The periodic table groups the elements in order of increasing atomic number in such a way that elements with similar properties fall near each other. As the atomic number increases, the number of electrons in each atom also increases. A full appreciation of electronic structure - how the electrons are arranged in atoms - is essential for understanding the similarities and periodicities of the elements. Understanding electronic structure, in turn, requires a brief excursion into classical and modern physics. To begin, we must look at the behaviour of light, the energy of light and some aspects of the interaction of light with matter.

Wavelength (l ) = distance between points in same place on adjacent waves (i.e. distance per cycles).

Frequency (n ) = number of complete waves, also known as cycles, passing a given point in a unit of time.

Wavelength has units of length, usually nanometres (nm) or 10^-9 m.

Frequency has cycles per unit time or cycles per second (s^-1), SI unit is of course Hertz (Hz).

Speed of light in a vacuum or c = 2.998 x 10⁸ m/s.

The wavelength and frequency of light are related to the speed of light by the equation:

c = l n

The wavenumber is the number of wavelengths per unit of length covered or

so its units are cm^-1.

 

• Proof of light as waves came in the form of diffraction or the spreading of waves as they pass obstacles or openings comparable in size to their wavelength.

   

  

   

• An example of constructive and destructive light interference comes with a diffraction pattern.

Light can apply a force to an object so it must also consist of particles but when two light beams are crossed they don't scatter! This existence of two ways of describing the behaviour of light, as both waves and small particles is referred to as wave-particle duality. It was also discovered that electrons in atoms can best be described as both waves and particles.

The new description of light began with the realisation that at each wavelength, light does not have available a continuum of energy values, as classical theory had assumed. Rather, light can be regarded as made up of particles, each of which carries a definite amount of energy, referred to as a quantum. A quantum is like a package that is available only in specific and separate amounts. Something that is quantised is restricted to amounts that are whole-number multiplets of the basic unit, or quantum, for a particular system. Quantum Theory is a general term for the idea that energy is quantised and the consequences of that idea. Like wave-particle duality, quantum theory has also been extended to electrons in atoms.

The quantisation of energy was introduced by Max Planck in 1900. The energy of groups of atoms could be related to their frequency by a constant h, called Planck's constant, according to the following equation:

E = hn

energy = Planck's constant x frequency.

Planck proposed that radiation could only be absorbed or given off in amounts of hn .

A modern value for h = 6.6262 x 10^-34 J.s.

A quantum of radiation energy is called a photon. Individual photons of high-intensity light of a given frequency have no greater energy than individual photons of a low-intensity light of the same frequency. High intensity light just has more photons, each with the same energy.

Information provided by: http://www.bpx.cov.ac.uk