Lanthanoids and actinoids are f-block elements in which the last electron enters the anti-penultimate energy level. The most common oxidation state of lanthanoids is +3, though +2 and +4 oxidation states are exhibited by some of the elements. In lanthanoid series, there is a progressive decrease in atomic and ionic radii, known as lanthanoid contraction. Unlike lanthanoids, actinoids show a large number of oxidation states.
Write the general electronic configuration of lanthanoids.

Explanation

To determine the general electronic configuration of lanthanoids, we can follow these steps:

### Step 1: Identify the Series
Lanthanoids are a series of 15 elements in the periodic table, ranging from atomic number 57 (Lanthanum, La) to atomic number 71 (Lutetium, Lu).

### Step 2: Understand the Electron Filling Order
Lanthanoids are f-block elements, meaning their last electrons fill the f-orbitals. Specifically, for lanthanoids, the 4f subshell is being filled.

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