Core electron
Core electrons are the electrons in an atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

 that are not valence electron
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...

s and therefore do not participate in bonding. An example: the carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom has a total of 6 electrons, 4 of them being valence electrons. So the remaining 2 electrons must be core electrons.

They are so tightly bound to the nucleus as to be negligibly perturbed by the environment of the atom when in the solid state. Therefore on the contrary of the valence electrons, the core electrons usually play a secondary role on chemical bonding and reactions and their main role is to screen the positive charge of the atomic nucleus.
In transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...

s, however, the distinction between core and valence electrons is more subtle and it could be very important to consider the electrons in the highest d-shells as valence rather than core electrons.

Electron transition

A core electron can be removed from its core-level upon absorption of electromagnetic radiation (X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

) and excited to an empty outer shell or emitted as photoelectron (photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...

). The resulting atom with one of its core-level (a so-called core-hole) empty is in a metastable state and decays within 10−15 s by x-ray fluorescence
X-ray fluorescence
X-ray fluorescence is the emission of characteristic "secondary" X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays...

 or by Auger effect.

By detecting the emitted photoelectrons (photoemission spectroscopy
Photoemission spectroscopy
Photoemission spectroscopy , also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in a substance...

), the X-ray photons (XAS
X-ray absorption spectroscopy is a widely-used technique for determining the local geometric and/or electronic structure of matter. The experiment is usually performed at synchrotron radiation sources, which provide intense and tunable X-ray beams. Samples can be in the gas-phase, solution, or...

 and fluorescence spectroscopy
Fluorescence spectroscopy
Fluorescence spectroscopy aka fluorometry or spectrofluorometry, is a type of electromagnetic spectroscopy which analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electrons in molecules of certain compounds and causes them to emit...

) or the Auger electrons (Auger electron spectroscopy
Auger electron spectroscopy
Auger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science...

) useful information on the electronic and the local lattice structures of a material can be obtained.

The atoms of such techniques results from the fact that since every atom has core-level electrons with well-defined binding energies, it is possible to select the element to probe by tuning the x-ray energy to the appropriate absorption edge. For the same reason the spectra of the radiation emitted (electrons or photons) can be used to determine the elemental composition of a material.
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