Which important property did Mendeleev use to classify the element in his periodic table ?

Explanation

To answer the question about the important property that Mendeleev used to classify elements in his periodic table, follow these steps:

### Step-by-Step Solution:

1. **Understanding Mendeleev's Contribution**: Mendeleev was a pioneer in the development of the periodic table of elements. His work laid the foundation for how we understand the relationships between different elements.

2. **Key Property Used for Classification**: The primary property that Mendeleev utilized to classify elements was their **atomic mass**. He arranged the elements in order of increasing atomic mass.

...