Question

The main experiments that led to the atomic theory ,both what they showed and how they showed it.

Answer 1

An atomic theory is a model developed to explain the properties and behaviors of atoms.

A proponent of the ultimate particle concept was the philosopher Democritus (c. 470–c. 380 b.c.), who named those particles atomos. In Greek, atomos means "indivisible."

in the first decade of the 1800s, the idea was revived. English chemist John Dalton (1766–1844) proposed the first modern atomic theory. Dalton's theory can be called modern because it contained statements about atoms that could be tested experimentally. Dalton's theory had five major parts. He said:

1. ll matter is composed of very small particles called atoms.
2. All atoms of a given element are identical.
3. Atoms cannot be created, destroyed, or subdivided.
4. In chemical reactions, atoms combine with or separate from other atoms.
5. In chemical reactions, atoms combine with each other in simple, whole-number ratios to form combined atoms.

Discovery of electron

Electron was discovered by J. J. Thomson in 1897 when he was studying the properties of cathode ray.

J. J. Thomson constructed a glass tube which was partially evacuated i.e. much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of particle (ray) was coming out from the negatively charged electrode (cathode) to positively charged electrode (anode). This ray is called cathode ray and the whole construction is called cathode ray tube.

Discovery of Proton

In 1909, Rutherford discovered proton in his famous gold foil experiment.

Gold Foil experiment

In his gold foil experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide (ZnS) screen.

Results

1. Most of the particles pass through the foil without any deflection.
2. Some of the alpha particles deflect at small angle.
3. Very few even bounce back (1 in 20,000).

Conclusion

Based on his observations, Rutherford proposed the following structural features of an atom:

1. Most of the atom’s mass and its entire positive charge are confined in a small core, called nucleus. The positively charged particle is called proton.
2. Most of the volume of an atom is empty space.
3. The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.