Chemical bonding deals with the study of formation of compounds out of elemental forms of atoms. A chemical bond is the result of the attraction between atoms or ions. Chemical compounds are formed by the joining of two or more atoms. The type of bonds that a molecule contains will determine its physical properties, such as melting point, hardness, electrical and thermal conductivity, and solubility. Various forms of bonding may be classified into the following types.
Ionic Bonding: The other name of Ionic bonding is electrovalent bonding; the molecular linkages are formed by the electrostatic attraction among oppositely charged ions. The attraction phenomenon is influenced by the formation of bond between molecules with the help of complete valence electron transfer from one atom to another. The atom which loses electron(s) are positively charged and named as cation and the one that gains electron(s) are named as anion which have negative charge. The best ionic bonding can be seen in the combination of non-metals, alkali and alkaline-earth metals. In molecular formation, like charges gets repulsed and those being oppositely charged, gets attracted in which each positive ion is surrounded by negative ions in attraction forming a net zero balance of ions for the stable ionic compound. The ionic bonds are also referred as extreme form of covalent bond, which happens only during a great difference in electronegativity of the participating atoms.
Covalent and Metallic Bonding: The sharing of electron pair among two atoms results in interatomic linkage termed as covalent bonding where the electrostatic occurs between the atoms having similar electronegativity resulting in binding process. Less the difference in the electronegativities, more is the strength of the covalent bond. A covalent bond is expected to form the bonding with lowest energy.The covalent bonds can also be formed other than in organic compounds. Inorganic substances like water, chlorine, ammonia, nitrogen and hydrogen also exhibit covalent bonding.
The single line represents single bond, two lines for double bonds, three lines for triple bonds which are represented symbolically as . The metallic bonding not being like ionic or covalent bonding, it is formed by the attraction of valence electrons and metal ions. In simple representation, many electrons surrounding the metal ions acts as glue in bond formation termed as metallic bonding. The loosely bound electrons forming a “sea” of floating valence electrons is the characteristic of metallic bonding. This very nature accounts for the highly electrical conductivity to metals.
COVALENT BONDS
Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds leads to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.
Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds. The nitrogen and oxygen which makes up the bulk of the atmosphere also exhibit covalent bonding in forming diatomic molecules.
Examples of covalent bonding
1) Chlorine: Two chlorine atoms could both achieve stable structures by sharing their single unpaired electrons. The two chlorine atoms are said to be joined by a covalent bond. The reason that the two chlorine atoms stick together is that the shared pair of electrons is atttached to the nucleus of both chlorine atoms.
2) Hydrogen: Hydrogen atoms only need two electrons in their outer level to reach the noble gas structure of helium. Once again, the covalent bond holds the two atoms together because the pair of electrons is attracted to both nuclei.
3) Hydrogen Chloride: The hydrogen has a helium structure and the chlorine has an argon structure.
IONIC BONDING (ELECTROVALENT BONDS)
In an ionic bonds, an electron is actually transferred from the less electronegative atoms (metals) to the more electronegative elements (non metals). Ionic bonds are the result of an electrostatic attraction between ions that have opposite charges; in other words, cations and anions. Ionic bonds usually form between metals and non metals. Ionic bonds are very strong, so compounds containing thses types of bonds have high melting and boiling point and exist in a solid state under standard conditions.
Ionic Bonding in Sodium Chloride
Sodium (2,8,1) has 1 electron more than a stable noble gas structure (2,8). If it gave away that electron it would become more stable while chlorine on the other side (2,8,7) has 1 electron short of a stable noble gas structure(2,8,8). If it could gain an electron from somewhere it too would become stable.
The answer is obvious. If a sodium atom gives an electron to a chlorine atom, both become more stable. The sodium has lost an electron, so it no longer has equal numbers of electrons and protons. Because it has one more proton than electron, it has a charge of +1. If electrons are lost from an atom, positive ions are formed. Positive ions are sometimes called cations.
The chlorine has gained an electron, so it now has one more electron than proton. If therefore has a charge of 1- . If electrons are gained by an atom, negative ions are formed. A negative ion is sometimes called an anion. The sodium ions and chloride ions are held together by the strong electrostatic attractions between the positive and negative charges. One sodium atom to provide the extra electron forone chlorine atom, so they combine together 1:1 . The formula is therefore NaCl.
Some other examples of ionic bonding
1) magnesium oxide: Again, noble gas structures are formed, and the magnesium oxide is held together by very strong attractions between the ions. The ionic bonding is stronger than in sodium chloride because this time you have 2+ ions attracting 2- ions. The greater the charge, the greater the attraction.
Mg 2,8,2 Mg²⁺ 2,8
O 2,6 -----> O²⁻ 2,8
The formula of magnesium oxide is MgO
2) Calcium chloride:
Cl 2,8,7 Cl⁻ 2,8,8
Ca 2,8,8,2 ----> Ca²⁺ 2,8,8
This time you need two chlorines to use up the two outer electrons in the calcium. The formula is therefore CaCl₂.
3) Potassium oxide:
K 2,8,8,1. K⁺. 2,8,8
O. 2,6. O²⁻ 2,8
Again, noble gas structure are formed. It takes two potassiums to supply the electrons the oxygen needs. The formula of potassium oxide is K₂O.
The atoms in solid state like ionic bonded molecules arrange in patterns of order termed as lattice which is the space of minimum energy stored. The ionic lattice forms in ionic bonding while in covalent bonds a bigger lattice structure is formed and molecular lattice is formed in intermolecular bonding. In general, there are fourteen types of lattice commonly named as Bravais lattice and the types of lattice formed in atoms and molecules are:
1) Cubic crystal with at all axes
2) Hexagonal with four crystallographic axes forming at three horizontal axes
3) Tetragonal with one perpendicular axes and other two in.
Key Points:
Ionic bonds are formed by complete transfer of valence electrons from one atom to another, between atoms having the greatest difference in their electronegativities.
Covalent bonds are formed by sharing of valence electrons, between atoms having minimal difference in electronegativities.
Metallic bonding occurs between atoms within a metal and has a “sea” of floating electrons on the surface.
In the solid state, the ionic, covalent and metallic bonding, all are arranged in definite geometrical shapes which are known as lattices.
The measurement of the atoms tendency to attract the bonding pair of electrons are termed as electronegativity and the formal charge among the atoms / molecules represents its resonance state. The tool used to measure electronegativity of the atoms / molecules is the Pauling scale where least electronegative atoms are Caesium and Francium with the value of 0.7 and the most electronegative atom is Fluorine with 4.
The polar and non-polar bonding of the molecules are determined using electronegativity values, in polar bonds the one of the molecule is slightly positive and other end of the molecule is slightly positive favouring the dissolution of the molecule where as in non-polar molecules the charge in both the ends are not balanced it exists in uneven distribution results in non-dissolution of the atoms / molecules.
There are three main points about electronegativity
The non-polar covalent bond exists when no electronegativity difference occurs.
Only a small electronegative difference results in the formation of polar covalent bonds.
The ionic bond results from the large electronegative difference.
The existence of molecular and ionic shapes occurs by the repulsive and attractive forces that exist within molecules and ions and generally accepted view is “The shape adopted is the one which keeps repulsive force to a minimum”. The types of molecular shape existence are listed below with the appropriate bond angle reference,
Linear with 2 bond pairs of bond angle
Trigonal planar with 3 bond pairs of bond angle
Tetrahedral with 4 bond pairs of bond angle
Trigonal bipyramidal with 5 bond pairs of bond angle combination
Octahedral with 6 bond pairs of bond angle
Key points:
Electronegativity of atoms determines the nature of chemical bond formed between them.
Attractive and repulsive forces operating within molecules and ions determine their shapes.
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