Yes, you are correct that not all elements require eight valence electrons to be stable. The concept of the octet rule, which suggests that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with eight valence electrons, primarily applies to elements in the second period (period 2) and beyond.
However, there are exceptions to the octet rule. Some elements can have stable configurations with fewer than eight valence electrons due to their unique electron configurations and the specific bonding patterns they exhibit. These elements include:
Hydrogen (H): Hydrogen only requires two valence electrons to achieve a stable electron configuration, as it has only one energy level.
Helium (He): Helium is an exception to the octet rule as it has only two electrons in total, which fills its first and only energy level.
Beryllium (Be): Beryllium can achieve a stable configuration with only four valence electrons, as it tends to lose its two valence electrons to form Be2+ cations.
Boron (B): Boron tends to form stable configurations with six valence electrons, as it often forms compounds in which it gains an additional electron or shares electrons to complete its octet.
Elements from period 3 and beyond: Elements such as phosphorus (P), sulfur (S), and chlorine (Cl) can also have expanded valence shells and form stable configurations with more than eight valence electrons. This is because they have d orbitals available for electron accommodation in addition to their valence p orbitals.
These are just a few examples of elements that don't strictly follow the octet rule. Different elements exhibit different bonding patterns and electron configurations, leading to variations in the number of valence electrons required for stability.