Bond polarity comes from different elements having different electronegativity; the innate ability to attract electrons within a covalent bond.
Each element has a value for electronegativity to compare how two atoms distribute negative charge when they share electrons.
Electronegativity Property of Atoms
Electronegativity expresses an element’s ability to attract electrons. This attraction comes from the positive charge within the nucleus.
A scale was devised by Linus Pauling to rate elements by relative electronegativity, Figure 1.
Charge and Distance
The basis of electronegativity comes from two opposing factors: the distance of valence electrons from a positive nuclear charge and the size of the positive nuclear charge, Figure 2.
The higher the nuclear charge, the greater the attraction of valence electrons.
The nuclear charge is offset when the principal quantum number of valence electrons increases. The valence electrons are further from the nuclear charge and shielded by inner electrons.
The valence electrons experience less of the positive nuclear charge.
Electronegativity increases to the right across a period, and decreases further and further from the nucleus, Figure 3.
This makes fluorine the most electronegative element with silicon being the least electronegative. Treat hydrogen and carbon as if a bond between them has no polarity.
In A Bond
When two different elements share a bond, a state of polarity exists between them. A polar bond means the covalent electrons between the atoms are not equally shared between atoms.
The electron density around each atom is shown by a gradient, where the darker shade shows more electron density.
The more electronegative element has more electron density than the less electronegative element. This creates a partial negative charge, -δ, on one side, and a partial positive charge, +δ, on the other, Figure 4.
Two Identical Elements
When the two elements are identical like Cl and Cl, the two chlorines share electrons equally. The bond is nonpolar.
Two Elements: Small Difference
The difference between carbon and nitrogen is not that great, but there is a difference. Nitrogen is more electronegative than carbon. This imparts a partial negative charge on nitrogen. As a result, carbon holds a partial positive charge.
Two Elements: Large Difference
The last example shows a hydrogen which shares electrons with fluorine. A large difference in electronegativity between the two elements gathers a partial positive charge around fluorine. With electron density pulled away from hydrogen, it holds a strong partial positive charge.
Polarity and Vectors
To show ta polar bond between two elements, an arrow indicates the direction of a partial charge. The arrow points towards more electron density.
The head of the arrow shows the partial negative charge, -δ. The base of the arrow points away from the positive charge, +δ. The size of the arrow shows the strength of the dipole between two elements, μ.
Dipoles can finally be shown as an arrow without a gradient to show electron density. When an arrow represents more than one variable, it is called a vector.