The two series Lanthanides and Actinides constitute ‘f’ block elements. They are group of 15 elements each. The most common oxidation state for the lanthanides and some actinides is +3. They are similar to each other in properties. The filling of 4f orbital is known as lanthanides and filling of 5f orbital is known as actinides. The lanthanides were known as rare earths earlier but it was not appropriate as many of the lanthanides are not particularly rare. The fourteen elements from actinium are called actinides. The actinides do not show chemical uniformity as in the case of lanthanides. All the actinides are radioactive but the most abundant isotopes of Thorium and Uranium have very long half lives. Except Actinium, Thorium, Protactinium and Uranium, all the elements are synthetic elements or manmade elements or transuranic elements. Lanthanum, the first member of lanthanides is a true member of Gr III, with atomic number 57. The fourteen members which follow lanthanum e.g. from atomic number 58 to 71, in which the 4f electrons are successively added to the La configuration are the rest. The term lanthanide is taken to include La, as this element is the prototype for the succeeding fourteen elements. Actinium is similarly, though being an element of Group III with atomic number 89 is the first member of actinide series. The fourteen elements which follow actinium with atomic number 90 to 103 constitute actinide series. Both lanthanides (Ln) and actinides(An) have variable valence’s. The values are mostly in agreement with L–S coupling scheme rather than only spin values. The magnetic properties of An show overall similarity to those of the corresponding Ln ions but are somewhat lower due to quenching of orbital contribution by crystal field effects. The periodic trends of ‘d’ and ‘f’ block elements are different from ‘s’ and ‘p’ block elements. In the sixth period, the 4‘f’ orbitals are very deeply buried under the core and valence orbitals, Thus there is no double bonding. Again‘d’ and ‘f’ block elements can form  bonds. Maximum two bonds can be formed per pair of atoms using dxy and d x y 2 2  orbitals. f block elements show spectra with pale colours due to Laporte’s forbidden f–f transitions. However, charge transfer spectra of MLCT and LMCT show intense colours. Ruby lasers and phosphors are based on ‘f’ block elements. These phosphors are present in colour televisions. ‘f’ block elements are also substituted isomorphorously for Ca2+ ions in enzymes. Modern Chemistry and Applications is an open access platform on all the aspects of Chemistry which include organic, inorganic, analytical, physical, material, environmental chemistry etc., with emphasis on current trends of computational and forensic chemistry and their applications and aims to publish most complete and reliable source of information on the discoveries and current developments in the mode of original articles, review articles, case reports, short communications, etc. in all areas of the field and making them freely available through online without any restrictions or any other subscriptions to researchers worldwide. Visit: https://www.walshmedicalmedia.com/modern-chemistry-applications.html Submit Manuscript: https://www.walshmedicalmedia.com/modern-chemistry-applications/submit-manuscript.html Instructions for Authors: https://www.walshmedicalmedia.com/modern-chemistry-applications/instructionsforauthors.html Archive page: https://www.walshmedicalmedia.com/modern-chemistry-applications/archive.html