of periodic trends such as atomic radius, first ionization energy (IE), electronegativity, and oxidizing. Core charge can also be calculated as 'atomic number' minus 'all electrons except those in the outer shell'. For example, chlorine (element 17), with electron configuration 1s2 2s2 2p6 3s2 3p5, has 17 protons and 10 inner shell electrons (2 in the first shell, and 8 in the second) so: Core charge = 17 − 10 = +7 A core charge is the net charge of a nucleus, considering the completed shells of electrons to act as a 'shield.' As a core charge increases, the valence electrons are more strongly attracted to the nucleus, and the atomic radius decreases across the period. Relativistic effects For elements with high atomic number Z, relativistic effects can be observed for core electrons. The velocities of core s electrons reach relativistic momentum which leads to contraction of 6s orbitals relative to 5d orbitals. Physical properties effected by these relativistic effects include lowered melting temperature of mercury and the observed golden colour of gold and caesium due to narrowing of energy gap. Gold appears yellow because it absorbs blue light more than it absorbs other visible wavelengths of light and so reflects back yellow-toned light. Electron transition A core electron can be removed from its core-level upon absorption of electromagnetic radiation. This will either excite the electron to an empty valence shell or cause it to be emitted as a photoelectron due to the photoelectric effect. The resulting atom will have an empty space in the core electron shell, often referred to as a core-hole. It is in a metastable state and will decay within 10−15 s, releasing the excess energy via X-ray fluorescence (as a characteristic X-ray) or by the Auger effect. Detection of the energy emitted by a valence electron falling into a lower-energy orbital provides useful information on the electronic and local lattice structures of a material. Although most of the time this energy is released in the form of a photon, the energy can also be transferred to another electron, which is ejected from the atom. This second ejected electron is called an Auger electron and this process of electronic transition with indirect radiation emission is known as the Auger effect. Every atom except hydrogen has core-level electrons with well-defined binding energies. It is therefore possible to select an element to probe by tuning the X-ray energy