Atomic Number of Lanthanum and Actinium

The f block elements appear in two series like 4f blocks and 5f blocks names as lanthanides and actinides in the periodic table. The electronic configuration of lanthanum and actinium has done filling electron in deep-seated 4f and 5f orbital with the increasing atomic number of the element. The trivalent oxidation number of lanthanides and actinides is a stable oxidation state.

Thus 4f block contains fourteen elements cerium to lutetium with the atomic number from 58 to 71. These are Lanthanum’s as they appear after lanthanum.

Atomic number electron configuration of f block elements Lanthanides in periodic table

Also, the 5f block contains fourteen elements thorium to lawrencium with atomic number 90 to 103. These elements are called actinides because they appear after actinium.

Therefore the general electron configuration of these elements and ion of actinium or actinides.

f Block Element Lanthanides

The 4f block in the periodic table has been variously called rare earth, lanthanum’s, and lanthanum.

And the lanthanum atoms and their trivalent ions have the following general electron configuration.

Lanthanum atoms
[Pd] 4fn 5S2 5P6 5d1 6S2
where n has values 1 to 14

Lanthanum (M+3) ions
[Pd] 4fn 5S2 5P6
where n has values 1 to 14

4f-block also called inner transition elements. Because the electrons are added to the deep-seated 4f-orbital with the increasing atomic number of lanthanum.

Thus the outer electronic configuration of 4f block elements 6S2 and inner orbitals contain f -electrons.

Cerium Gadolinium Lutetium in Periodic Table

Cerium, Gadolinium, and Lutetium in the periodic table contain one electron in 5d orbital with atomic number 58, 64, and 87.

Therefore the electron configuration of cerium, gadolinium, and lutetium outside of the general electronic configuration.

Because electrons of similar spin developed an exchange interaction which leads to stabilizing the system. Hence the electrons of similar spin, repulsion is less by an amount called the exchange energy of an electron.

But the greater the number of electrons with parallel spins the greater of exchanged interaction and greater stability. It is the basis of Hound’s rules of maximum spin municipality.

Electron Configuration of Cerium

The electron configuration of cerium
[Pd] 4f1 5S2 5P6 5d1 6S2

Cerium atom contains one electron in a 5d orbital. But 4f and 5d are very close in terms of energy.

Thus the half-filled orbital of cerium slightly more stable than orbital with one additional electron by increasing exchange energy.

Electron Configuration of Gadolinium

The electron configuration gadolinium
[Pd] 4f7 5S2 5P6 5d1 6S2

Gadolinium contains seven electrons with a parallel spin in the seven f-orbitals with the maximum stability of the f orbital.

This half-filled energy level stabilizes by exchange energy in 4f orbital of gadolinium atom.

Electron Configuration of Lutetium

Lutetium also has the f14d1 electron configuration where the last electrons have added the capacity of the f-shell.

The electronic configuration Lutetium
[Pd] 4f14 5S2 5P6 5d1 6S2

Praseodymium electronic configuration

Praseodymium possesses electronic configuration 4f3 6s2 instead of the expected one 4f2 5d1 6s2. This can be explained by (n + l) rules, the orbital which has a higher value of (n + l) is the higher energy orbitals.

For 4f orbital, (n + l) = 4+3
= 7

For 5d orbital, (n + l) = 5+7

Thus for 4f and 5d-orbital, the sum of principal and azimuthal quantum numbers are the same.

In this case, the highest number of principal quantum numbers is the higher energy quantum systems of an atom. Thus 5d-orbital is the higher energy quantum shell.

Again electrons fed into orbitals in order of increasing energy until all the electrons have been accommodated.

Thus the electron filling process for f block element praseodymium f-electron filling first and possess electronic configuration 4f3 6s2.

Oxidation State of Lanthanides

The trivalent oxidation state is the common oxidation state of lanthanum. Because of the removal of three electrons from lanthanides is easier than the removal of greater than three electrons.

The ground state electronic configuration of lanthanum’s atoms and trivalent lanthanum’s ions

Lanthanum’s element
[Pd] 4fn 5S2 5P6 5d1 6S2

Lanthanum ions
[Pd] 4fn 5S2 5P6

where n is 0 to 14 from Lanthanum to Lutetium.

The stability electrons in the f orbital of lanthanides greater than the d and s electrons. Therefore the f electrons can not participate in the chemical reactions and the +3 oxidation number or state is common for lanthanides.

f Block Elements Names Actinides

Name and symbol f block elements in periodic table element

The second series of 5f block elements results from the filling of 5f-orbital. Actinides consist of element thorium to lawrencium with atomic number 90 to 103.

Actinides atoms
[Rn] 4fn 5d1-2 6S2
where n has values 1 to 14

Actinides (M+3) ions
[Rn] 4fn
where n has values 1 to 14

All of these f block elements in the periodic table are radioactive but most abundant isotopes of thorium and uranium have very long half-lives.