Chemistry Lesson Note On Structure of the Atom and Electronic Configuration

Topic: Structure of the Atom and Electronic Configuration

Class: SS1

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Specific Objectives:

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

1. Describe the basic structure of the atom.
2. Identify the subatomic particles (proton, neutron, and electron) and their properties.
3. State and explain Bohr’s atomic model.
4. Write electronic configurations for elements (1–20).
5. Relate atomic structure to the periodic table.

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Instructional Materials:

• Periodic table chart
• Atomic structure models
• Projector/slides showing Bohr’s model
• Flashcards of elements and their atomic numbers
• Worksheets on electronic configuration

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Lesson Content:

Step 1: Structure of the Atom

• An atom is the smallest particle of an element that retains its chemical properties.
• Basic components of the atom:
  • Nucleus: Center of the atom; contains protons and neutrons.
  • Electrons: Negatively charged particles orbiting the nucleus in energy levels or shells.

Subatomic Particle	Symbol	Charge	Mass (kg)	Location
Proton	p⁺	+1	1.67 × 10⁻²⁷	Nucleus
Neutron	n⁰	0	1.67 × 10⁻²⁷	Nucleus
Electron	e⁻	–1	9.11 × 10⁻³¹	Orbitals/Shells

• Atomic number (Z): Number of protons (equal to number of electrons in a neutral atom)
• Mass number (A): Number of protons + neutrons
• Isotopes: Atoms of the same element with different numbers of neutrons.

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Step 2: Bohr’s Atomic Model

• Proposed by Niels Bohr in 1913.
• Electrons revolve around the nucleus in fixed paths or energy levels (shells).
• These shells are labeled K, L, M, N... or 1, 2, 3, 4...
• Maximum number of electrons per shell = 2n² where n = shell number.

Shell	Symbol	Max Electrons
1	K	2
2	L	8
3	M	18
4	N	32

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Step 3: Electronic Configuration

• Arrangement of electrons in shells or orbitals.
• Based on Aufbau principle – electrons fill from lowest energy levels first.

Examples:

• Hydrogen (Z = 1): 1
• Carbon (Z = 6): 2, 4
• Oxygen (Z = 8): 2, 6
• Sodium (Z = 11): 2, 8, 1
• Calcium (Z = 20): 2, 8, 8, 2

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Step 4: Relationship to Periodic Table

• Group number = number of valence electrons.
• Period number = number of electron shells.
• Atoms in the same group have similar chemical properties due to similar valence electrons.

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Evaluation:

1. Draw and label the basic structure of an atom.
2. State three properties each of protons, neutrons, and electrons.
3. What is the atomic number and mass number of an atom with 11 protons and 12 neutrons?
4. Write the electronic configuration of the following: (a) Fluorine (Z=9), (b) Magnesium (Z=12), (c) Argon (Z=18).
5. How many shells are there in an atom of potassium (Z=19)? How many valence electrons?

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Homework:

1. Explain Bohr’s model of the atom with a diagram.
2. Write the electronic configuration for the first 20 elements of the periodic table.
3. Distinguish between isotopes and ions with two examples each.
4. Why is an atom electrically neutral?

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Conclusion:

Atoms consist of a dense nucleus surrounded by electrons in defined energy levels. Understanding atomic structure and electronic configuration is essential for grasping chemical bonding, reactivity, and the periodic table.