ATOMIC BONDS

3.0 ATOMIC BONDS

  Engineering materials are mostly solids and liquids. The properties of the material are dependent on the type of inter atomic bonding. It has been observed that when the atom of the element combines to form molecules, a force of attraction is developed between the atoms, which holds them together. This force of attraction is known as “Atomic Bond”.

Cause of Bonding: -

It is obvious that the atoms of most of the elements form bonds with one another in order to achieve a stable electronic configuration. It has been observed that the atoms can achieve stable configuration in the following three ways.

By loosing one or more electrons to another ion.

By gaining one or more electron fro, another atom.

By sharing one or more election with other atom.

3.1 Classification of atomic bonds: -

Atomic bonds between atoms can be divided into two groups namely, primary or strong bonds and secondary or weak bonds

PRIMARY BONDS: -

A primary bond is an inter atomic bond in which electrostatic force of attraction holds the atoms together. It may be noted that a primary bond is stronger and more stable.

SECONDARY BONDS: -

A secondary bond is an inter molecular bond in which the weak force holds the molecule together. The weak force is also known as ‘VANDER-WAALS’ force it may be noted that a secondary bond is weak and less stable than the Primary bond.

3.1.1. Types of Primary Bonds: -

The Primary Bonds are of three types

-Ionic Bonds

-Covalent Bonds

-Metallic Bonds

IONIC BONDS: - When one or more electrons, from the outer most shell of one atom are transferred to the outer most shell of the other atom, an ionic bond is formed.

We know that the atom, which loses the electron, acquires a positive charge and becomes a positive ion. And the atom which gains the electron acquires a negative charge becomes a negative ion. In this way both the atoms becomes oppositely charged ions and an electrostatic attraction is created due to which the ionic bond is formed. Following are the two conditions required in the formation of ionic bonds.

• The atom, which loses electron, should need a small amount of energy to remove the electron form its outermost energy level.
• The atom, which gains electron, should be able to readily accept an electron in its outermost energy level (affinity).

The formation of the ionic bond is illustrated by the formation of sodium chloride.
- Atomic number of sodium is 11 and its electronic configuration is 1s2   2s2   2p6  3s1

- Atomic number of chlorine is 17 and its electronic configuration is 1s2  2s2   2p6  3s2   3p5

- The atomic structures of both the atoms are shown in figure 3.1

-Sodium has one electron in outermost energy level and chlorine has seven electron in it’s in its outermost energy level.
-Sodium loses the electron and the chlorine accepts the lost electron of sodium thereby creating an electrostatic force of attraction between each other to form a new solid called sodium chloride.
Properties of Ionic Solids: -
We know that the ionic solids are oppositely charged and due to electrostatic force of attraction they are bounded to each other. Some of the important properties of ionic bonds are: -
Rigid, Unidirectional, and crystalline: - They are rigid, Unidirectional, and crystalline in nature. Since they are held together by electrostatic force of attraction and form a regular crystal.
Bad conducting capacity:- Since atoms are tightly bound together, there is no possibility of movement of ions. So they are bad conductors of electricity.
Boiling and Melting Temperature: - As a considerable energy is required to break the bonds between them, it has high melting and boiling points.
Covalent Bond: -
Covalent bond is also known as Homopolar bond. The electronic structure of an atom is relatively stable if it has eight electrons by sharing electrons in its outer valence shell. Sometimes an atom may acquire these eight electrons by sharing two electrons with the adjacent similar atom. Thus, when electrons are shared between atoms, it gives rise to “Covalent Bond”
The formation of a covalent bond is illustrated by the following example:
If we look into the chlorine atom the outer shells of each atom have seven electrons. Each chlorine atom would to try to gain one electron, and thus form a stable octet. This can be done by sharing of two electrons between pair of chlorine atoms thereby producing stable diatomic molecules. In other words, each atom contributes one electron for the sharing process. Figure 3.2 & 3.3

Ionic Bond

Covalent Bond

METALIC BOND: -

In the metal atoms, the electrons in their outer most energy levels are loosely held by their nucleii. Due to which a very less amount of energy is required to detach themselves from their nucleii. At room temperature all the metal atoms lose their electron from their outermost energy levels, which form an electron cloud or common pool of electrons. These atoms leaving outermost electron level, acquires positive charge and becomes positive ion, surrounded by a large number of free electrons, forming electron cloud or electron gas as shown in Figure 3.4. The electrostatic force of attraction between the electron cloud and the positive ions forms a bond called as “Metallic Bond”.
Properties of Metallic Bonds: -
We know that free electrons are bounded to different atoms at different times and that too for an extremely small period. It is due to this reason that a metallic bond is weaker than the ionic and covalent bonds. Some of the important properties of metallic solids from the subject point of view are:
Electrical & Thermal conductivity: - Due to the presence of free electrons and variable electrostatic force of attraction, ions move freely through large distance when electric field is applied, thereby having more electrical and thermal conductivity.
Malleability & Ductility: - As the metallic bond is weaker and the planes of atom can slip over each other while a little force and moreover the surrounding of each ion remain same even after slip due to the availability of electron cloud. Thus the metallic solids are malleable and ductile.

Melting & Boiling temperature:- We know that the electrostatic force of attraction between the positive ions and electrons is less than ionic and covalent bonds. Therefore less energy is required to melt and boil them.

Types of Secondary Bonds

                 We know primary bond results from either a transfer of electrons or from sharing of electrons, but secondary bond do not generally involve the valence electron at all. In these bonds comparatively weak inter molecular bonds formed as a result of dipole attraction, which is formed as a result of the unbalanced distribution of electrons in an asymmetrical molecule. These bonds may be permanent or temporary and these bonds are called as Van-Der Waals Bonds.

          Three types of intermolecular or secondary bonds are

• Dispersion bonds
• Dipole bonds
• Hydrogen bonds

DISPERSION BONDS: -

                    Bonds of these are made possible largely because of the repulsion action of adjacent electron. But in a symmetrical molecule, the center of positive charge and negative charge coincides with each other as shown in fig (a). But it has been observed that at certain times the distribution of electron in the molecule is not symmetrical around the nuclei. This results in the displacement of the center of positive and negative charge as shown in  fig (b). The electronic imbalance is known as ‘Polarization’. This polarization is of the fluctuating nature and is known as dispersion effect due to which there exists a weak force of attraction between two molecules of the same element and a bond is formed between them, which is known as Dispersion Bond.

DIPOLE BONDS: -
                      We have seen that a Covalent bond is formed when two or more electrons of an atom share the outermost electron of an atom. But some times there is an unequal sharing of electrons in those atoms, which has a high affinity to attract an electron than the other. The effect of this unequal sharing is to create opposite charge on the parent atom. As result of this, permanent dipoles are produced.  It has also been observed that such dipoles attract each other and a bond is formed known as Dipole Bond.
The formation of Dipole bond in case of hydrogen fluoride molecule is illustrated in fig 3.6
            We know that atomic number of hydrogen is one and atomic number of fluorine

is nine and their electronic configuration are 1s1 and 1s2 2s2 2p5 respectively. Number of electrons in the outermost shell of hydrogen and fluorine are one and seven. To have more stable configuration fluorine needs one more electrons and also hydrogen needs one electron. Therefore both the hydrogen and fluorine shares electrons and forms a covalent bond. But the fluorine has more affinity than hydrogen. Thus the shared electron pair shifts towards the fluorine atom, which produces a dipole.

 HYDROGEN BOND: -  

            The formation of hydrogen bond is illustrated with the water molecule.

          We know that atomic number of hydrogen is one and atomic number of oxygen is eight.  The electronic configuration is 1s1 & 1s2 2s2 2p4 responsively. We see that there is only one electron outermost energy level of hydrogen and six in oxygen. Therefore the oxygen and hydrogen will share their electrons to acquire stable configuration. This leads to the formation of covalent bond and molecules so formed are known as water molecules. But oxygen atom has higher affinity than the hydrogen atom that tends to shift the hydrogen atom towards the oxygen atom by forming a dipole. Thus hydrogen bonds are formed in water. Refer Fig 3.7

3.2 COMPARISION BETWEEN IONIC, COVALENT AND METLIC BOND: -
Comparison between Ionic, Covalent and metallic bonds:
The following table gives the comparison between, ionic, covalent, and metallic bond: -

S.NO	Ionic Bond	Covalent Bond	Metallic Bond
1.	It exists due to electrostatic force of attraction between Positive and negative ions of different elements.	It exists due to the electrostatic force of attraction between Atoms, which shared electrons pairs to form a covalent bond.	It exists due to the electrostatic force of attraction between Electron cloud and positive ions of same or Different metals.
2.	It is formed between two different elements. One of the atoms looses its valence electron and other accepts it. The ions so formed attract each other to from an ionic bond.	It is formed due to the sharing of electron pairs between the atoms of same or different elements.	It is formed when the valence electrons detach themselves from their parent electrons and form common pool. The force, which binds the electron cloud and the positive ions of the metal, forms the metallic bond.
3.	The ionic solids have very low electrical and thermal conductivity.	The covalent solids have low electrical and thermal conductivity.	The metallic solids have high electrical and thermal conductivity.
4.	The ionic solids have very high hardness due to their crystalline structure.	The covalent bond have low hardness except diamond, silicon, carbide etc,	The metallic solids have soft crystalline structure.
5.	The ionic solids are not malleable and ductile.	Covalent solids are also not malleable and ductile.	The metallic solids are malleable and ductile.
6.	The ionic solids have high melting and boiling temperature.	The covalent solids have lower melting and boiling temperature than the ionic solids.	The metallic solids have slightly lower melting and boiling temperature than the ionic solids.
7.	The ionic compounds exist in the form of solids only.	The covalent compound exists in the form of solids, liquids and gases.	The metallic compound also exists in the form of solids only.
8.	The ionic solids are soluble in water	The covalent solids are soluble in benzene and toluene etc,	The metallic solids are neither soluble in water nor in benzene etc,

Secondary Bonds