Core electrons are the electrons in an atom that are not valence electrons and therefore do not participate in bonding.
 The number of valence electrons of an element can be determined by the periodic table group of the element.
 With the exception of the transition metals in groups 3-12 and the Lanthanide and Actinide series, the number of valence electrons ranges from 0-8 electrons.
 All the non-valence electrons for an atom of that element are considered core electrons.
In chemistry, molecular orbital (MO) theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.
 The spatial and energetic properties of electrons within atoms are fixed by quantum mechanics to form orbitals that contain these electrons.
 While atomic orbitals contain electrons ascribed to a single atom, molecular orbitals, which surround a number of atoms in a molecule, contain valence electrons between atoms.
 Molecular orbital theory, which was proposed in the early twentieth century, revolutionized the study of bonding by approximating the positions of bonded electrons—the molecular orbitals—as linear combinations of atomic orbitals (LCAO).
 These approximations are now made by applying the density functional theory (DFT) or Hartree–Fock (HF) models to the Schrödinger equation.
In chemistry, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.
 The presence of valence electrons can determine the element's chemical properties, such as its valence—whether it may bond with other elements and, if so, how readily and with how many.
 For a main group element, a valence electron can exist only in the outermost electron shell; in a transition metal, a valence electron can also be in an inner shell.
The actinide or actinoid (IUPAC nomenclature) series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom and is sometimes called a non-bonding pair.
 Lone pairs are found in the outermost electron shell of atoms.
 They can be identified by using a Lewis structure.
 Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding.
 Thus, the number of lone pair electrons plus the number of bonding electrons equals the total number of valence electrons around an atom.
In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons.
 That is, the atoms share one pair of electrons where the bond forms.
 Therefore, a single bond is a type of covalent bond.
 When shared, each of the two electrons involved is no longer in the sole possession of the orbital in which it originated.
 Rather, both of the two electrons spend time in either of the orbitals which overlap in the bonding process.
 As a Lewis structure, a single bond is denoted as AːA or A-A, for which A represents an element (Moore, Stanitski, and Jurs 329).
 In the first rendition, each dot represents a shared electron, and in the second rendition, the bar represents both of the electrons shared in the single bond.
The 18-electron rule is a rule used primarily for predicting and rationalizing formulae for stable metal complexes, especially organometallic compounds.
 The rule is based on the fact that the valence shells of transition metals consist of nine valence orbitals (one s orbital, three p orbitals and five d orbitals), which collectively can accommodate 18 electrons as either bonding or nonbonding electron pairs.
 This means that, the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding.
 When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period.
 The rule and its exceptions are similar to the application of the octet rule to main group elements.
 The rule is not helpful for complexes of metals that are not transition metals, and "interesting or useful transition metal complexes will violate the rule" because of the consequences deviating from the rule bears on reactivity.
 The rule was first proposed by American chemist Irving Langmuir in 1921.
Iodanes are chemical compounds containing hypervalent iodine.
 These iodine compounds are hypervalent because the iodine atom in it contains more than the 8 electrons in the valence shell required for the octet rule.
 When iodine is complexed with a monodentate electronegative ligand such as chlorine, iodine compounds occur with a +3 oxidation number as iodine(III) or λ-iodanes or as a +5 oxidation number as iodine(V) or λ-iodanes.
 Iodine itself contains 7 valence electrons and in a λ-iodane three more are donated by the ligands making it a decet structure.
 λ-iodanes are dodecet molecules.
 In an ordinary iodine compound such as iodobenzene the number of valence electrons is eight as expected.
 In order to get from iodine to a hypervalent iodine compound it gets oxidized with removal of first 3 electrons and then 5 electrons.
 The ligands in turn contribute electrons pairs and form coordinate covalent bonds by adding a total of 6 or 10 electrons back to iodine.
 In the L-I-N notation L stands for the number of electrons donated by ligands and N the number of ligands.
In the context of atomic orbitals, an open shell is a valence shell which is not completely filled with electrons or that has not given all of its valence electrons through chemical bonds with other atoms or molecules during a chemical reaction.
 Atoms generally reach a noble gas configuration in a molecule.
 The noble gases (He, Ne, Ar, Kr, Xe, Rn) are less reactive and have configurations 1s (He),
Electronegativity, symbol "χ", is a chemical property that describes the tendency of an atom to attract electrons (or electron density) towards itself.
 An atom's electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus.
 The higher the associated electronegativity number, the more an element or compound attracts electrons towards it.

Question: Aside from the series that ranges from atomic numbers 89 to 103, what series is an exception to the rule that the number of valence electrons ranges from 0-8 electrons?
Answer: Lanthanide