`=>` CANAL RAYS: Particles carrying positive charge.

`=>` ATOMIC NUMBER:The number of protons presents in the nucleus of an atom is called atomic number.
`=>` MASS NUMBER: Mass number is the total number of protons and neutrons present in the nucleus of an atom.

`=>` ISO ELECTRONIC SPECIES: These are the species which have same number of electrons.
`=>` ISOBARS: These are atoms with same mass number but different atomic number.
`=>` ISOTOPES: These are the atoms with same atomic number but different atomic mass.

`=>` RADIOACTIVITY: Henri Becqueral observed that certain elements emit radiation on their own and named this phenomenon as radioactivity. And these elements are known as radioactive elements.

`=>` ELECTROMAGNETIC RADIATIONS: These are the radiations which do not need any medium for propagation e.g. ultraviolet, infrared etc.
`=>` FREQUENCY: The number of waves which pass through a point in one second. Its unit is `s^(-1)` or Hertz.
`=>` WAVENUMBER: This is reciprocal of wavelength and defined as the number of wavelenghts per unit length. Its unit is `m^(-1)` or `cm^(-1)`

`=>` BLACK BODY: The ideal body which emits and absorbs radiation of all frequencies is called a black body and the radiation emitted by such a body is called a black body radiation.

`=>` PHOTOELECTRIC EFFECT: H. Hertz performed a very interesting experiment (1887) in which electrons (or electric current) were ejected when certain metals (for example K, Rb, Cs, etc.) were exposed to a beam of light as shown in fig. 2.9. The phenomenon is called photoelectric effect.

`=>` SPECTRUM: The splitting of a beam of light into radiations of different wavelengths or frequencies after passing through a prism or diffraction grating is called dispersion and pattern of radiation observed after the dispersion is called spectrum.

CONTINUOUS SPECTRUM: When white light is allowed to pass through a prism, it gets resolved into several colours. The spectrum is a rainbow of colours i.e. there is no sharp boundary between two colours.

`=>` LINE SPECTRUM: When gases or vapours of a chemical substances are heated in an electric arc or in a bunsen flame, light is emitted. If the ray of this light is passed through a prism, a line spectrum is produced. The lines are seperated from each other by dark spaces.

`=>` EMISSION SPECTRUM: The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum.

`=>` ABSORPTION SPECTRUM: A continuum of radiation is passed through a sample which absorbs radiation of certain wavelength. The missing wavelength which corresponds to the radiation absorbed by the matter, leave dark spaces in the bright continuous spectrum.

`=>` SPECTROSCOPY: The study of emission or absorption spectra is called spectroscopy.

`=>` ORBIT: It is well-defined circular path followed by electron around nucleus.

`=>` HEISENBERG'S UNCERTAINITY PRINCIPLE: It states that it is impossible to determine simultaneously the exact position and exact momentum (or velocity) of an electron.

`=>` QUANTUM MECHANICS: The branch of science which considers the dual behaviour of matter is called quantum mechanics.
`=>` WAVE FUNCTION: It is simply a mathematical function of the coordinates of the electron.

`=>` BOUNDARY SURFACE DIAGRAM: For a given orbital, only that boundary surface diagram of constant probability density is taken which encloses a region or volume in which the probability of finding the electron is `90%`.
`=>` ANGULAR NODES: The probability density functions at the plane passing through the nucleus is zero. This is called angular node.

`=>` DEGENERATE ORBITALS; Orbitals having same energy are called degenerate orbitals.
`=>` AUFBAU PRINCIPLE: In the ground state of the atoms, the orbitals are filled in order of the their increasing energies.
`=>` PAULI EXCLUSION PRINCIPLE: No two electrons in an atom can have the same set of four quantum numbers.
`=>` HUND'S RULE OF MAXIMUM MULTIPLICITY : Pairing of electrons in a orbital of a specific subshell (`s`, `p`, `d`, `f`) does not take place until each orbital of that subshell is singly occupied.

`=>` ELECTRONIC CONFIGURATION: The distribution of electrons into orbitals of an atom is called its electronic configuration.