AI Tutor
Drop a tiny chunk of pure sodium into water, and it bursts into violent flames within seconds. Yet neon, sitting just one proton away in atomic weight, can be trapped in a glowing glass tube for decades without reacting with anything.
This dramatic difference in chemical behavior is not determined by an atom's nucleus, but entirely by the electrons orbiting in its outermost shell.

Valence Electrons

Valence electrons are the electrons located in an atom's outermost occupied energy level (the highest principal quantum number, nn). All inner electrons are called core electrons, which shield the outer electrons from the full positive pull of the nucleus.
How can you instantly find the number of valence electrons for any representative element without drawing every electron shell from scratch?

Reading the Periodic Table

For main-group elements (Groups 1, 2, and 13–18), the group number reveals the exact number of valence electrons. In 1916, American chemist Gilbert N. Lewis introduced dot diagrams to represent these outer electrons after noticing that elements in the same column share identical bonding behaviors.
Why do atoms with eight valence electrons remain completely inert, while atoms with one or seven valence electrons react explosively?

The Octet Rule and Stability

The octet rule states that main-group atoms tend to gain, lose, or share electrons until their outermost ss and pp subshells are completely filled with eight valence electrons (ns2np6ns^2 np^6). A full valence shell represents a state of maximum electrostatic stability and minimum potential energy, exactly matching a noble gas configuration.