Rutherford's Nuclear Model

After the Geiger–Marsden experiment (also called the Gold foil experiment or the Rutherford experiment), conducted by Rutherford, and his students Hans Geiger and Ernest Marsden, by bombarding alpha particles on a very thin gold foil, Rutherford concluded that:

• Most of the space in the atom is empty
• The positive charge and most of the mass of the atom was concentrated in a small volume called nucleus
• The size of the nucleus (10^-15) is very small compared to that of the atom (10^-10)
• Electrons move around the nucleus with very high speed in circular paths called orbits
• Electrons and nucleus are held together by electrostatic forces of attraction
  

Radioactivity and Radioactive Elements

Henri Becqueral observed that certain elements emit radiation on their own. This phenomenon is known as radioactivity. And such elements are called radioactive elements.
Other scientists in this field include Marie Curie, Piere Curie, Rutherford and Fredrick Soddy.

α-rays = 2 protons +2 neutrons
β-rays = -ve particles (electron or positron)
γ-rays are electromagnetic radiatons of very high energy (do not contain particles)
α-rays have the least penetrating power. β-rays have a penetrating power 100 times that of α-rays.
And γ-rays have 1000 times that of α-rays

X-rays

When electrons strike a material in the CRT, rays are produced which can cause fluorescence in fluorescent materials. These were called X-rays by its discoverer (Wilhalm Roentgen)(1895)
X-rays are produced effectively when electrons strike dense metal anode (called targets). These are not reflected by electric or magnetic fields (but they possess electro-magnetic character) and have very high penetrating power.
Wavelength = ~0.1 nm

Thomson Model of Atom

JJ Thomson proposed in 1898 that atom was spherical (radius approximately 10^-10 m). That the positive charge was uniformly distributed in this sphere. And that the electrons were embedded in it like the plum in pudding, in such a manner that the atom is in a stable electrostatic state.
It is called plum pudding, raisin pudding or watermelon model.
Mass of the atom was assumed to be uniformly distributed over the atom (Rutherford said it to be localized in nucleus)

Thomson got Nobel in 1906 not for this no non-sense model (which not only helped in explaining the neutrality of the atom, but is also accepted as a standard for computational models involving spherical systems like golf balls, global weather models, and biological viruses), but for his investigations on the conduction of electricity by gases.

Neutron

Discovered by Chadwick (1932) by bombarding a thin sheet of beryllium (Be) by alpha particles. Thus neutral particles of mass slightly greater than protons (neutrons) were emitted.

Protons

Canal rays (rays containing positively charged particles) :

• depend upon the nature of the gas present in the CRT. These are the positively charged ions (so, will have as many protons as the gas in the tube)
• charge to mass ratio depends on gas
• some particles carry a multiple of the fundamental unit of electrical charge.
• opposite behaviour to cathode rays in electrical or magnetic field (charge different)

Proton from hydrogen was characterised in 1919.