The horizontal rowsin the modern periodic table are called:

periods

The correct answer is option 1. periods.

The modern periodic table is organized in a way that elements with similar chemical properties are grouped together. The horizontal rows in the periodic table are known as periods. Each period represents the energy levels or shells that electrons occupy in an atom.

1. Energy Levels:  Each period corresponds to a specific energy level in an atom. As you move from left to right across a period, the atomic number increases, indicating the addition of electrons to the same energy level.

2. Numbering: The periods are numbered from 1 to 7. The period number indicates the highest energy level (or shell) occupied by electrons in the elements of that period.

3. Properties:  Elements within the same period may vary widely in properties, but they share some commonalities due to being in the same energy level.

For example, in the 2nd period, elements like lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon are present. All of these elements have electrons filling the second energy level.

In summary, periods in the periodic table represent the horizontal rows, and each period corresponds to a specific energy level in the electron configuration of elements. The arrangement of elements in periods helps to observe patterns in their properties and behavior.