Valence
Definition and meaning of Valence in chemistry.
Valence is the combining capacity of an element, meaning the number of bonds one of its atoms typically forms with other atoms. It is usually determined by the number of electrons in the atom's outermost shell, called valence electrons.
In more detail
Atoms bond with each other to reach a more stable electron arrangement, often one where the outermost shell holds eight electrons, a pattern known as the octet rule. To reach this stable arrangement, atoms gain electrons, lose electrons, or share electrons with other atoms, and the number of electrons involved sets the atom's valence.
Elements in the same group of the periodic table usually share similar valences, because atoms in the same group have the same number of valence electrons. For example, every element in group 1 has one valence electron and typically forms one bond or loses one electron.
Transition metals behave differently. Many can show more than one valence, since their outer d-electrons can participate in bonding in varying numbers depending on reaction conditions and which compound is forming. Iron, for instance, commonly forms compounds where it acts as though it has a valence of two or three.
Oxygen typically shows a valence of two, forming compounds like water, while nitrogen typically shows a valence of three, forming compounds like ammonia and many other nitrogen-containing molecules.
Key facts
| Field | General Chemistry |
|---|---|
| Determining factor | Number of valence (outermost shell) electrons |
| Carbon valence | 4 |
| Group 1 valence | 1 (typically forms +1 ions) |
| Group 17 valence | 1 (typically forms -1 ions) |
| Note | Transition metals often show more than one valence |
Carbon has a valence of four, so each carbon atom typically forms four covalent bonds with other atoms. In methane (CH4), one carbon atom bonds to four separate hydrogen atoms, using all four of its valence bonds and completing a stable octet of electrons around the carbon. This same valence of four lets carbon atoms link together into the long chains and rings found throughout organic chemistry.
Frequently asked questions
Is valence the same as oxidation state?
No, valence is the number of bonds an atom forms, while oxidation state is the hypothetical charge an atom would have if all its bonds were completely ionic.
Why do noble gases have essentially no valence?
Noble gases already have a full outer shell of electrons, usually eight, so their atoms have little tendency to gain, lose, or share electrons. This near-zero valence explains why noble gases rarely form chemical bonds.
How is valence related to the group number on the periodic table?
For main-group elements, valence often matches the group number for groups 1 and 2, or eight minus the group number for groups 13 through 17. This pattern comes directly from each group's number of valence electrons.