Atomic Theory: Dalton, Thomson and Rutherford | Texas Gateway - Free Printable

Let's complete the table based on historical contributions to atomic theory by these scientists:

| Who? | Democritus | John Dalton | J.J. Thomson | Ernest Rutherford |
|----------|----------------|------------------|--------------------|------------------------|
| When? | 4th Century BC | Early 1800s | 1897 | 1911 |
| Remembered for? | Proposed the idea of atoms as indivisible particles (atomos = "uncuttable") | Developed the modern atomic theory; proposed that elements are made of atoms and that atoms combine in fixed ratios | Discovered the electron using cathode ray experiments | Discovered the nucleus through the gold foil experiment |
| Atom model? | Atoms are tiny, indivisible, indestructible spheres | Solid sphere model – atoms are like tiny, indivisible balls | Plum pudding model – electrons embedded in a positively charged sphere | Nuclear model – atom has a small, dense, positively charged nucleus with electrons orbiting around it |

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Explanation of Each Scientist’s Contribution:

1. Democritus (4th Century BC)
- Remembered for: He was one of the first to propose the concept of atoms as fundamental building blocks of matter.
- Atom model: He believed atoms were indivisible and eternal, differing only in shape and size. This was more philosophical than scientific.

2. John Dalton (Early 1800s)
- Remembered for: He formulated the first scientific atomic theory based on experimental evidence.
- Atom model: He proposed that:
- All matter is made of atoms.
- Atoms of the same element are identical.
- Atoms combine in simple whole-number ratios to form compounds.
- Atoms cannot be created or destroyed in chemical reactions.
- His model was a solid sphere—like tiny billiard balls.

3. J.J. Thomson (1897)
- Remembered for: He discovered the electron through experiments with cathode rays.
- Atom model: He proposed the "plum pudding" model, where negatively charged electrons were embedded in a positively charged sphere, like plums in a pudding.

4. Ernest Rutherford (1911)
- Remembered for: He conducted the gold foil experiment, which revealed that most of the atom is empty space and that the positive charge is concentrated in a small nucleus.
- Atom model: He introduced the nuclear model of the atom: a small, dense, positively charged nucleus surrounded by electrons in motion.

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✔ This progression shows how our understanding of the atom evolved from philosophical ideas to modern scientific models.