Atoms tend to bind with each other to get stability. It results the formation of molecules and the force which holds the bonded atoms in a molecule is called as chemical bond. It can be defined as the attraction between electrons of one atom and nucleus of another atom. The repulsion between electrons or protons of both atoms must be balanced with this attraction force. Then only a stable molecule will form.
There are different theories which make us understand the concept of chemical bonds in molecules. For example, octet concept, Lewis dot structure, valence bond theory, molecular orbital theory and quantum mechanics of chemical bonding help us to understand the chemical bonding in the molecules.
There are different types of chemical bonds like ionic bond, covalent bond, metallic bond, Van der Wall forces etc. The nature of chemical bond depends on the chemical nature of bonded atoms such as non-metals mainly form covalent bonds whereas metals involve in metallic bonding. The combination of metals and non-metals results the formation of ionic compounds in which ions are bonded with electrostatic force of attraction.
Two identical non-metals are bonded with covalent bond like hydrogen molecule (H2), nitrogen molecule (N2) and oxygen molecule (O2). They are also called as homo-atomic molecules. Ionic compounds have systematic arrangement of oppositely charged ions like in sodium chloride molecule, sodium ions and chloride ions are arranged in face centered cubic lattice. Metals are bonded with metallic bond which is an attraction force between metal ions and free electrons. Due to presence of free electrons metals are good conductor of heat and electricity. The concept of chemical bonding can be explained with the help of concept of valence which was purposed by E. Frankland in 1852. According to this concept, each element has a definite number of valances therefore they formed compounds with definite amounts. Later F.A. Kekule proposed tetra-valency of carbon atoms. The discoveries of electrons explained the use dots for the representation of valence electrons which further helped to predict the valence and the structure of molecules. Linus Pauling introduced the concepts of resonance, electro-negativity, ionic bond, and covalent bond. N.V. Sidgwick and H.E. Powell developed the valence bond theory and the VSEPR theory to explain the geometry of molecules with and without lone pairs of electrons.
involves in chemical bonding to get stability with the help of octet configuration. Octet configuration can be acquired either by sharing or complete transfer of valence electrons.
Remember, in any of the chemical bonding, only valence electrons which are placed in outermost shell of an atom involve in bonding and all atoms tend to get octet configuration in their valence shell only. A covalent bond can be defined as the chemical bond which is formed by equal sharing of valence electrons between bonded atoms. Such types of chemical bonds are usually formed with non-metals which have 4, 5, 6 or 7 valence electrons. The sharing of valence electrons can be represented with the help of Lewis dot structure. On the basis of number of electron pairs share between bonded atoms, covalent bonds can be three types; single, double and triple covalent bond
. As name suggested, a single covalent bond is formed by sharing of one pair of electrons, double bond contains two pairs and a triple bond contains three pairs of electrons between bonded atoms.
An ionic bond
can be defined as the chemical bond which is formed due to electrostatic force between two opposite charged ions; cation and anion. A cation has positive charge and is formed by lose of electrons whereas an anion carries negative charge and is formed by gain of electron.
These opposite charge ions attract each other and are arranged in a fix pattern which is called as crystal lattice. Sodium chloride is one of the best examples of ionic compounds in which sodium ions ad chloride ions are bonded through ionic bond. Usually ionic bond exists between metal and non-metals as metals have tendency to lose electrons and form cations. Similarly non-metals can accept electrons to form anion.
The combination of metals and non-metals form ionic compounds
in which metal ions are bonded with negatively charged anions formed from non-metals.
For example, in sodium chloride a sodium atom loses its one valence electron to form sodium ion and chlorine atom accepts one electron to form chloride ion to form chloride ion. Due to opposite charges, both ions attract each other and form sodium chloride; NaCl.
Another example is calcium chloride which is formed by calcium ion (Ca2+
) and chloride ion (Cl-
). Calcium is an alkaline earth metal with two electrons in its valence shell therefore it tends to lose these two electrons to get the octet configuration. Since an ionic compound would be neutral therefore one calcium ion will bond with two chloride ions to form calcium chloride.
One of the simplest examples of covalent bond is methane molecule in which center carbon atom is bonded with four hydrogen atoms through four single covalent bonds. Carbon atom exhibits tetra-valency and can form four covalent bonds to get the octet configuration.
Some other examples of covalent bond are phosphorus trichloride, water and carbon dioxide molecules.
The formation of ionic and covalent bond depends on the type of bonded atoms. Let’s discuss some common differences between ionic and covalent bonds.
|| Covalent Bonds
|| Ionic Bonds
|| They are non-polar chemical bonds as usually formed between elements with least difference in electro-negativity.
|| They are polar chemical bonds as formed by charged ions.
|| Form by equal sharing of valence electrons between bonded atoms.
|| Form by interaction between oppositely charged ions.
| Crystal lattice
|| They do not form crystal lattice.
|| They form crystal lattice.
| Bonded atoms
|| Such chemical bonds are formed between non-metals
|| They are usually formed between metals and non-metals
| Melting point
|| They have low melting points
|| They have high melting points
|| Water, Carbon dioxide
|| Calcium chloride, Sodium hydroxide
| Boiling point
|| They have low boiling point
|| They have high boiling point
| Physical state
|| Covalent compounds can exist in solid, liquid or gaseous states
|| They are usually formed ionic solids.|
Compare and Contrast Ionic and Covalent Bonds
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is an electrostatic force of attraction between cation and anions due to their opposite charges. Therefore the elements which can easily form cation like metals involve in ionic bonding.
Similarly non-metals can easily accept electrons to form anion and form ionic bond with metals. Ions of ionic compounds are arranged in fix pattern to form a crystal lattice like in sodium chloride crystal, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions.
On the contrary, covalent bonds
are formed by sharing of valence electrons between bonded atoms. Such type of chemical bonds is non-polar and usually forms between two same or different non-metals. For example; two chlorine atoms share one valence electron from each atom and form a single covalent bond to form chlorine molecule. Similarly in water molecule, center oxygen atom forms two covalent bonds, one with each hydrogen atom.