Valence electrons are the outermost, farthest from the nucleus, and the nuclear attraction. Consequently, the valence electrons are loosely held; therefore, they participate in chemical reactions by being gained, lost, or shared.
For instance, two atoms share any one valence electron to form a two-electron covalent bond.
The loss or gain of the valence electrons forms ions that electrostatically attract to form ionic bonds.
Most Lewis structures use valence electrons as dots above the atom’s symbol for structural representation and in chemical reactions.
However, not all valence electrons participate in bond formation or other chemical reactions. Some remain uninvolved as lone pair(s) or nonbonding electron(s).
The core electrons are those present closer to the nucleus. Their attractive interactions with the nucleus are stronger; therefore, they are bound tightly to the nucleus. This lowers their energy and contributes to stability, making them indifferent to bonding reactions.
Determining the valence electrons
1) Group number
The group number (column number) represents the number of valence electrons common to all the elements within that group.
Table A
Periodic table block | Periodic table group | Valence electrons |
s- block | Group 1 | 1 |
Group 2 | 2 | |
p-block | Group 13 (Boron family) | 3 |
Group 14 (Carbon Family) | 4 | |
Group 15 (Nitrogen family) | 5 | |
Group 16 (Oxygen family) | 6 | |
Group 17 (Halogen family) | 7 | |
Group 18 (Noble gases) | 8 | |
d-block | Group 3-12 (Transition metals) | 3-12 |
f-block | Lanthanides and actinides | 3-16 |
All elements belonging to the same group in the periodic table will have the same number of valence electrons (table A). The only difference will be their shell number due to the increase in atomic size.
For example, all halogens belong to group 17 of the periodic table. However, the size of the halogen increases down the group, and new electrons are added in new shells. Therefore, the valence electron shell of Fluorine is 2 (2s2 2p5), whereas for Chlorine, it is 3 (3s2 3p5), and Bromine is 4 (4s2 4p5).
2) Atomic Number
The atomic number represents the number of protons or electrons in an atom.
The electrons are distributed in different energy levels, and their representation is known as the electronic configuration of the element. The outermost shell containing the valence electrons is determined from the electronic configuration of the element.
A Carbon atom has an atomic number of 6. The electronic configuration of carbon is 1s2 2s2 2px1 2py1.
The 1 and 2 are two different shells of the Carbon atom. Since the valence electrons are the outermost shell electrons, here it is the electrons of shell 2.
Therefore, the number of valence electrons of Carbon is 4 (2s2 and 2px1 2py1). The 1s2 electrons are the core electrons of the Carbon atom.
An Argon atom has the atomic number 18. The electronic configuration of Argon is 1s2 2s2 2p6 3s2 3p6. Argon has electrons distributed among three shells (1, 2, and 3). The outermost shell is 3, and the number of valence electrons is 8 (3s2 3p6). Argon has ten core electrons present in shells 1 and 2.
Numerical Problem (solved)