1. Atoms & Elements
Concept
All matter is made up of atoms, the smallest unit of an element that retains its properties.
Example:
An atom with 16 protons is Sulfur (S).
Key Idea:
The number of protons = the atomic number, which defines the element.
Notes:
- • Neutral atom → protons = electrons
- • Isotopes differ by neutrons
2. Isotopes
Concept
Atoms of the same element with the same number of protons but different numbers of neutrons.
Example:
Key Idea:
Isotopes have different mass numbers but behave chemically the same.
Notes:
Isotopes can be stable or radioactive.
3. Dalton's Atomic Theory
Concept
First modern atomic theory (early 1800s).
Key Points:
- All elements are made of tiny indivisible particles called atoms.
- Atoms of the same element are identical.
- Atoms combine in whole-number ratios to form compounds.
Limitations:
- • Atoms are divisible (contain subatomic particles)
- • Isotopes exist, so not all atoms of an element are identical
Learning Type:
Foundational theory
4. Thomson's Model (Plum Pudding Model)
Experiment
Cathode Ray Tube
Discovery
Electrons (negatively charged subatomic particles)
Model
Atom is a sphere of positive charge with electrons scattered within it
Contribution:
"Discovered electrons, proposed 'plum pudding' model"
Learning Type:
Early model comprehension
5. Rutherford's Model (Nuclear Model)
Experiment
Gold Foil Experiment (1909)
Discovery
- • Atoms have a small, dense, positively charged nucleus
- • Atoms are mostly empty space
Improvement Over Thomson
- • Showed positive charge is concentrated in the nucleus
- • Replaced uniform "pudding" with a central nucleus
Learning Type:
Concept development
6. Bohr's Model (Planetary Model)
Concept
Electrons move in fixed orbits (energy levels) around the nucleus.
Key Contribution:
"Electrons exist in discrete energy levels"
Notes:
- • Explains how atoms absorb or emit light
- • Paved the way for modern quantum theory
Learning Type:
Model refinement
7. Comparison of Atomic Models
| Scientist | Main Discovery / Contribution | Model Name | Limitations / Improvements |
|---|---|---|---|
| Dalton | Atoms are indivisible; combine in whole ratios | Solid sphere | Didn't know about subatomic particles |
| Thomson | Discovered electron | Plum pudding | No nucleus; charge not explained |
| Rutherford | Discovered nucleus | Nuclear model | Couldn't explain electron movement |
| Bohr | Electrons in energy levels | Planetary model | Works only for hydrogen-like atoms |
8. Summary
- • Atomic theory evolved as new experiments revealed the structure of the atom.
- • Each model built on the last — from Dalton's solid sphere to Bohr's energy-level atom.
- • Understanding this progression helps explain how scientific models change as evidence grows.
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