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Atomic Structure
ICSE Class 8 Chemistry • Chapter 4 (Detailed Master Notes)
Chapter Overview
John Dalton once confidently believed that the atom was the absolute smallest, indivisible particle of matter. He was wrong. Modern atomic theory reveals that the atom is actually a complex miniature solar system, packed with even smaller subatomic particles. Let's look inside.
4.1 The Three Subatomic Particles
Every atom is constructed from three fundamental "building blocks".
| Particle | Symbol | Charge | Mass (Relative) | Location |
|---|---|---|---|---|
| Proton | $p^+$ | Positive (+1) | 1 amu (Heavy) | Inside the central Nucleus |
| Neutron | $n^0$ | Neutral (0) | 1 amu (Heavy) | Inside the central Nucleus |
| Electron | $e^-$ | Negative (-1) | $\approx \frac{1}{1840}$ amu (Nearly zero) | Revolving around Nucleus in shells |
4.2 Structure of an Atom
The Nucleus: The extremely small, dense, positively charged center of the atom. It contains all the protons and neutrons (collectively called nucleons). Almost the entire mass of the atom is concentrated here.
Electron Shells (Orbits): The electrons revolve rapidly around the nucleus in specific, fixed circular paths called energy shells or orbits. These shells are designated as K, L, M, N... starting from the innermost shell.
AI Image Prompt: A vivid 3D diagram of an atom (Bohr Model). At the center, a dense cluster of red (protons) and grey (neutrons) glowing spheres forming the nucleus. Surrounding it are distinct, thin circular blue orbital rings. Small, glowing blue spheres (electrons) sit on these rings. Label the Nucleus, K-shell, L-shell, and Electrons.
4.3 Atomic Number and Mass Number
How do we tell a Carbon atom apart from an Oxygen atom? We count the protons.
- Atomic Number ($Z$): The strictly total number of protons present inside the
nucleus of an atom.
Key Rule: In a stable, neutral atom: Number of Protons = Number of Electrons. - Mass Number ($A$): The total sum of protons and neutrons present in the nucleus.
Formula: $A = p + n$
Example: Sodium ($Na$) has $11$ protons and $12$ neutrons. Therefore, its Atomic Number is $11$, and its Mass Number is $23$ ($11+12$).
4.4 Electronic Configuration
Electrons do not just fly around randomly; they are systematically arranged in the orbital shells according to the Bohr-Bury Rules.
Rule 1 (Maximum Capacity): The maximum number of electrons that can be accommodated in a shell is deeply dictated by the formula $2n^2$ (where $n$ is the shell number).
- 1st shell (K): $2(1)^2 = 2$ electrons maximum.
- 2nd shell (L): $2(2)^2 = 8$ electrons maximum.
- 3rd shell (M): $2(3)^2 = 18$ electrons maximum (but is usually satisfied with 8 in lower classes).
Rule 2 (Octet Rule): The outermost shell of an atom cannot hold more than $8$ electrons, regardless of its total capacity. An atom with $8$ electrons in its valence shell is considered chemically highly stable (like the Noble Gases).
4.5 Valence Electrons and Valency
The electrons in the innermost shells are safely deeply buried and rarely participate in chemical reactions. Only the outermost electrons matter.
- Valence Shell: The outermost electron shell of an atom.
- Valence Electrons: The electrons present strictly in the valence shell. These dictate the chemical reactivity of the element.
- Valency: The combining capacity of an element. It is the number of electrons an atom strictly needs to legally gain, strictly lose, or share to completely achieve a stable octet (8) configuration.
Q1. An atom possesses 17 protons, 17 electrons, and 18 neutrons. What is its atomic number, mass number, and electronic configuration?
Answer:
- Atomic Number ($Z$) = Number of Protons = $17$.
- Mass Number ($A$) = Protons + Neutrons = $17 + 18 = 35$.
- Electronic Configuration indicates the arrangement of the $17$ electrons: K-shell ($2$), L-shell
($8$), M-shell ($7$). Written simply as: $2, 8, 7$.
Q2. Based on the configuration ($2, 8, 7$), what is the valency of this element?
Answer: The valence shell has $7$ electrons. To achieve a stable octet ($8$), it is intensely easier for the atom to specifically gain $1$ electron rather than firmly strictly safely lose $7$. Since it needs to gain $1$ electron to become stable, its valency is $1$.