The horizontal rows in Mendleev's table were called :

Explanation

To solve the question regarding what the horizontal rows in Mendeleev's table were called, follow these steps:

### Step-by-Step Solution:

1. **Understand Mendeleev's Table**: Mendeleev's periodic table organizes elements based on their atomic mass and properties. It consists of both horizontal and vertical arrangements.

2. **Identify the Arrangement**: In Mendeleev's table, the horizontal rows are referred to as "periods." Each period corresponds to a different energy level of electrons in the atoms of the elements.

3. **Count the Periods**: Mendeleev's periodic table contains a total of seven periods. This indicates the number of horizontal rows present.

4. **Differentiate Between Rows and Columns**: While horizontal rows are called periods, the vertical columns in Mendeleev's table are known as "groups."

5. **Conclude the Answer**: Therefore, the horizontal rows in Mendeleev's table are called "periods."

### Final Answer:
The horizontal rows in Mendeleev's table were called **periods**.

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