Core electron

Core electronsare theelectronsin anatomthat are notvalence electronsand do not participate inchemical bonding.Thenucleusand the core electrons of an atom form the atomic core. Core electrons are tightly bound to the nucleus. Therefore, unlike valence electrons, core electrons play a secondary role in chemical bonding and reactions by screening the positive charge of the atomic nucleus from the valence electrons.

The number of valence electrons of an element can be determined by theperiodic table groupof the element (seevalence electron):

• Formain-group elements,the number of valence electrons ranges from 1 to 8 (ns andnp orbitals).
• Fortransition metals,the number of valence electrons ranges from 3 to 12 (ns and (n−1)d orbitals).
• Forlanthanidesandactinides,the number of valence electrons ranges from 3 to 16 (ns, (n−2)f and (n−1)d orbitals).

All other non-valence electrons for an atom of that element are considered core electrons.

Orbital theory

A more complex explanation of the difference between core and valence electrons can be described withatomic orbital theory.

In atoms with a single electron the energy of an orbital is determined exclusively by the principal quantum numbern.Then= 1 orbital has the lowest possible energy in the atom. For largen,the energy increases so much that the electron can easily escape from the atom. In single electron atoms, all energy levels with the same principle quantum number are degenerate, and have the same energy.

In atoms with more than one electron, the energy of an electron depends not only on the properties of the orbital it resides in, but also on its interactions with the other electrons in other orbitals. This requires consideration of theℓquantum number. Higher values ofℓare associated with higher values of energy; for instance, the 2p state is higher than the 2s state. Whenℓ= 2, the increase in energy of the orbital becomes large enough to push the energy of orbital above the energy of the s-orbital in the next higher shell; whenℓ= 3 the energy is pushed into the shell two steps higher. The filling of the 3d orbitals does not occur until the 4s orbitals have been filled.

The increase in energy for subshells of increasing angular momentum in larger atoms is due to electron–electron interaction effects, and it is specifically related to the ability of low angular momentum electrons to penetrate more effectively toward the nucleus, where they are subject to less screening from the charge of intervening electrons. Thus, in atoms of higheratomic number,theℓof electrons becomes more and more of a determining factor in their energy, and the principal quantum numbersnof electrons becomes less and less important in their energy placement. The energy sequence of the first 35 subshells (e.g., 1s, 2s, 2p, 3s, etc.) is given in the following table [not shown?]. Each cell represents a subshell withnandℓgiven by its row and column indices, respectively. The number in the cell is the subshell's position in the sequence. See the periodic table below, organized by subshells.

Periodic Table organized by atomic orbitals.

Atomic core

The atomic core refers to the central part of theatomexcluding thevalence electrons.The atomic core has a positiveelectric chargecalled thecore chargeand is theeffective nuclear chargeexperienced by an outer shellelectron.In other words, core charge is an expression of the attractive force experienced by thevalence electronsto thecoreof anatomwhich takes into account theshielding effectof core electrons. Core charge can be calculated by taking the number ofprotonsin thenucleusminus the number of core electrons, also called inner shell electrons, and is always a positive value in neutral atoms.

The mass of the core is almost equal to the mass of the atom. The atomic core can be considered spherically symmetric with sufficient accuracy. The core radius is at least three times smaller than the radius of the corresponding atom (if we calculate the radii by the same methods). For heavy atoms, the core radius grows slightly with increasing number of electrons. The radius of the core of the heaviest naturally occurring element -uranium- is comparable to the radius of a lithium atom, although the latter has only three electrons.

Chemical methods cannot separate the electrons of the core from the atom. When ionized by flame orultravioletradiation, atomic cores, as a rule, also remain intact.

Core charge is a convenient way of explaining trends in the periodic table.Since the core charge increases as you move across a row of theperiodic table,the outer-shell electrons are pulled more and more strongly towards the nucleus and theatomic radiusdecreases. This can be used to explain a number ofperiodic trendssuch as atomic radius,first ionization energy(IE),electronegativity,andoxidizing.

Core charge can also be calculated as 'atomic number' minus 'all electrons except those in the outer shell'. For example,chlorine(element 17), withelectron configuration1s²2s²2p⁶3s²3p⁵,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 completedshellsof electrons to act as a 'shield.' As a core charge increases, thevalence electronsare more strongly attracted to the nucleus, and theatomic radiusdecreases across the period.

Relativistic effects

For elements with high atomic numberZ,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 affected by these relativistic effects include lowered melting temperature of mercury and the observed golden colour ofgoldandcaesiumdue 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.

Gold Spectrum

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 aphotoelectrondue to thephotoelectric effect.The resulting atom will have an empty space in the core electron shell, often referred to as acore-hole.It is in a metastable state and will decay within 10⁻¹⁵s, releasing the excess energy viaX-ray fluorescence(as acharacteristic X-ray) or by theAuger 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 aphoton,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 theAuger 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 to the appropriate absorption edge. The spectra of the radiation emitted can be used to determine the elemental composition of a material.

See also

• Atomic orbital
• Auger effect
• Lanthanide contraction
• Relativistic quantum chemistry
• Shielding effect
• Surface core level shift
• Valence electron