A bond, as its names suggests, holds atoms together to form molecules.
In this article we study a few elementary types of bonds.
Ionic Bond
An ionic bond is formed as consequence of the electrostatic forces of attraction between oppositely charged ions.
If the difference in electronegativity of the atoms is over 1.7 the bond is likely to be ionic.
Covalent Bond
A covalent bond is formed due to the sharing of valence electrons or in terms of the orbital concept, a covalent bond is formed due to the overlapping of atomic orbitals.
If the difference in electronegativity of the atoms is less than 1.7 the bond is likely to be covalent.
In practice most bonds are not 100% covalent or ionic.
If the difference in electronegativity between the 2 bonded atoms is zero, then the bond is purely covalent. If the difference in electronegativity between the 2 bonded atoms ranges from zero to 1.7 the bond is a polar covalent bond and exhibits some ionic character. If the difference in electronegativity between the 2 bonded atoms is 1.7 then the bond is 50% covalent and 50% ionic and if the difference in electronegativity is over 1.7 then the bond is said to be ionic.
Ready-Reference Chart (ΔE is the difference in electronegativity)
ΔE Value Bond Character
ΔE = 0; 100% Covalent
ΔE > 1.7; ionic
Dipole moment is a vector quantity, therefore if a molecule with polar bonds is symmetrical then the opposite pulls will cancel each other resulting in zero net dipole. eg. Carbon dioxide, pictured below.
Fajans' Rules: Account for the covalent nature of ionic bonds. According to these small cations (or high positive charge) have greater polarising power and large anions have greater polarisibility, thus bonds formed between such species involves overlap of their electron clouds resulting in covalent character.
Quick Tip: Picture a large bunch of teenage girls (electrons) on a field trip with their teacher (nucleus) and let's call this group our anion. Now a small cation, hmm Justin Beiber for instance, walks in, and some of the girls move away from the group towards him. Thus the small cation, Justin Bieber in this case has pulled the electron cloud of the large anion towards himself and this mingling of electron clouds accounts for the covalent character of the bond.
If the group were smaller (i.e the anion was smaller) the teacher would've been able to check the movement of the girls. And if the celebrity was somebody more important (i.e a larger cation), like Kazimierz Fajans(yes, he is the guy behind fajans rule) for instance, again fewer girls (electrons) would've strayed away from the group.
Ready-Reference Chart
Ionic Covalent
Low +ve charge High +ve charge
small anion large anion
large cation small cation
Metallic Bonding
Since Ionic and Covalent bonds cannot account for many physical properties of metals, such as strength, malleability, ductility, thermal and electrical conductivity, opacity, and luster, the concept of metallic bonding has been devised.
The positive charge in a metal aggregates to form a "kernel" whereas the electron separate out forming an electron cloud.
Metallic bonding constitutes the electrostatic attractive forces between these delocalized electrons, gathered in an electron cloud, and the kernel of positively charged metal ions.
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In this article we study a few elementary types of bonds.
Ionic Bond
An ionic bond is formed as consequence of the electrostatic forces of attraction between oppositely charged ions.
If the difference in electronegativity of the atoms is over 1.7 the bond is likely to be ionic.
Covalent Bond
A covalent bond is formed due to the sharing of valence electrons or in terms of the orbital concept, a covalent bond is formed due to the overlapping of atomic orbitals.
If the difference in electronegativity of the atoms is less than 1.7 the bond is likely to be covalent.
If the difference in electronegativity between the 2 bonded atoms is zero, then the bond is purely covalent. If the difference in electronegativity between the 2 bonded atoms ranges from zero to 1.7 the bond is a polar covalent bond and exhibits some ionic character. If the difference in electronegativity between the 2 bonded atoms is 1.7 then the bond is 50% covalent and 50% ionic and if the difference in electronegativity is over 1.7 then the bond is said to be ionic.
Ready-Reference Chart (ΔE is the difference in electronegativity)
ΔE Value Bond Character
ΔE = 0; 100% Covalent
ΔE > 1.7; ionic
Dipole Moment: Accounts for the ionic character of covalent bonds.
Homo-nuclear, diatomic molecules (eg. H2 , Cl2 etc.) are non-polar.
In case of hetero-nuclear molecules, for a molecule to be non polar, the central atom mustn't have a lone pair of electrons and the central atom must be surrounded by the same atoms i.e the molecule must be symmetrical (explained below). (both conditions must be met).
However, if these conditions aren't met, then the more electronegative element will pull the electrons towards itself and thus acquire a slightly negative charge (given as δ- ) and the other atom shall acquire a slight positive charge (given as δ+ ).
Quick Tip: Think of the bond as a match of tug of war, the stronger (in this case more electronegative element) player will pull the rope (in this case electrons) towards himself.
Such a bond is termed a polar covalent bond and its polarity is given in terms of dipole moment.
Dipole moment (m) = electric charge x distance of charge seperation (bond length)
m = q x d
Dipole moment is measured in ‘Debye’ unit (D)
Dipole moment is a vector quantity, therefore if a molecule with polar bonds is symmetrical then the opposite pulls will cancel each other resulting in zero net dipole. eg. Carbon dioxide, pictured below.
Quick Tip: Picture a large bunch of teenage girls (electrons) on a field trip with their teacher (nucleus) and let's call this group our anion. Now a small cation, hmm Justin Beiber for instance, walks in, and some of the girls move away from the group towards him. Thus the small cation, Justin Bieber in this case has pulled the electron cloud of the large anion towards himself and this mingling of electron clouds accounts for the covalent character of the bond.
If the group were smaller (i.e the anion was smaller) the teacher would've been able to check the movement of the girls. And if the celebrity was somebody more important (i.e a larger cation), like Kazimierz Fajans(yes, he is the guy behind fajans rule) for instance, again fewer girls (electrons) would've strayed away from the group.
Ready-Reference Chart
Ionic Covalent
Low +ve charge High +ve charge
small anion large anion
large cation small cation
Metallic Bonding
Since Ionic and Covalent bonds cannot account for many physical properties of metals, such as strength, malleability, ductility, thermal and electrical conductivity, opacity, and luster, the concept of metallic bonding has been devised.
The positive charge in a metal aggregates to form a "kernel" whereas the electron separate out forming an electron cloud.
Metallic bonding constitutes the electrostatic attractive forces between these delocalized electrons, gathered in an electron cloud, and the kernel of positively charged metal ions.
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| Shitty illustration depicting kernel of positive charge and cloud of electrons. |
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That's all folks!
The Passive Observer
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