Modern Physics
Modern Physics
The unit in which atomic and nuclear masses are measured is called atomic mass unit.
One atomic mass unit is defined as one-twelfth of the mass of an atom of carbon twelve isotope.
i.e., one atomic mass unit equals one point six six times ten to the negative twenty-seven kilograms.
Atomic masses can be measured using a mass spectrometer.
The different types of atoms of the same element which exhibit similar chemical properties, but have different masses are called isotopes.
Isotopes are the atoms of an element whose nuclei have the same number of protons, but have different numbers of neutrons.
Isobars are the nuclei with the same mass number, but with different atomic numbers.
Isotones are the atoms of different elements with the same atomic weight, but with different atomic numbers.
Nucleus
Nucleus
The nucleus has the positive charge possessed by the protons. For an element of atomic number Z, the total charge on an atomic electron is negative Z times e, while the charge of the nucleus is positive Z times e.
The composition of a nucleus is described using the following terms and symbols:
Z equals atomic number equals number of protons. N equals neutron number equals number of neutrons.
A equals mass number equals Z plus N equals total number of protons and neutrons.
The elements that emit highly penetrating and high energy radiation beams are known as radioactive elements. Example, uranium, radium, and thorium.
Becquerel rays: Radiations given out by radioactive elements.
Properties:
o Affect photographic plate.
o Ionize the gas.
o Penetrate through matter.
o Affected by electrostatic and magnetic fields.
During radioactivity, nucleons are ejected from the nucleus.
Alpha decay: If nucleus ejects alpha particles.
Beta decay: If nucleus ejects beta particles.
Experiment to demonstrate the properties of Becquerel Rays.
The sample of radioactive element is placed in a small cavity.
A photographic plate is placed over the cavity.
It was found that the photographic plate developed the following pattern.
Alpha particles: The particles deflected towards the negative plate and having a dark spot.
Properties:
It is similar to a doubly ionized helium atom and has the speed of the order of one hundred million meters per second.
Alpha particles have large kinetic energy and momentum.
It strongly ionizes the gas through which it passes.
It rapidly dissipates its energy as it moves through a medium and therefore its penetrating power is quite small.
As alpha particles are positively charged, so they are deflected by electric and magnetic fields.
Alpha particles cause fluorescence on striking a fluorescent material.
Beta particles: The particles deflected towards the positive plate and having a lighter spot.
Properties:
These particles have speed of the order of one hundred million meters per second. Different beta particles emitted from the same radioactive substance have different speeds.
The rest mass of the beta particle is equal to the mass of an electron and charge on it is equal to the charge on an electron.
These particles ionize the gas through which they pass. Also, their ionizing power is equal to one-hundredth times that of the alpha particles.
Their penetrating power is more than the alpha particles.
As these particles are negatively charged, so they are deflected by electric and magnetic fields.
Beta particles cause fluorescence on striking a fluorescent material.
Gamma particles: The particles which were not deflected towards either plate and having a very light spot.
Properties:
The speed of these particles is of the order of the speed of light, three times one hundred million meters per second.
Their ionizing power is very low and it is one-thousandth times that of the alpha particle.
Penetration power is very high for these particles.
As these particles have no charge on them, so they do not get deflected by electric and magnetic fields.
Gamma particles cause fluorescence on striking a fluorescent material.
Gamma radiations are very useful in the treatment of cancer.