In most cases, atoms are not capable of having more than eight valence electrons. This is known as the octet rule, which states that atoms tend to gain, lose, or share electrons in order to achieve a stable electron configuration with a full valence shell of eight electrons. This configuration is often referred to as an octet.
However, there are some exceptions to the octet rule. Elements beyond the second period of the periodic table, such as phosphorus, sulfur, and chlorine, can accommodate more than eight valence electrons due to the availability of empty d orbitals in their valence shells. These elements can expand their octet and form compounds in which they have ten or even twelve valence electrons.
For example, phosphorus pentachloride (PCl5) has phosphorus with ten valence electrons—five chlorine atoms bonded to it and one lone pair of electrons. Similarly, sulfur hexafluoride (SF6) has sulfur with twelve valence electrons—six fluorine atoms bonded to it and no lone pairs.
It's important to note that these exceptions are more common for elements in the third period and beyond, and they do not apply to all elements. The octet rule remains a useful guideline for predicting the bonding behavior of most elements.