The size of the trivalent cations in the lanthanoid series decreases steadily as the atomic number increases. What is this known as ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the phenomenon observed in the lanthanoid series where the size of trivalent cations decreases as the atomic number increases.

2. **Identifying the Phenomenon**: The decrease in size of trivalent cations in the lanthanoid series is referred to as "lanthanide contraction."

3. **Explaining the Reason**: The reason for lanthanide contraction is the filling of the inner f-orbitals. As we move across the lanthanoid series, the f-orbitals are progressively filled, which leads to an increase in the effective nuclear charge experienced by the outer electrons.

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