An element with atomic number 9 belongs to the second period of the periodic table. The periods in the periodic table represent the number of energy levels (shells) occupied by electrons in an atom of the element. Each period corresponds to a specific principal quantum number (n), which indicates the energy level of the electron shells.
In the case of atomic number 9, the element is fluorine (F). Fluorine has two electron shells: the first shell can hold a maximum of 2 electrons, and the second shell can hold a maximum of 8 electrons. Since fluorine has 9 electrons, 2 of them occupy the first shell, and the remaining 7 electrons occupy the second shell.
The elements in the first period (period 1) have only one energy level (shell) occupied by electrons, while the elements in the second period (period 2) have two energy levels (shells) occupied by electrons. Therefore, an element with atomic number 9 belongs to the second period.