Answer:
8
Step-by-step explanation:
The octet rule explains reactivity in relation to valence electrons.
Valence Electrons
Firstly, let's define valence electrons. Across the periodic table, elements can have a wide range of total electrons. However, when discussing reactions, valence electrons are the most important. Electrons arrange themselves according to energy levels. Any electrons that are on the outermost energy level are considered valence electrons. Most valence energy levels, also called valence shells, hold 8 electrons.
Octet Rule
Now that we know what valence electrons are we can use them to determine reactivity. It is important to note that atoms are most stable when their valence shell is full (as supported by Hund's rule). The octet rule explains atoms will always attempt to fill their valence shell in order to become stable. If an atom has only a few valence electrons, then it will attempt to lose its electrons. On the other hand, atoms that almost have a full valence shell will attempt to gain new electrons.
The losing or gaining of electrons occurs during reactions. So, if an atom already has 8 valence electrons then it will not react. It is already stable, so it has no need to lose or gain more electrons.
Exceptions
While the octet rule explains a lot, like all rules in chemistry it has exceptions. The most common exceptions are hydrogen and helium. Due to the size of these atoms their valence shell can only hold 2 electrons. So, these atoms are actually unreactive at 2 valence electrons.
Another exception can be seen with sulfur and xenon. Both of these atoms are hypervalent. This means they can react even if they have 8 valence electrons. However, this exception is less common.