f block elements

The elements whose last electron enters into any one of the seven f-orbitals of the ante-penultimate shell are called f-block elements. This block consists of two series of elements placed at the bottom of the periodic table. Each series comprises fourteen elements.

1)The first series is called Lanthanide

2) second series is known as Actinide.

The f-block elements are also known as inner transition elements.

LANTHANIDES

The series which involves filling of 4-f orbitals and follows lanthanum in the periodic table is called lanthanide series. The 14 elements present in this series are called lanthanides. They are also called rare earth elements because they occur rarely in the earth’s crust. This series forms a part of the 6th period .

CHARACTERISTICS

GENERAL ELECTRONIC CONFIGURATION

[Xe] 4f 1-14 5d0-1 6s2

ATOMIC RADIUS

IONIC RADIUS

Q:- What is lanthanide contraction ?

Ans:- The phenomenon of slow and steady decrease in atomic and ionic sizes of lanthanides with the increase in atomic number is called lanthanide contraction. E.g. Ionic radius decreases from Ce3+ (103 pm) to Lu3+ (86 pm)

CAUSE OF LANTHANIDE CONTRACTION

As we move along the series the nuclear charge keeps on increasing and electrons are added to the same inner 4f subshell. As a result of which the attraction on the electrons by the nucleus increases and this further tends to decrease the size. Due to diffused shape of f-orbitals ,4f electrons have very poor shielding effect. This poor shielding is unable to counterbalance the effect of increased nuclear charge. Hence the valence shell keeps on contracting and the net effect is slow decrease in the sizes of lanthanides.

CONSEQUENCES OF LANTHANIDE CONTRACTION

1. All lanthanides show similar chemical properties.
2. Separation of lanthanides is very difficult.
3. 4d and 5d series resemble each other.
4. Zr-Hf, Nb-Ta, Mo-W etc. have almost similar sizes.
5. The basic strength of hydroxides decreases with the increase in atomic number.
6. Their standard reduction potential increases slowly and steadly.

DIFFERENCES B/W LANTHNOIDS AND ACTINOIDS

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