Question 18

Periods are the horizontal rows of the periodic table.

In the periodic table, the horizontal rows are referred to as periods. Each period corresponds to the filling of a new electron shell.

A) groups

Groups are the vertical columns of the periodic table, not the horizontal rows. Each group contains elements that have similar chemical properties due to their similar valence electron configurations.

B) periods

Periods are indeed the correct term for the horizontal rows in the periodic table. Each period signifies a new principal energy level of electrons, reflecting the periodic nature of elements as they are arranged by increasing atomic number.

C) families

Families is another term often used to describe groups, but it does not refer to horizontal rows. Families can indicate elements that share similar characteristics but do not define the arrangement of the periodic table horizontally.

D) sets

Sets is not a standard term used to describe any part of the periodic table. This term does not have a specific meaning in the context of the organization of elements in the periodic table.

Conclusion

The correct answer, periods, accurately describes the horizontal rows of the periodic table where elements are arranged based on increasing atomic number and periodic properties. All other options either refer to different aspects of the periodic table or do not apply to the horizontal arrangement, confirming that periods are the only correct designation in this context.