A chemical bond is the physical phenomenon and interaction of chemical substances that are held together by the attraction of atoms to each other.
This association of atoms helps for the formation of molecules, ions, crystals, etc. via sharing, as well as exchanging, of electrons -or electrostatic forces.
There are different types of chemical bonding including Covalent bonds, ionic bonds, metallic bonds, hydrogen bonds, etc. Covalent and ionic bonds are the main cases of chemical bonds.
It is also called a molecular bond, mutual sharing of one or more pairs of electrons between two atoms. These electron pairs that participating in bonding are known as shared pairs or bonding pairs, and shared electrons located in the space between the two nuclei are called bonding electrons and the stable balance of attractive and repulsive forces between atoms when they share electrons, is known as covalent bonding.
The nature of the interaction between the atoms depends on their relative electronegativity (the ability of an atom to attract the atom or bonding pair). When the difference between the electronegativities of two atoms is too small for an electron transfer to occur to form ions covalent bond is formed. These atoms have very high ionization energy.
The atoms share their electrons to obtain the octet configuration in their valence shell. It normally contains the energy of about ~80 kilocalories per mole (kcal/mol). Covalent bonds rarely break spontaneously after it is formed.
A covalent bond formed between the two non-metals or between two of the same (or similar) elements. The covalent bonds within the molecules are very strong and Covalent interactions are highly directional and depend on the orbital overlap.
Properties of covalent bond
Covalent compound contains the following properties:
- In the physical state, they may exist as solids, liquids, or gases.
- Covalent bond molecules have definite shapes.
- They are not hard usually they are soft and waxy by nature. This is because of the presence of a cloud of electrons between each layer of carbons atoms.
- They are readily soluble in non-polar solvents and insoluble in polar solvents.
- The compounds containing covalent bonds are non-conductors of electrical charge or have very low conductivity because of the absence of charged ions or free electrons. But the graphite is the good conductor as there we see a cloud of electrons.
- They are bad conductors of heat. Their molecules lack free electrons and that obstructs the flow of heat energy.
- They are very low or nonmalleable or non-ductile. Smaller covalent compounds with weak bonds are frequently soft and malleable
- Covalent compounds have low boiling points. This can be attributed to their weak force of attraction between the various bonded atoms.
HCl, H2O, PCl5 etc are example of covalent bonds.
Types of covalent bonds
There are following types of covalent bonds based on electronegativity that plays a vital role in determining the different types of covalent bonding.
- Polar covalent bond
- Non polar covalent bond
Polar covalent bond
It is formed between two nonmetal atoms having different electronegativities and share their electrons (unequal sharing of electrons) in a covalent bond. It is formed between two different atoms.
In it a part of the electron density of the bonding electron pair is closer to one of the bound nuclei, creating partially positive and negative atomic centers with the magnitude of the charge transfer. This depends on the relative electronegativities of the two atoms. Electrons cloud will shift to that atom which has high electronegativity.
These compounds can exist as solids due to the greater force of interactions and have high melting and boiling points. They are soluble in polar compounds such as water.
Example: Example of polar-covalent bonding are:
- Bonds between hydrogen and other elements such as oxygen (H2 O).
- Bond between hydrogen and other atoms such as Cl (HCl)and F (HF)
Non Polar covalent bond
It is formed between two same atoms having same electronegativities and share their equal electrons in a covalent compound. The difference in electronegativity is mostly negligible in non-polar covalent bonds.
They exist as gas form but rarely as liquid form and are very soft in nature. They have low boiling and melting points and soluble in non-polar solvents.
H2, N2, O2, Cl2 etc. are example of non-polar covalent bond
Other types of Covalent Bond
There are different other types of covalent bond based on number of shared electron paired.
- Single covalent bond
- Double covalent bond
- Triple covalent bond
Single covalent bond
In single covalent bond only one pair of electron is shared between two atoms. It is represented by one dash (-). It is weaker bond as compared to double and triple bond. It has less density. It is most stable bond
Example: The bond between hydrogen and hydrogen (H-H) is an example of single covalent bond. Other example of covalent bond is F2, HCl etc.
Double covalent bond
When two pairs of electrons are shared between the two atoms the bond formed is called double bond. It is represented by one dash (=). It is formed by one Pi bond and one sigma bond. It is a strong bond as compared to single but it is a less stable bond.
Example: The bond between two oxygen atoms (O=O) is an example of a double covalent bond. Other examples are CO2, C2H4, acetone, ozone, etc.
Triple covalent bond
In a triple covalent bond, three pairs of electrons are shared between two atoms. It is the least stable than general types of covalent bonds. It is represented by three dashes (≡).
Example: N≡N is an example of triple covalent bond.
An ionic bond is also called an electron covalent bond, formed by the complete transfer of some electrons (from the outermost orbital) from one atom to another. Electron transfer produces negative ions called anions (The atom gaining one or more electron) and positive ions called cations (The atom losing one or more electrons).
These ions attract each other. They are developed by the electrostatic attraction of elements with opposite electric charges. They form between atoms with large differences in electronegativity. In the formation of the ionic bond, by the gaining or losing electrons from the atoms the octet is achieved. In this bond, Ions are arranged in a three-dimensional array, or crystals and they dissociate into ions in solution.
It is a common feature of the inorganic compounds and the salts of organic molecules. Ionic bonds depend on the radius of the atom, the larger the radius, the more likely the compound will have ionic bonding. It is mostly formed between a metal and a nonmetal atom.
Properties of ionic bonds
There are following properties of ionic bond.
- They exist as solid-state.
- ionic bonds are hard because of crystalline nature and also having Haigh melting and boiling points.
- This type of bond has high bond energy than the metallic bond.
- They are non-malleable and non-ductile.
- As compared to other bonds, this is considered not a good conductor of electricity but in the molten state can conduct electricity due to the presence of ions which act as charge carriers.
- Ionic bond dissociated into an ion, as these soluble in water.
- This bond is considered the strongest bond than others and is highly brittle.
Example: Following are some example of ionic bonds
KCl (Potassium Chloride)), CsF (Cesium Fluoride), BeS (Beryllium Sulfide), NaCl (Sodium chloride) etc.
It is the type of chemical bond that holds atoms together among the metals and shares the free electrons among the lattice of cations. As it occurred in metals or alloys so-called metallic bond. It is different from the covalent bond because ionization energy for electrons occupying the outer orbitals of the metallic elements is much smaller.
When the hybridization is absent s-orbital (allowing overlapping with up to 12 further s-orbitals of the surrounding atoms) leads to the formation of ‘metallic’ bond (non-directional) whereas d-orbital leads to the formation of ‘covalent’ bond (directional).
Different factors affect the strength of a metallic bond include, the total number of delocalized electrons, Magnitude of positive charge held by the metal cation, Ionic radius of the cation.
- Examples: Alloy are formed through metallic bonding. Example of alloy are brass (Cu and Zn) and steel (C and Fe) The other examples of metallic bond Iron, Cobalt, calcium and magnesium, silver, gold etc.
Properties of metallic bonds
Following are the properties of metallic bond containing compounds
- Metallic bonds are usually in the solid-state.
- They are usually hard in nature and have no definite shape.
- If we see its solubility in non-polar and polar solvents are insoluble
- Most metals are excellent electrical conductors because the electrons in the electron sea are free to move and carry the charge.
- Metallic bonds are malleable and ductile with high melting and boiling points as well as low volatility.
Main difference between ionic, covalent and metallic bond
|Covalent bond||Ionic bond||Metallic bond|
|Sharing of electrons between two|
|Complete transfer of electrons||Sharing of electrons between metals lattice|
|Present among non-metals||Present among metals and non-metals||Present among the metals|
|Strong bond than metallic bond||Most strong bond||Weak bond than other bond|
|Exist in solid, liquid and gas state||Exit as solid||Exit in solid state|
|Bond is directional||Bond is non directional||Bond is non directional|
|Electronegativity for polar covalent is 0.5-1.7 and for non-polar covalent is ˂ 0.5.||˃ 0.7 is electronegativity||Electronegativity not required.|