Valence Electron
Valence electrons are the electrons located in the outermost energy shell (valence shell) of an atom, and they are the primary participants in chemical bonding. The number of valence electrons determines an element's chemical reactivity and the types of bonds it can form. In main-group elements, the number of valence electrons equals the group number (using the 1–18 IUPAC numbering), making them a key predictor of periodic trends.
Valence Electrons for Selected Main-Group Elements
| Element | Symbol | Group | Valence Electrons | Typical Bonds Formed |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Carbon | C | 14 | 4 | 4 |
| Nitrogen | N | 15 | 5 | 3 |
| Oxygen | O | 16 | 6 | 2 |
| Chlorine | Cl | 17 | 7 | 1 |
| Neon | Ne | 18 | 8 | 0 |
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Related Terms
Electron Configuration
Electron configuration is the distribution of electrons of an atom or molecule among its atomic orbitals, described by the principal quantum number (n), sublevel (s, p, d, f), and the number of electrons in each sublevel. Electron configuration determines how an element behaves chemically — its valence electrons (outermost shell) govern reactivity, oxidation states, and bonding capacity. The Aufbau principle, Pauli exclusion principle, and Hund's rule govern how electrons fill orbitals.
Atomic Orbital
An atomic orbital is a mathematical function describing the wave-like behaviour and probable location of an electron in an atom, representing a region of space where there is a high probability (typically 90–95%) of finding the electron. Orbitals are characterised by three quantum numbers (n, l, mₗ) and have distinct shapes: s-orbitals are spherical, p-orbitals are dumbbell-shaped, and d- and f-orbitals have more complex geometries. Atomic orbitals form the basis for understanding electron configurations, chemical bonding, and molecular orbital theory.
Ionization Energy
Ionization energy (IE) is the minimum energy required to remove the most loosely bound electron from a gaseous atom or ion in its ground state, producing a positive ion. The first ionization energy (IE₁) removes the first electron; successive ionization energies increase because each removal leaves behind a more positively charged species that holds remaining electrons more tightly. Ionization energy increases across a period (due to greater effective nuclear charge) and decreases down a group (due to greater atomic radius and electron shielding), making it a key periodic trend.
From Latin "valentia" meaning strength or capacity, used in chemistry since the mid-19th century to describe an atom's combining power. The concept was formalized by Edward Frankland around 1852.