How are electrons distributed in different orbits?

Content Standards

In this lesson, the students understand the arrangement of electrons in different shells/orbits.
They learn to apply Bohr–Bury rules to write electronic configurations of elements and relate electron distribution to valency and chemical reactivity

Performance Standards

Students will be able to:

Explain Bohr–Bury rules for electron distribution.
Write electronic configurations of the first 18 elements correctly.
Illustrate electron shells through diagrams/models.
Connect electron distribution with valency and stability of atoms.

Alignment Standards

Reference: NCERT Class 9 Science
The lesson is aligned with the NCERT Grade 9 Science Book-Chapter 4: Structure of Atom, Section:3- How electrons are distributed in different orbits (shells)?

Learning Objectives

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

Recall the concept of shells (K, L, M, N…).
Apply the formula 2n² to find maximum electrons in a shell.
Distribute electrons step by step for given elements.
Correlate outermost shell electrons with valency.

Prerequisites (Prior Knowledge)

Students should already know:

Knowledge of sub-atomic particles (electrons, protons, neutrons).
Rutherford’s and Bohr’s atomic models.
Atomic number and its significance.

Introduction

In this lesson, the concept of electron distribution is a crucial link between atomic structure and chemical behavior. Students have already learned about subatomic particles and Bohr’s model. This lesson takes them deeper into how electrons occupy shells (K, L, M, N…) around the nucleus following specific rules (Bohr–Bury scheme).

Understanding electron distribution helps explain:

Why do atoms show specific valencies?
Why some elements are chemically reactive (like sodium and chlorine), while others are inert (like neon and argon).
The basis of the octet rule and chemical bonding.

This topic builds the foundation for understanding valency, periodicity, and bonding, which will be revisited in higher classes (Class 10–11). The aim is not just to make students memorize configurations, but to help them see the logic behind chemical behavior.

Timeline (40 Minutes)

Title	Approximate Duration	Procedure	Reference Material
Engage	5	Ask: “Why does sodium (Na) react so vigorously, but neon (Ne) does not?” Show atomic structures of Na (2,8,1) and Ne (2,8). Lead students to think about the role of electron arrangement.	Slides
Explore	10	Ask students to use Virtual Lab / Simulation to visualize and learn about the 2n² rule and draw the structure of the following elements in the notebook: C=6, O=8, Na=11, Cl=17.	Virtual lab
Explain	10	Teacher explains Bohr–Bury rules: Maximum electrons in a shell = 2n². The outer shell can have max 8 electrons. Inner shells must be filled first. Demonstrate with examples: Carbon (Z=6): 2,4 Oxygen (Z=8): 2,6 Sodium (Z=11): 2,8,1	Slides + Virtual Lab
Evaluate	10	Students will attempt the Self Evaluation task on LMS	Virtual Lab
Extend	5	Students compare electron configurations of reactive elements (Na, Cl, O) with inert gases (He, Ne, Ar). Discuss how achieving 8 electrons (octet) explains stability. Link concept to valency and bonding.	Slides

How are electrons distributed in different orbits?

Introduction

Have you ever wondered why some elements, like sodium (Na), react so quickly, while others, like neon (Ne), remain completely unreactive? The answer lies in the way electrons are arranged inside an atom.

Electrons are not placed randomly around the nucleus. They occupy fixed paths or regions called shells or energy levels. Just like people sitting in rows in a theatre, electrons also “sit” in these shells according to certain rules.

This arrangement of electrons in different shells is called the distribution of electrons. Scientists Niels Bohr and Bury gave a simple set of rules, called the Bohr–Bury scheme, to explain how electrons are filled in these shells. Understanding this arrangement helps us explain why atoms combine, why some are stable, and why some are very reactive.

Theory

1. The Need for Electron Distribution

Every atom is made up of three types of particles – protons, neutrons, and electrons. While protons and neutrons stay packed in the nucleus, electrons revolve around the nucleus in fixed paths called shells or energy levels.

But electrons cannot just sit anywhere; they follow strict rules of arrangement. This arrangement, called electron distribution or electronic configuration, decides how an element behaves chemically.

For example:

Sodium (Na) has the configuration 2,8,1. Since it has 1 electron in its outermost shell, it easily loses that electron, making it highly reactive.
Neon (Ne), on the other hand, has the configuration 2,8. Its outer shell is full, so it remains stable and unreactive.

Thus, knowing electron distribution helps us explain valency, reactivity, and chemical bonding.

2. Shells and Energy Levels

The regions around the nucleus where electrons are found are called shells. They are labeled with letters and numbers:

K shell (n = 1) → closest to nucleus
L shell (n = 2) → second shell
M shell (n = 3) → third shell
N shell (n = 4) → fourth shell

Here, n is the shell number (principal quantum number). The shells farther from the nucleus have more space and can hold more electrons.

3. Bohr–Bury Rules for Electron Distribution

To know how many electrons go in each shell, scientists Niels Bohr and Bury suggested rules. These are called the Bohr–Bury scheme:

Maximum number of electrons in a shell = 2n²
- Formula: 2n², where n = shell number.
- Examples:
  - K shell (n=1): 2(1)² = 2 electrons max.
  - L shell (n=2): 2(2)² = 8 electrons max.
  - M shell (n=3): 2(3)² = 18 electrons max.
  - N shell (n=4): 2(4)² = 32 electrons max.
The outermost shell can hold only up to 8 electrons.
Even if the formula allows more (like 18 in M shell), the last shell cannot exceed 8 electrons.
Electrons fill inner shells first.
Lower shells (closer to nucleus) must be completely filled before electrons start entering the next shell.

4. Step-by-Step Examples

Hydrogen (Z = 1):
1 electron → goes to K shell → configuration = 1
Helium (Z = 2):
2 electrons → K shell can hold 2 → configuration = 2
Carbon (Z = 6):
6 electrons → K = 2, L = 4 → configuration = 2, 4
Oxygen (Z = 8):
8 electrons → K = 2, L = 6 → configuration = 2, 6
Sodium (Z = 11):
11 electrons → K = 2, L = 8, M = 1 → configuration = 2, 8, 1
Magnesium (Z = 12):
12 electrons → K = 2, L = 8, M = 2 → configuration = 2, 8, 2
Argon (Z = 18):
18 electrons → K = 2, L = 8, M = 8 → configuration = 2, 8, 8

5. Connection with Valency and Stability

Valency: The number of electrons in the outermost shell (valence electrons) decides how an atom combines with others.
- If an atom has 1, 2, or 3 electrons in its outermost shell, it tends to lose them → positive valency.
- If it has 5, 6, or 7 electrons, it tends to gain electrons → negative valency.
- If it has 4, it may share electrons.
Stability (Octet Rule): Atoms with 8 electrons in their outermost shell are very stable.
- Example: Noble gases like Neon (2,8) and Argon (2,8,8).
- This explains why they rarely take part in chemical reactions.

In short: The electron distribution explains why elements combine, why some are highly reactive (like Na, Cl), and why some remain inert (like He, Ne, Ar).

Vocabulary

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

Electron Shell (Orbit): The fixed path or energy level around the nucleus where electrons are found (K, L, M, N…).
Bohr–Bury Rules: Rules that explain how electrons are arranged in shells.
2n² Rule: Formula to calculate maximum number of electrons in a given shell.
Valence Shell: The outermost shell of an atom.
Valence Electrons: Electrons present in the outermost shell.
Valency: The combining capacity of an atom, based on its valence electrons.
Stable Configuration: When an atom has 8 electrons in its outermost shell (octet rule).
Inert/Noble Gases: Elements like helium, neon, and argon that already have stable electron configurations and do not react easily.

How are electrons distributed in different orbits?

Introduction

This VR lab takes students inside a 3D atomic world where they can visualize and interact with the structure of an atom. Instead of just reading from a textbook, learners can explore the nucleus, orbiting shells (K, L, M, N), and understand how electrons are distributed according to the 2n² rule. The lab provides a step-by-step journey—starting from the structure of the atom to practicing electronic configurations—making abstract concepts easy, visual, and engaging.

Key Features

Immersive 3D Environment – Students experience atoms in a realistic and interactive way.

Exploration of Atomic Structure – Clearly observe the nucleus and surrounding electron shells.

Electron Distribution Practice – Learn to write electronic configuration following the 2n² rule.

Guided Questions – Step-by-step prompts help learners distribute electrons for different elements.

Instant Feedback – Wrong attempts are corrected to strengthen learning.

MCQs are integrated at the end of each module for engagement.

Step-by-Step Procedure for VR Experience

Step 1: Understanding Structure of Atom

Enter the VR lab and observe a 3D model of an atom.
Visualize the nucleus at the center and the circular paths (shells) around it.

Step 2: Understanding K, L, M, N Shells

Move closer to see the shells labeled as K, L, M, and N.

Step 3: Number of Electrons in Each Shell

Notice that K holds 2, L holds 8, M holds 18, and N holds 32 electrons.

Step 4: Questions on Electronic Configuration of Atoms

Different atoms will appear with their atomic numbers.
Distribute the electrons step by step into K, L, M, and N shells using the 2n² rule.

Step 5: Evaluation

After interaction, students proceed to the quiz:
- 2 MCQs