What does a period in the periodic table refer to?

A period in the periodic table refers to a horizontal row of elements. Each period represents a principal energy level of electrons in an atom. As you move from left to right across a period, the atomic number increases, meaning that each subsequent element has one more proton and one more electron than the one before it. This organization helps demonstrate trends in element properties, such as electronegativity, ionization energy, and atomic radius, which vary predictably across a period.

In contrast, a vertical column pertains to groups or families of elements that share similar chemical properties. This grouping is based on the similar valence electron configuration of the elements within that column. While the idea of isotopes relates to variations of the same element with different numbers of neutrons, it is not relevant in defining periods in the periodic table. The choice regarding isotopes is particularly unrelated as it does not pertain to how elements are arranged in the periodic table’s structure.