Valence Electron
A valence electron is an electron in the outermost energy level of an atom; valence electrons decide how an element bonds and reacts.
Definition
Valence electrons are the electrons on the outside edge of an atom, in its highest occupied energy level, and because they are the ones that touch other atoms they decide how an element bonds. For main-group elements the count follows the periodic table: groups $1$ and $2$ have $1$ and $2$, and groups $13$ through $18$ have $3$ through $8$. Atoms bond by transferring or sharing valence electrons to reach a stable octet, an $ns^2 np^6$ configuration. Valence electrons occupy the outermost $s$ and $p$ subshells (plus partially filled $d$ subshells in transition metals), have the lowest ionization energies, are tracked by Lewis dot structures, and determine oxidation states, bonding capacity, and molecular geometry through VSEPR theory.
Example
Sodium has $1$ valence electron and chlorine has $7$; sodium gives its one to chlorine and the two stick together as table salt. Oxygen, in group $16$, has $6$ and needs $2$ more, so it shares with two hydrogens to form $H_2O$, while neon in group $18$ has a full $8$ and forms no bonds. Chlorine, $[Ne]3s^2 3p^5$, has a first ionization energy of $1251\text{ kJ/mol}$ and gains an electron instead of losing one, whereas sodium, $[Ne]3s^1$, gives up its electron at only $496\text{ kJ/mol}$.
Key Insight
Atoms are most stable with a full outer shell, usually $8$ valence electrons, and most bonding is atoms working toward that number. Elements in the same column react alike because they share a valence count, which is why lithium, sodium, and potassium all react with water. Core electrons shield the valence electrons from the nucleus, so effective nuclear charge, not total charge, governs how tightly they are held.