What are Chemical Bonds?
The attractive force which has the ability of holding various constituent elements like atoms, ions, molecules, etc. together in different chemical species is termed as a chemical bond. Chemical compounds are dependent on the strength of chemical bonds between its constituents. Stronger the chemical bond, more will be the stability in the chemical compounds. Hence, it can be said that bonding defines the stability of chemical compounds.
Important terms of Chemical Bonding
Different elements undergo different chemical reactions in order to complete their octet to attain the noble gas configuration.
Electrons which are present in the outermost shell undergo chemical bonding to attain stability.
Ions having a positive charge on them.
Ions having a negative charge on them.
Types of Bonds
Any substance undergoes chemical bonding to attain stability; this stability depends on the type of chemical bond they form. Different types of chemical bonds show different types of strength and property. Mainly there are three types of chemical bonds which are formed to attain stability and yield compounds. They are:
- Ionic bonds
- Covalent bonds
- Hydrogen bonds
- The term ionic means the electrical pull between positive and negative ions. A type of chemical bond formation which occurs because of the transfer of electrons from one atom or molecules or ions to another is called an ionic bond. During the formation of this bond, an atom loses an electron which is in turn, gained by another atom. This losing and gaining process leads to the formation of ionic species. During this exchange of an electron, one atom which develops a negative charge is called an anion whereas the one that develops a positive charge is called a cation.
- The strength which is gained by this bond is because of the difference in charge between two atoms. The more the disparity between the cation and the anion, the stronger will be the bond.
Favourable conditions for a stable ionic bond:
- Low ionization energy of atoms forming cations: Ionization energy is the amount of energy required to remove electrons from the outermost shell of an isolated gaseous atom to convert it into a cation. Hence ionization energy or ionization enthalpy is in inverse relation with formation of bond i.e. lesser the ionization energy, stronger will be the bond or vice-versa.
- High electron affinity of atoms to form an anion: Electron affinity is the energy which is released when an isolated gaseous atom takes up an electron to form anion. Hence electron affinity is in direct relation with ionic bonding. More the electron affinity, more favourable will be the formation of such bonds.
- High lattice energy in the crystals which are formed: As lattice energy is the amount of energy required to separate 1 mole of ionic compound into separate oppositely charged ions. More of this energy releases, more strong is the bond.
Characteristic features of ionic compounds
- Ionic compounds have high melting and boiling point.
- Ionic compounds are hard and brittle in nature.
- Ionic compounds are good insulators.
- Ionic compounds have higher enthalpies of fusion and vaporization than molecular compounds.
- Ionic bonds conduct electricity easily when they are dissolved in water.
Bond which is formed by sharing of electrons between atoms is called a covalent bond. This bond is formed by those compounds which contain carbon atoms. The shared pair of electrons by two atoms extends around the nuclei of atoms, which further form a molecule.
Conditions favourable for a stable covalent bonding
- For covalent bond formation each of the two atoms should have 5, 6 or 7 electrons in their outermost shell.
- In covalent bonds, both atoms attain the stability of octet by sharing 3, 2 or 1 electron pair respectively.
- For the formation of covalent bonds, proper overlapping of atoms should be there.
Characteristic features of covalent compounds
- Covalent compounds have low melting and boiling point.
- Covalent compounds are insoluble in water but soluble in organic compounds like benzene.
- Covalent compounds do not conduct electricity.
- Covalent compounds are formed from neutral molecules.
Types of covalent bonds
There are two types of covalent bond based on bonding pattern they have:
- Polar covalent bond
Such bonds are formed when there is an unequal attraction for the shared electron pairs between two atoms which are undergoing bond formation. In such a bond two atoms of different electronegative values combine.
- Non- polar covalent bond
Such bonds are formed when there is equal attraction for the shared electrons between two atoms. In this two atoms of the same electronegativity combine together.
Example: bond between two Cl to form Cl2 molecule, bond between two hydrogen atoms to form H2 molecule.
- Hydrogen bonds are considered weaker bonds than ionic and covalent bonds.
- For instance, in water, hydrogen forms a type of polar covalent bond because hydrogen and oxygen have different values of electronegativity and combine with each other for bond formation. During this bond formation, hydrogen develops a partial positive charge. Electrons are pulled closer to the more electronegative oxygen atom. As a result, hydrogen atoms can be attracted by the negative charges of any neighbouring atoms. Such type of chemical bonding is called hydrogen bonding.
Conditions favorable for a stable hydrogen bond
- Hydrogen atom should get attached to a highly electronegative atom, i.e. N, O, and F.
- Atoms with high electronegativity should have lone pairs of electrons.
Context and Applications
This topic is useful for Bachelors and Masters in Chemistry.
1. Explain coordinate bonding?
Ans. Bond which is formed by sharing an electron pair from a single atom is called co-ordinate bond. In this bond, both shared electrons are donated by the same atom. These bonds are formed when there is a reaction between two non-metals like hydrogen atoms or during bond formation between metals, ions and ligands.
2. Explain the term bond length along with factors affecting size of bond length?
Ans. The distance between the centres of two nuclei of the bonded atom is called bond length.
Factors which affect bond length are:
- Size of the atom.
- Hybridization of the atom
- The multiplicity of the bond.
3. What is bond order in terms of bonding?
Ans. The number of bonds between two atoms in a molecule is termed as its bond order.
Bond order of H2 molecules i.e. between two atoms of Hydrogen (H—H) is 1.
Bond order of 02 molecule (O=O) is 2.
4. What is bond enthalpy along with the factors affecting it?
Ans. During the formation of bond, energy is released in a similar way as during the dissociation of bond, energy is required. So, the amount of energy required to break one mole of a compound is termed as bond dissociation enthalpy or bond enthalpy.
Factors affecting bond enthalpy are:
- Size of atoms.
- The multiplicity of the bond.
- Bond angle.
- Bond order.
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