explain Thomson’s model, Rutherford’s model, and their limitations.
  • Interpret observations from Rutherford’s scattering experiment.
  • Represent atoms using standard notation (A / Z format).
  • Differentiate between isotopes and isobars with examples.
  • Calculate the number of protons, neutrons, and electrons from given atomic numbers and mass numbers.

Alignment Standards

Reference: NCERT Book Alignment 

The lesson is aligned with Chapter 2 of the NCERT Grade 11 Chemistry Textbook, Structure of Atom

Section 2.2 – Structure of Atom – Atomic Models

Learning Objectives

By the end of the lesson, students will be able to:

  • Describe the discovery of sub-atomic particles and the challenges they presented.
  • Explain Millikan’s Oil Drop Experiment and state the formula: q = n × e.
  • Illustrate Thomson’s Model and identify its shortcomings.
  • Analyse Rutherford’s alpha-particle scattering observations and conclusions.
  • Define atomic number (Z), mass number (A), and nucleons.
  • Distinguish clearly between Isotopes and Isobars.
  • Perform calculations to find protons, neutrons, and electrons in any neutral atom or ion.

Prerequisites (Prior Knowledge)

  • Basic idea that matter is made of particles (Dalton’s atomic theory).
  • Simple structure of an atom (electron, proton, neutron).
  • How to use atomic numbers from the periodic table.
  • Concept of charge (+, –) and basic mathematical subtraction.

Introduction

In this session, students will explore how atomic models evolved—from Dalton’s indivisible atom to Thomson’s “plum pudding” structure and Rutherford’s nuclear model. They will also learn how to determine protons, neutrons, and electrons, understand isotopes, isobars, and apply atomic notation in problem-solving. Real-world discoveries, such as Millikan’s Oil Drop Experiment and radioactivity, will help them connect theory with scientific evidence.

Timeline (40 Minutes)

TitleApproximate DurationProcedureReference Material
Engage5

Ask:“Draw a quick sketch of what you think the inside of an atom looks like.” 

Slides

Explore10

Students observe different atomic models in virtual lab.

VR lab

Explain10


Explains: Thomson’s model, Rutherford’s conclusions, drawbacks of nuclear model, isotopes, isobars, atomic number & mass number. Practice calculating neutrons from A–Z.

Slides and Virtual Lab

Evaluate10

Students will attempt the Self Evaluation task on LMS

Virtual Lab

Extend5

Challenge: Answer the MCQs.

Slides

Atomic Models

Introduction

In this lesson, students will learn about how the model of an atom evolved based on experimental discoveries. Through real experiments, visual demonstrations, and reasoning, students will explore how scientists understood the structure of atoms, sub-atomic particles, atomic numbers, isotopes, and isobars, and why these concepts form the foundation of modern chemistry.

Theory

Introduction: Why Study Atomic Models?

Have you ever wondered what an atom actually looks like?
Scientists discovered electrons, protons, and neutrons—but then faced major questions:

  • Why is the atom stable?
  • How do atoms combine to form different molecules?
  • Why do elements behave differently chemically and physically?

Experiments like Millikan’s Oil Drop and Rutherford’s Gold Foil Scattering led to models that tried to answer these questions.

A relatable example:
Just like improving versions of a smartphone, atomic models improved whenever new discoveries were made.

What are Atomic Models?

Atomic models are scientific explanations that show how charges and mass are arranged within an atom.

Key Parts:

  1. Sub-atomic Particles
    • Electrons (negative), Protons (positive), Neutrons (neutral)
  2. Model / Arrangement
    • How these particles are placed inside an atom (e.g., uniform distribution, central nucleus, circular orbits)

Simple Example:
Thomson visualized an atom like a watermelon—positive charge like the red pulp, and electrons like black seeds.

Steps / Process 

Millikan’s Oil Drop Method

Step 1: Fine oil droplets fall between charged plates.
Step 2: X-rays ionise air → droplets pick up extra charges.
Step 3: By adjusting the electric field, droplets can be stopped or moved.
Step 4: By measuring forces, Millikan found:
q = n × e (charge is a whole-number multiple of e)

Solved Example:
If an oil drop carries 3 extra electrons, charge = 3e = 3 × 1.6×10⁻¹⁹ C

Thomson’s Model of the Atom

  • An atom is a spherical ball of positive charge.
  • Electrons are embedded like seeds in a pudding.
  • Explains neutrality, but not scattering behavior.

Rutherford’s Nuclear Model

Step 1: Shoot α-particles at thin gold foil.
Step 2: Observe flashes on zinc sulphide screen.
Step 3: Key observations:

  • Most passed straight → atom is mostly empty space.
  • A few deflected → positive charge is concentrated.
  • Rare ones bounced back → small, dense nucleus.

Conclusion:
Electrons revolve around the nucleus like planets around the sun.

Atomic Number & Mass Number

  • Atomic Number (Z) = number of protons = number of electrons (neutral atom)
  • Mass Number (A) = number of protons + neutrons

Neutrons = A – Z

Example:
Protons = 12
Neutrons = 12 – 6 = 6
Electrons = 6 (neutral)

Isotopes & Isobars

  • Isotopes → Same Z, different A (different neutrons)
    Example: ₁¹H, ₁²D, ₁³T
  • Isobars → Same A, different Z
    Example: ₆¹⁴C and ₇¹⁴N
  1. Solved Example:

Problem 2.1 Calculate the number of protons, neutrons and electrons in Br. 

Solution 

In this case, Br, Z = 35, A = 80, species is neutral

  • Number of protons = number of electrons = Z = 35.
  • Number of neutrons = 80 – 35 = 45.

Applications / Why is it Useful?

  • Understanding chemical reactions (electrons decide reactivity).
  • Medical imaging (X-rays) – based on atomic interactions.
  • Nuclear energy and radioactivity (alpha, beta, gamma rays).
  • Carbon Dating – uses isotopes like C-14.
  • Identifying elements using spectral lines.

Vocabulary

This is the list of vocabulary terms used throughout the lesson.

  1. Sub-atomic Particles: The electrons, protons, and neutrons that make up an atom. Example: Electrons revolve around the nucleus.
  2. Atomic Number (Z): The number of protons present in the nucleus of an atom. Example: Oxygen has an atomic number of 8.
  3. Mass Number (A): The total number of protons and neutrons in an atom. Example: Helium has a mass number of 4.
  4. Isotopes: Atoms that have the same atomic number but different mass numbers. Example: Uranium-235 and Uranium-238.
  5. Isobars: Atoms that have the same mass number but different atomic numbers. Example: Potassium-40 and Argon-40.
  6. Alpha (α) Particles: High-energy particles with a +2 charge and mass of 4 u. Example: Released during radioactive decay.
  7. Nuclear Model: Rutherford’s model that shows a central nucleus with electrons revolving around it. Example: Based on the gold foil experiment.
  8. Electron Charge (e): The fundamental negative charge of an electron = 1.602 × 10⁻¹⁹ C. Example: All electrons carry the same charge.

Atomic Models

Category

Introduction

This Virtual Lab helps students explore atomic models, isotopes, isobars, and Rutherford’s experiment through visuals, interactions, and quick quizzes. Learners understand how atoms are structured and how protons, neutrons, and electrons vary across isotopes and isobars.

Key Features

  • Simple and intuitive interface for easy navigation.
  • Visual explanations of sample space, events, and probability formulas.
  • Interactive activities where students drag and drop correct probability blocks.
  • Real-time feedback after each step or input.
  • Gamified quiz section to boost engagement and reinforce learning.
  • Clear step-by-step demonstrations of probability concepts such as complementary events.

Step-by-Step Procedure for VR Experience

  1. Open the Navigation Menu and select Atomic Models & Isotopes.
  2. Explore the visuals explaining Thomson, Rutherford, and Bohr models.
  3. View simulations such as Rutherford’s scattering experiment to see particle paths.
  4. Learn isotope concepts with examples like Carbon-12, Carbon-13, and Carbon-14.
  5. Enter values (protons, neutrons, electrons) in interactive input fields and get feedback.
  6. Understand Isobars using Argon, Potassium, and Calcium through guided visuals.
  7. Complete the Interactive Check by solving neutron or proton-number tasks.
  8. Finish the Quiz to test your understanding and receive instant feedback.
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