In the periodic table, the term ‘period’ refers to the horizontal rows, while ‘group’ denotes the vertical columns.

A ‘period’ in the periodic table of elements represents a horizontal row. The elements found within a single period do not exhibit similar properties. The first period contains only two elements: hydrogen and helium. In contrast, both the second and third periods consist of $8$ elements each. The fourth and fifth periods are longer, comprising $18$ elements. The sixth and seventh periods contain $32$ elements each, as they include the two separate rows at the bottom of the table, known as the lanthanides and actinides. The length of each period is determined by the number of electrons that can occupy the sublevels being filled during that period.

Conversely, a ‘group’ in the periodic table refers to a vertical column. Elements within the same group share similar physical and chemical characteristics due to the comparable arrangement of their outermost electron shells (i.e., they exhibit the same core charge). This similarity arises because the chemical properties of elements are predominantly influenced by the orbital locations of their outermost electrons. There are $18$ numbered groups in the periodic table; however, the f-block columns, which lie between groups $3$ and $4$, are not assigned numbers. Elements within a group tend to exhibit similar properties. For instance, the alkali metals are located in group $1$, and they share common characteristics such as high reactivity and a tendency to lose one electron, achieving a noble-gas electron configuration.

In summary, the periodic table is systematically organized into periods and groups to classify elements based on their shared properties and atomic structure. This arrangement is invaluable for chemists, as it aids in understanding and predicting the behavior of elements, making it an essential resource in the study of chemistry.