The valence electrons of an atom are those electrons that are in its outer energy shell or that are available for bonding. The American chemist Gilbert Newton Lewis (1875-1946) introduced a useful way to represent electrons in the valence shell of an atom. The elements symbol represents the atomic nucleus together with the core electrons.
The valence electron configuration of an atom helps us understand its properties and can be ascertained from the periodic table. When chemists study chemical reactions, they study the transfer or sharing of electrons. The electrons more loosely held by the nucleus - the electrons in the energy level farthest away from the nucleus - are the ones that are gained, lost or shared.
Valence Electrons DefinitionBack to Top
To define valence electrons "the electrons in the last shell (outermost shell) are called valence electrons." These electrons govern the chemical properties of the atoms. Valence electrons are mainly responsible for the emission spectra of the elements.
The electrons called valence electrons are particularly important in determining the bonding characteristics of a given atom. A valence electron is an electron in the outermost electron shell of a representative element or noble gas element. The number of valence electrons in a representative element can be determined by its electronic configuration.
Number of Valence ElectronsBack to Top
The valence electron periodic table is given below.
- For elements in Group IA and IIA, only the outermost s electrons are valence electrons.
- For elements in Groups IIIA through VIIIA the outermost s and p electrons in the highest energy shell are valence electrons.
- For transition elements the valence electrons are those in the outermost s sub-shell and in the d sub-shell of the next to outermost energy shell.
- For inner transition elements the valence electrons are those in the s sub-shell of the outermost energy shell the d sub-shell of the next to outermost energy shell, and the f sub-shell of the energy shell two levels below the outermost shell.
IIIA-VIIA elements beyond period II might under some circumstances accept electrons into their empty d sub-shell which gives them more than 8 valence electrons.
Oxygen Valence ElectronsBack to Top
Oxygen has two shared pairs and two pairs. Therefore oxygen has access to eight electrons and is stable.
Carbon Valence ElectronsBack to Top
Valence Electrons ChartBack to Top
The valence electrons for some of the atoms are tabulated below.
|1||Nitrogen Valence Electrons
||Sulfur Valence Electrons
||Chlorine Valence Electrons
||Hydrogen Valence Electrons
||Boron Valence Electrons
||Phosphorus Valence Electrons
||Sodium Valence Electrons
|8||Aluminum Valence Electrons ||3|
|9||Iron Valence Electrons ||2|
|10||Fluorine Valence Electrons ||7|
|11||Iodine Valence Electrons||7|
|12 ||Bromine Valence Electrons ||7|
|13 ||Copper Valence Electrons ||1|
|14 ||Zinc Valence Electrons ||2|
|15 ||Lithium Valence Electrons ||1|
|16 ||Helium Valence Electrons ||2|
|17 ||Potassium Valence Electrons ||1|
|18 ||Magnesium Valence Electrons ||2|
|19 ||Calcium Valence Electrons ||2|
Determining Valence ElectronsBack to Top
To determine the quantity of valence electrons an element has one must look at the periodic table group in which the element is categorized. With the exception of groups 3-12, the number within the units place identifies how many valence electrons are contained within the elements listed under that particular column.
- Group 1 (I) (alkaline metals) - 1
- Group 2 (II) (alkaline earth metals) - 2
- Group 3 - 12 (transition metals) - 1 or 2
- Group 13 (III) (boron group) - 3
- Group 14 (IV) (carbon group) - 4
- Group 15 (V) (nitrogen group) - 5
- Group 16 (VI) (chalcogens) - 6
- Group 17 (VII) (halogens) - 7
- Group 18 (VIII or 0) (noble gases) - 8 [except for helium which has only two valence electrons)
Valence Electrons of Transition MetalsBack to Top
Bonding between metal atoms will thus be weak and the pure elements may not exist as diatomic molecules. Instead stability is achieved by the sharing of valence electrons between many atoms. The metallic solid exists as a lattice arrangement of positive ions held together by de-localized electrons from the ns and (n-1) d orbitals.
The 18-electron rule states that 18 electrons are required from the metal and its associated ligands too attain a noble gas configuration. It is derived from the fact that transition metals have nine valence atomic orbitals.
which can be used either for metal-ligand bonding or for the accommodation of non bonding electrons.
Counting Valence ElectronsBack to Top
- The total of the A group numbers equals the total number of valence electrons of the atoms in a neutral molecule.
- For a negative ion, add electrons equal to the ions charge.
- For a positive ion subtract the number of electrons equal to the charge.
For example consider PCl5, P coming from Group 15/VA contributes 5 valence electrons. Each Cl (Group 17/VIIA) contributes 7 valence electrons for a total of 26 valence electrons.
Importance of Valence ElectronsBack to Top
- The valence electrons are most important in chemical bonding.
- Valence electrons are transferred from one atom to another to form ionic bonds or shared between atoms to form covalent bonds.
- When valence electrons are transferred or shared, the atoms in the compound form full outer Bohr orbits and therefore gain stable electron configurations. Because a stable electron configuration usually involves eight electrons, this is known as the octet rule.
Examples of Valence ElectronsBack to Top
- Hydrogen - This is the simplest element with only 1 electron in th k shell. This electron is the valence electron.
- Lithium - There are 2 electrons in the K shell and 1 electron in the L (valence) shell. The electron configuration is thus K2, L1 or more simply 2, 1.
- Helium - This element has 2 electrons, both of which occupy the K shell. The shell is now full and helium is very stable, unreactive element.