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Quick Summary
Atomic structure is fundamental to understanding chemistry. Class 9 covers the history of atomic models from Dalton to Bohr, the three subatomic particles, electronic configuration, and the relationship between atomic number, mass number, and isotopes. This is a high-weightage chapter for 3-mark and 5-mark questions.
Key Definitions
- Atom: Smallest unit of an element that retains the chemical properties of that element.
- Atomic Number (Z): Number of protons in the nucleus. Determines the element.
- Mass Number (A): Total number of protons and neutrons. A = Z + N.
- Isotopes: Atoms of the same element with same atomic number but different mass numbers (e.g., ¹H, ²H, ³H).
- Isobars: Atoms of different elements with the same mass number but different atomic numbers.
- Valence Electrons: Electrons in the outermost shell. Determine chemical reactivity.
- Valency: Combining capacity of an element.
Subatomic Particles
| Particle | Symbol | Charge | Mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 amu | Nucleus |
| Neutron | n | 0 | 1 amu | Nucleus |
| Electron | e⁻ | -1 | 1/1840 amu | Shells (orbitals) |
History of Atomic Models
| Scientist | Year | Model | Key Feature |
|---|---|---|---|
| Dalton | 1808 | Solid sphere | Atom is indivisible |
| Thomson | 1897 | Plum pudding | Electrons embedded in positive sphere |
| Rutherford | 1911 | Nuclear model | Nucleus at centre; electrons orbit it |
| Bohr | 1913 | Planetary model | Electrons in fixed energy levels (shells) |
Electronic Configuration Rules
1. Electrons fill shells in order: K → L → M → N. 2. Maximum electrons per shell: 2n² (K=2, L=8, M=18, N=32). 3. Outermost shell cannot have more than 8 electrons. 4. Second last shell cannot have more than 18 electrons.
Examples
| Element | Atomic No. | Configuration |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2, 4 |
| Oxygen | 8 | 2, 6 |
| Sodium | 11 | 2, 8, 1 |
| Chlorine | 17 | 2, 8, 7 |
| Calcium | 20 | 2, 8, 8, 2 |
Isotopes and Applications
| Isotope | Application |
|---|---|
| Carbon-14 | Radiocarbon dating |
| Uranium-235 | Nuclear fuel |
| Cobalt-60 | Cancer treatment |
| Iodine-131 | Thyroid treatment |
Important Questions
One Mark
- What is the atomic number of oxygen?
- Define isotopes with one example.
- What is the maximum number of electrons in the L shell?
- Who proposed the nuclear model of the atom?
- What does the mass number of an atom represent?
Short Answer (3 marks)
- Write the electronic configuration of Na (11) and Cl (17). What are their valencies?
- Distinguish between isotopes and isobars with examples.
- Describe Rutherford's gold foil experiment and its observations.
Long Answer (5 marks)
- Describe the Bohr model of the atom. State its postulates. How does it explain the stability of atoms?
- Compare and contrast Thomson's, Rutherford's, and Bohr's models of the atom.
Common Mistakes
- Confusing atomic number (protons) with mass number (protons + neutrons).
- Writing electronic configurations without following the 2n² rule and the 8-electron outer shell limit.
- Confusing isotopes (same element, different mass) with isobars (different elements, same mass).
Revision Checklist
- [ ] Know properties and locations of proton, neutron, electron
- [ ] Know electronic configurations of first 20 elements
- [ ] Understand all four atomic models and their key features
- [ ] Know definition and examples of isotopes and isobars
- [ ] Can find valence electrons and valency from configuration
Frequently Asked Questions
Proton: positive charge (+1), mass ≈ 1 amu, found in nucleus. Neutron: no charge (0), mass ≈ 1 amu, found in nucleus. Electron: negative charge (-1), negligible mass (1/1840 amu), found in orbitals around nucleus.