Learning Objective: To study about the electrons and judge its reactivity based on its placement around an atom’s orbital. We will also determine how to find the outermost shell electrons from its group number and electronic configurations.
Skill Level – Intermediate
Prerequisites:
Orbitals
Shells in Chemistry
Electronic configuration
Related –
Definition of core electrons
Chapter: Atom
Author's Note: In the previous section, we learnt how atomic orbitals are occupied by the electrons using Aufbau's principle. We also learnt how to apply Hund's rule when electrons are to be filled in similar energy orbitals, followed by tips on writing the electronic configuration of simple p-block elements.
In this section, we explore valence and core electrons from an organic chemistry perspective, examining how an electron's position in an atom determines its inertness versus reactivity. It is considered that the origin of an atom's chemical reactivity is in its valence electrons. We illustrate this with examples, showing how they dictate reactivity, and also offer a counter perspective to demonstrate that not all valence electrons are inclined towards bond-forming reactions.
Another parameter suggestive of an electron's reactivity is the number of outermost valence shell electrons, which will be covered in the next chapter – Bonding in Atoms.
Core and valence electrons - How to Determine Them
The electrons in an atom are the only ones involved in bonding and other chemical reactions when compared to other subatomic particles.
The electrons, based on their placements in an atom, are divided into two parts: core electrons and valence electrons.
The core electrons are electrons present in the inner shell, closer to the nucleus. Their attractive interactions with the nucleus are stronger; therefore, they are bound tightly to it. This lowers their energy and contributes to stability, making them indifferent to bond-forming chemical reactions.
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. It is only the valence electrons that are responsible for the chemical characteristics of an element.
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.
A shell is the number ‘n’ that is used to identify each energy level. It is also called the principal quantum number. Therefore, the energy level 1 closest to the nucleus is shell 1, energy level 2 farther from the nucleus is shell 2, and so on.
For example, all halogens belong to group 17 of the periodic table, will have 7 valence electrons. Similarly, the elements belonging to the Carbon family will have 4 valence electrons.
Only change within a group is in the shell number as the size of the halogen increases down the group, and new electrons are added in new shells. Therefore, within a group from top to bottom, the valence shell number changes. As an example, for the halogens, 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 1s22s2 2px1 2py1. The 1 and 2 are two different shells of the Carbon atom, whereas the s and p are the subshells. Since the valence electrons are the electrons of the outermost shell, here it is the electrons of the shell numbered as 2. Therefore, the number of valence electrons of Carbon is 4 (2s2 and 2px1 2py1), whereas the 1s2 electrons are the core electrons of the Carbon atom.
Let’s look at another example of an Argon atom that has the atomic number 18. The electronic configuration of Argon is 1s2 2s2 2p6 3s23p6. 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 10 core electrons present in shells 1 and 2.
Bonding Preferences of the Valence Electrons
It is important to note that not all valence electrons participate in the bond formation or other chemical reactions. Even amongst the valence electrons, some electrons choose to refrain from bond-forming reactions, and they are called the non-bonding electrons or the lone pair.
G. N. Lewis proposed an octet rule to indicate an atom’s chemical stability. According to him, an atom is said to be stable if the number of outermost electrons is eight. So, in order to achieve that, an atom loses, accepts, or shares electrons to attain a filled outer shell containing eight electrons.
For instance, two atoms share any one valence electron to form a two-electron covalent bond. In the example below, the Fluorine atom, by sharing, now has a total of 8 valence electrons in its outermost shell required for stability.
The loss or gain of the valence electrons forms ions having stable octets (He core having duplet state, like in the case of Li ion below, is an exception to the octet rule) that electrostatically attract to form ionic bonds, satisfying the octet rule.
Most Lewis structures use valence electrons as dots above the atom’s symbol for structural representation and in chemical reactions.
Therefore, the study of valence electrons forms an integral part of the study of organic chemistry. All chemical transformations are deeply indebted to these reactive electrons.
This is an excerpt from CurlyArrows' Introduction to Organic Chemistry Premium Tutorials, Chapter Atom. Subscribe to learn all about the atom, its components, reactive elements, and plenty of more interesting concepts with examples and illustrations.
Related Reading- Difference between valence and core electrons
Numerical Problem (solved)
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