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Valence Electrons

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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 Definition

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Definition of Valence Electrons

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 Electrons

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The number of valence electrons of an atom is important in deriving some of the fundamental periodic variables used in organizing the chemistry of the elements. The elements in a group of the periodic table have the same number of valence electrons in the same shapes of orbitals. In general, the number of valence electrons of an atom equals its group number.

The valence electron periodic table is given below.
Number of Valence Electrons

A group number in the periodic table is used as a guide to indicate the number of valence electrons in each atom.
  1. For elements in Group IA and IIA, only the outermost s electrons are valence electrons.
  2. For elements in Groups IIIA through VIIIA the outermost s and p electrons in the highest energy shell are valence electrons.
  3. 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.
  4. 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.
  5. 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 Electrons

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An oxygen atom has 6 valence electrons. The number of valence electrons in oxygen is 6. Oxygen bonding electrons is 2 and the non-bonding electrons of oxygen is 6.

Oxygen Valence Electrons

Valency of oxygen = (8 - 6) = 2

For example, lets consider the water molecule. In water, the atom connectivity is H-O-H. Each hydrogen has one valence electron and oxygen has six. Thus there are eight valence electrons, or four electrons pairs. The of the electron pairs are used to give single bonds between the oxygen and each hydrogen. The two remaining electron pairs are placed on oxygen as lone pairs of electrons.

Oxygen has two shared pairs and two pairs. Therefore oxygen has access to eight electrons and is stable.

Carbon Valence Electrons

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The number of valence electrons in carbon is 4. It needs four electrons to complete its octet. It shares four electrons with four hydrogen atoms and forms four single covalent bonds. It cannot lose four electrons since very high amount of energy will be required to lose four electrons to form C4+ ion. There is strong force of attraction between nucleus and valence electrons.

Carbon Valence Electrons

Valency of carbon = (8 - 4) = 4

Carbon cannot gain four electrons to form C4- ion because six protons cannot hold 10 electrons easily and there will be strong inter electronic repulsion. Carbon can share four electrons easily with other atoms of carbon and other elements to acquire stable electronic configuration.

Valence Electrons Chart

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The valence electrons which are the outermost electrons of an atom. The identical number of valence electrons primarily accounts for the similar properties of elements in the same group.

The valence electrons for some of the atoms are tabulated below.

S.No
Element
Valence electrons
1 Nitrogen Valence Electrons
5
2
Sulfur Valence Electrons
6
3
Chlorine Valence Electrons
7
4
Hydrogen Valence Electrons
1
5
Boron Valence Electrons
3
6
Phosphorus Valence Electrons
5
7
Sodium Valence Electrons
1
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 Electrons

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Valence electrons are the electrons located within the outermost energy level of an atom. These electrons participate in chemical reactions.

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.
  1. Group 1 (I) (alkaline metals) - 1
  2. Group 2 (II) (alkaline earth metals) - 2
  3. Group 3 - 12 (transition metals) - 1 or 2
  4. Group 13  (III) (boron group) - 3
  5. Group 14 (IV) (carbon group) - 4
  6. Group 15 (V) (nitrogen group) - 5
  7. Group 16 (VI) (chalcogens) - 6
  8. Group 17 (VII) (halogens) - 7
  9. Group 18 (VIII or 0) (noble gases) - 8 [except for helium which has only two valence electrons)

Valence Electrons of Transition Metals

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The transition elements are all metals, with high melting and boiling points and good thermal and electrical conductivities. The valence electrons of the transition metal atoms are held rather loosely by the nuclei and the atoms have no great affinity for more electrons.

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.

[5 $\times$ nd, (n+1)s and 3 $\times$ (n+1+p)]

which can be used either for metal-ligand bonding or for the accommodation of non bonding electrons.

Counting Valence Electrons

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A group number in the periodic table is used as a guide to indicate the number of valence electrons in each atom.
  1. The total of the A group numbers equals the total number of valence electrons of the atoms in a neutral molecule.
  2. For a negative ion, add electrons equal to the ions charge.
  3. 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 Electrons

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The importance of valence electrons are listed below.
  1. The valence electrons are most important in chemical bonding.
  2. Valence electrons are transferred from one atom to another to form ionic bonds or shared between atoms to form covalent bonds.
  3. 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 Electrons

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The outer shell that contains electrons is called the valence shell and it can have a maximum of 8 electrons. Electrons in the valence shell are called valence electrons. Some of the examples of valence electrons of different atoms are given below.
  1. Hydrogen - This is the simplest element with only 1 electron in th k shell. This electron is the valence electron.
  2. 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.
  3. 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.