Which important property did Mendeleev use to classify the elements in his periodic table and did he stick to that?

Explanation

**Step-by-Step Solution:**

1. **Identify the Classification Property:**
- Mendeleev used the **atomic mass** of elements as the primary property for classifying them in his periodic table. He arranged the elements in order of increasing atomic mass.

2. **Grouping of Elements:**
- Mendeleev placed elements with similar properties into the same **group** or column. This means that elements that exhibited similar chemical behaviors were categorized together.

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