Which was the incomplete period in the long-form of the modern periodic table?

7^th period

The correct answer is option 1. 7^th period.

The \(7^{th}\) period is the incomplete period in the long-form of the modern periodic table.

While all periods have elements still under study and with ongoing research, the \(7^{th}\) period is considered incomplete due to the following reasons:

Most elements are unstable: Elements in the 7th period are generally very unstable and exist for only fractions of a second before decaying into lighter elements. This makes studying their properties quite challenging.

Fewer elements discovered: Compared to other periods, fewer elements have been discovered and confirmed in the 7th period. As of today, only 32 elements have been placed in this period, while other periods can hold up to 32 elements each.

Therefore, while the 7th period contains elements and continues to be a subject of active research, its "incompleteness" refers to the difficulty of studying these elements due to their short lifespans and the ongoing discovery process.