The question of how many unpaired electrons chlorine possesses requires an understanding of how electrons are structured within an atom. Chlorine (Cl) is a halogen, a reactive nonmetal with an atomic number of 17. The precise arrangement of these 17 electrons determines the atom’s chemical behavior and provides a definitive answer to the count of its unpaired electrons.
Chlorine’s Place in the Periodic Table
A neutral chlorine atom has an atomic number of 17, meaning it contains 17 protons and 17 electrons. Located in Group 17 of the periodic table, chlorine is classified as a halogen. It is also in the third period, indicating its electrons occupy three primary energy levels, or shells. The outermost energy level, known as the valence shell, holds seven electrons, which are responsible for chemical bonding.
Mapping Electron Shells and Orbitals
The 17 electrons are organized into distinct energy shells and subshells, which are regions of space called orbitals. Each shell contains specific types of subshells: \(s\), \(p\), \(d\), and so on. The spherical \(s\) subshell holds one orbital (two electrons), while the \(p\) subshell contains three orbitals (six electrons). Electrons fill these orbitals following specific rules, including Hund’s rule. Hund’s rule dictates that electrons must occupy separate orbitals within a subshell singly before they begin to pair up, which is fundamental to determining the number of unpaired electrons.
The first two shells of chlorine are completely filled, accounting for 10 electrons (\(1s^2\) and \(2s^2 2p^6\)). This leaves seven electrons for the third and outermost valence shell (\(n=3\)). Within the valence shell, the electrons are distributed as \(3s^2 3p^5\). The \(3s\) orbital is completely filled with two paired electrons, meaning the remaining five electrons must be placed into the three available \(3p\) orbitals.
Identifying Chlorine’s Unpaired Electrons
To determine the count of unpaired electrons, the five electrons in the \(3p\) subshell must be distributed across the three \(p\) orbitals. Following Hund’s rule, one electron is placed into each of the three orbitals first. This accounts for three electrons. The remaining two electrons are then paired with the first two electrons in two of the \(p\) orbitals, following the Pauli exclusion principle.
This distribution results in a \(3p\) subshell where two of the three orbitals are completely filled (paired electrons), and the third orbital contains only one electron. The full electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^5\). Since four electrons are paired and one remains single, a neutral chlorine atom in its ground state has only one unpaired electron.
How Unpaired Electrons Drive Chemical Reactivity
The presence of a single unpaired electron explains chlorine’s chemical reactivity. Atoms seek stability by achieving a full outer valence shell, a condition often referred to as the octet rule, which means having eight valence electrons. Since chlorine has seven valence electrons, it is just one electron short of this stable configuration.
The single unpaired electron represents an open slot in chlorine’s valence shell that the atom is motivated to fill. Chlorine will readily accept an electron from another atom to form a stable ion, or it will share its unpaired electron to form a single covalent bond. This drive to acquire one additional electron classifies chlorine as a strong oxidizing agent. This behavior allows it to form compounds like sodium chloride (NaCl) and hydrogen chloride (HCl), completing its electron octet.