f block elements in periodic table

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Number of f-block elements in the periodic table

The f-block in the periodic table appears in two series characterized by the filling of 4f and 5f sublevel in the respective third inner principal quantum number from outermost.

The 4f block contains fourteen elements cerium to lutetium with the atomic number from 58 to 71 and is called Lanthanum's as they appear after lanthanum.

The 5f block contains fourteen elements thorium to lawrencium with the atomic number from 90 to 103 and is called actinides as they appear after actinium.

Electronic structure of 4f-block in the periodic table

The 4f block in the periodic table has been variously called rare earth, lanthanum’s, and lanthanum. The lanthanum atoms and their trivalent ions have the following general electronic 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.
Electronic configuration of 4f-block elements in the periodic table
4f-block elements in the periodic table
4f-block or inner transition elements with increasing atomic number, electrons are added to the deep-seated 4f-orbital. The outer electronic configuration of 4f-elements is 6S² and inner orbitals contain f -electrons.
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Cerium, Gadolinium, and Lutetium

Only Cerium, Gadolinium, and Lutetium contain one electron in 5d orbital with atomic number 58, 64 and 87. The electronic configuration of these elements is outside of the general electronic configuration.

Electrons of similar spin developed an exchange interaction which leads to the stabilization of the system. For the electrons of similar spin, repulsion is less by an amount called the exchange energy of an electron.

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

Cerium
[Pd] 4f¹ 5S² 5P⁶ 5d¹ 6S²

Gadolinium atom contains one electron in a 5d orbital. 4f and 5d are very close in terms of the energy of 4f and 5d orbital. In such a case, the half-filled orbital is slightly more stable than orbital with one additional electron by increasing exchange energy.

Maximum stability of f-shell when there are seven electrons with parallel spins in the seven f-orbital. This half-filled energy level stabilizes by exchange energy.

Gadolinium
[Pd] 4f⁷ 5S² 5P⁶ 5d¹ 6S²

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

Lutetium
[Pd] 4f¹⁴ 5S² 5P⁶ 5d¹ 6S².

Electronic configuration of praseodymium

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.

n + l = 4+3 = 7 for 4f-orbital.
n + l = 5+7 =7 for 5d-orbital
.

4f and 5d-shell sum of principal and azimuthal quantum number the same. In this case, the highest number of principal quantum numbers is the higher energy quantum level of an atom and 5d-orbital is the higher energy quantum shell.

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

Electron filling process for praseodymium f-electron filling first and possess electronic configuration 4f³ 6s².

Question
Why +3 oxidation number is so common and stable in lanthanum?

Answer
The nature of lanthanum's elements is such that three electrons are removed comparatively easy to give the normal trivalent state.

The ground state electronic configuration of the natural lanthanum's atoms and trivalent ions

Lanthanum's [Pd] 4fn 5S² 5P⁶ 5d¹ 6S²

Lanthanum ions [Pd] 4fn 5S² 5P⁶
where n is 0 to 14 from Lanthanum to Lutetium.

As a consequence, the f-electrons can not participate in the chemical reactions thus +3 oxidation numbers common and stable in lanthanums.

Electronic structure of 5f-block elements in the periodic table

Electronic configuration of 5f-block elements in the periodic table
5f-block elements in the periodic table
The second series f block elements result from the filling of 5f-orbital and consist of elements thorium to lawrencium with atomic number 90 to 103.

All of them are radioactive but most abundant isotopes of thorium and uranium have very long half-lives.
General electronic configuration of these elements and ion are

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.

Number of f-block elements in the periodic table, study the electronic structure of 4f-block, and 5f-block for online schools and colleges courses

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