If an atom's outermost electron shell has a capacity to hold more than 8 electrons, such as in the case of elements beyond the second period (row) of the periodic table, the additional electrons would occupy the next available energy level or subshell.
The octet rule, which states that atoms tend to gain, lose, or share electrons in order to achieve a stable configuration with 8 valence electrons, applies mainly to elements up to the second period (row) of the periodic table. These elements have their outermost shell, known as the valence shell, as the second energy level (n = 2) with an s and p subshell.
However, elements beyond the second period, starting from the third period (row), have additional energy levels and subshells available for electron occupation. These include the d and f subshells. For example, elements in the third period have a third energy level (n = 3) and include the 3s, 3p, and 3d subshells. Elements in the fourth period have a fourth energy level (n = 4) and include the 4s, 4p, and 4d subshells.
When an atom in these periods gains an extra electron beyond the 8 valence electrons, it would typically fill the next available subshell or energy level. For instance, if an atom in the third period gains an additional electron after filling its 3s² and 3p⁶ subshells, it would move to the 4s subshell to accommodate the additional electron. Similarly, for elements in the fourth period, the additional electron would go to the next available subshell, such as the 4p or 4d subshell.
It's important to note that while the octet rule provides a useful guideline for chemical bonding and stability for many elements, there are exceptions and variations that arise when considering elements with expanded electron configurations beyond the second period. These elements exhibit diverse electron arrangements and can form a variety of chemical bonds.