Valence Electrons

The number of valence electrons determines the reactivity of the atom. They also determine the atom’s electronegativity, electron affinity, and ionization energy, which lead to things such as covalent and ionic bonds.

Atoms are most stable if they possess a filled valence shell, with each shell having 8 valence electrons. An exception to this phenomenon is hydrogen and helium, which become stable with 2 electrons in their outermost shell.

• They exist only in the outermost electron shell. However, they can exist in the inner shell of a transition metal.
• Atoms with valence shells filled with electrons (octet or duplet) are chemically inert.
• They absorb or release energy as small packets of energy (photons).
• They categorize elements into metal, non-metal, or metalloid based on the electrical conductivity of an element.

There are two ways of calculating the number of valence electrons in an element.

1. By Using Periodic Table

It is the most widely used method to determine the number of valence electrons in an element. Here, we just refer to the periodic table and search for the position of the element in it. As we proceed downwards in a group, the numbers of valence electrons are same, although the number of shells increases. In contrast, the number of valence electrons across a period increases by one as we move left to right of a period.

Exception: As discussed, the period number indicates the number of shells, whereas the group number specifies the valence electron number in the outermost shell of an atom. However, this only holds for the main group elements, groups 1-2 and 13-18. The rule applies to the transition and inner transition elements in groups 3-12.

Valence Electrons Periodic Table Chart

The table below depicts the number of valence electrons in the different groups of the periodic table:

Periodic Table Group	Valence Electrons
Group 1 (I) – Alkali metals	1
Group 2 (II) – Alkaline earth metals	2
Group 13 (III) – Boron group	3
Group 14 (IV) – Carbon group	4
Group 15 (V) – Nitrogen group	5
Group 16 (VI) – Oxygen group	6
Group 17 (VII) – Halogens	7
Group 18 (VIII or 0) – Noble gases	8

2. By Using Electronic Configuration of the Element

Within the shells, electrons occupy a special place called atomic orbitals. The arrangement of electrons in such orbitals is known as electronic configuration. The configuration of electrons gives a quick overview of the number of electrons present in the last shell. So, just by writing the electronic configuration of that element, we can quickly determine its number of valence electrons.

Here is a table representing the number of valence electrons of elements belonging to the second period and their electronic configuration.

Name of the element	Symbol	Atomic Number	Electron Configuration	No. of valence electrons
Lithium	Li	3	1s22s1	1
Beryllium	Be	4	1s22s2	2
Boron	B	5	1s22s22p1	3
Carbon	C	6	1s22s22p	4
Nitrogen	N	7	1s22s22p3	5
Oxygen	O	8	1s22s22p4	6
Fluorine	F	9	1s22s22p5	7
Neon	Ne	10	1s22s22p6	8

Lewis structure is an easier representation of the valence shell electrons in a molecule. Here, the atom is represented using a special Lewis symbol.

First, write the element’s chemical symbol, and then show its valence electrons surrounding it using dots. Show only the electrons residing in the valence shell as it solely determines the chemical reactivity of an atom.

For example, the Lewis symbol of oxygen depicts a ‘O’ surrounded by 6 valence electrons because oxygen has an electron configuration of 1s²2s²2p⁴.