The modern periodic table has been divided into different blocks. These blocks consist of elements that share similar physical and chemical properties and follow a particular trend in their properties. There are s-block elements, p-block elements, d-block elements and then there are f-block elements. Our interest for this article lies in the f-block elements. The f-block elements, sometimes also referred to as the inner transition metals,include the list of rare earth metals called Lanthanoids and Actinoids. There are total 14 elements in Lanthanoids and 14 in Actinoids. The presence of electrons in f-orbitals differentiates these elements from the main group elements in the periodic table.

Electronic Configuration
The study of Lanthanoids often includes lanthanum due to its close resemblance to the properties of lanthanoid elements. Starting from lanthanum with atomic number 57, these soft silvery white metals continue till lutetium with atomic number 71. The elements belonging to lanthanoid series (57-71) have their atomic configuration with a variable occupancy of 4f energy level and a common 6s2 level. Post lanthanum, the energy level of the 5d sub-shell is found to be greater than that of the 4f sub-shell. As a result of the low energy of 4f sub-shell electrons occupy these sub-shells before occupying the 5d sub-shell. As we progress from atomic number 57 to 71, an electron is added in 4f energy level with increase in atomic number with the exception of some metals where the electron occupies a 5d energy level. Tri-positive ions being the most stable oxidation state, the electronic configuration takes the form of 4fn for all the tri-positive ions with n progressively increasing from 1 to 14 with increase in atomic number. The electronic configuration of all the 14 metals belonging to lanthanoid series is given below along with their name, atomic number and symbol.

Atomic Number Symbol Name Electronic Configuration
57 La Lanthanum [Xe] 5d16s2
58 Ce Cerium [Xe] 4f15d16s2
59 Pr Praseodymium [Xe] 4f36s2
60 Nd Neodymium [Xe] 4f46s2
61 Pm Promethium [Xe] 4f56s2
62 Sm samarium [Xe] 4f6 6s2
63 Eu Europium [Xe] 4f7 6s2
64 Gd Gadolinium [Xe] 4f75d16s2
65 Tb Terbium [Xe] 4f9 6s2
66 Dy Dysprosium [Xe] 4f10 6s2
67 Ho Holmium [Xe] 4f11 6s2
68 Er Erbium [Xe] 4f12 6s2
69 Tm Thulium [Xe] 4f13 6s2
70 Yb Ytterbium [Xe] 4f14 6s2
71 Lu Lutetium [Xe] 4f14 5d1 6s2

The determination of electronic configuration and inner transition metal properties has been a great task for the scientists due to the low difference in the energy levels of 5d sub-shell and 4f sub-shell. Lanthanoids and their compounds find great application as catalysts, in mining and metallurgical processes and medical sciences.