Formation of Compounds - Part I

Compounds are formed from atoms reacting together. Now the question that might arise in your mind is that what is the underlying mechanism that holds the atoms together? There are millions of compounds that are formed from combination of different types of elements. For example Hydrogen (H), Nitrogen (N), Oxygen (O) are basic elements, but in nature they are found to have a stable chemical formula H₂, N₂, and O₂. These are the stable molecules of the Hydrogen, Nitrogen and Oxygen gases. Sodium (Na) is a metal, Chlorine (Cl) is a non-metal, and salt, that is NaCl is a compound. 

There is a definite way in which atoms of one element interacts with other atoms. You will not have Sodium (Na) and Potassium (K) interacting with each other!! To understand how compounds are formed, we have to first know the types of bonds that can be forged between atoms. Electronic configuration and valence of the atoms play a crucial role in the types of bonds. 

What we will study in this chapter : 

1. Ionic Bonds
2. Covalent Bonds
3. Other Bonds 

1. Ionic Bonds
Let us see how NaCl is formed. The electronic configuration of Na has 2 electrons in the first orbit (K shell), 8 electrons in the second orbit (L shell) and only one electron in the third (M shell). The last is loosely bound and an Na atom willingly wants to give up its last electron.

      Na                                  Na ⁺                 +         e^-         
     (neutral sodium atom)                 (positive sodium ion)            (electron)         

The electronic configuration of Cl has 2 electrons in the K shell, 8 electrons in the L shell and 7 electrons in the M shell. The Cl atom is short of one electron for completely filling the last orbit. It has the habit of picking up one electron from any one who is willing to donate one!!   

       Cl                          +                 e^-                         Cl^-                                         
       (neutral chlorine atom)                       (electron)                 (negative chlorine ion)

When a sodium and a chlorine atom come close, they complement each other’s needs to loan-borrow electrons and therefore easily bond together. The electron given up by a Na atom is easily accommodated in the last orbit of a Cl atom. The net reaction between Na and Cl is Na⁺ - Cl^- , and the atoms are attracted to each other because of opposite electrical charges. Now you can see why Sodium (Na) and Potassium (K) do not form bonds.     

Atomic bonds where electrons are transferred between the constituent atoms of a compound are known as ionic bonds (or electrovalent bonds). The compound thus formed is called an ionic or electrovalent compound. If you see the electronic configuration of Na⁺ and Cl^- ions, they are completely filled. Therefore they easily separate or dissociate if dissolved in water which is a polar solvent[1]. 

Generally ionic bonds are formed between metallic atoms having extra electron to spare and non-metallic atoms which are electron deficient. Magnesium Oxide (MgO), Potassium Chloride (KCl), Iron Oxide (FeO) are some of the examples of ionic compounds.

Ionic compounds are crystalline in nature. Because of availability of ions, such compounds are good conductors of  heat. Due to their crystalline nature, such compounds also have high melting points. They form electrolytes when molten or in solution. They are non-conductors of electricity when solid - no free ions - all locked together in a crystal. 

The ion that looses an electron is called a positive ion or a cation. The ion that gets an electron is called a negative ion or an anion. In the examples above Na⁺, Mg⁺⁺, K⁺, Fe⁺⁺ are cations; Cl^-, O^-- are anions.  

In chemistry there is a certain way to find out whether a particular atom will become a cation or an anion. The method is known as an electron-dot structure. Table below illustrates a few of these. 

Table showing electron dot structures of some atoms

Element (Symbol)	Atomic Number (Z)	Electronic Configuration   K   L   M   N	Number of Outermost Electrons	Valence	Ion Formed	Electron dot structure (for the atom)
Sodium  (Na)	11	2   8    1	1	1+	Na+   (cation)
Magnesium      (Mg)	12	2   8    2	2	2+	Mg++   (cation)
Aluminum (Al)	13	2   8    3	3	3+	Al+++   (cation)
Fluorine   (F)	9	2   7	7	1-	F-   (anion)
Chlorine   (Cl)	17	2   8    7	7	1-	Cl-   (anion)
Oxygen   (O)	8	2   6	6	2-	O--   (anion)

Using the electron-dot structure the reaction Na⁺  +  Cl^-       NaCl can be written as

The Cl shown in boxed bracket represents the dot structure for a chlorine ion.

[1] In a water molecule H₂O , the atoms are bonded at an angle with the result the electrical charges are not evenly distributed. That is why water is electrically polarized or not symmetrical. Hence water is called a polar molecule. 

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