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Electronegativity
The electronegativity of an atom describes how much force its nucleus pulls on bonding electrons.
Higher Electronegativity Value: Stronger pull on electrons.
Lower Electronegativity Value: Weaker pull on electrons.
Electronegativity Chart of Elements (Pauling Scale)
Polarity
A bond is polar when the two atoms have a difference in electronegativity. When atoms pull on electrons with unequal strength, it results in unequal charge distribution.
When two atoms bond:
Small/No difference in Covalent Bond: Even pull of electrons → Nonpolar covalent bond.
Large difference in Covalent Bond: Uneven pull of electrons → Polar covalent bond.
Cations and anions of ionic compounds have extreme differences in electronegativity.
An unequal charge distribution creates dipole moments (e.g. bond dipoles from polar covalent), meaning the subject has a positive and negative end.
Molecular Compounds: Polar if it has a net dipole. Molecular polarity depends on both bond polarity and molecular geometry.
If bond dipoles cancel out or if there aren't any: Nonpolar molecule.
If bond dipoles don't cancel out: Polar molecule, has partially positive (δ⁺) and negative (δ⁻) ends.
Ionic Compounds: Because they are composed of ions, they have discretely positive and negative ends. The strength of their polarity is determined by the charge number of the ions.
Properties of Polar Substances
Polar bonds are generally stronger than nonpolar bonds.
Polar compounds have stronger interactions than nonpolar molecules due to electrostatic attraction. Therefore, they usually exhibit properties such as:
Higher melting/boiling points, requires more energy input to weaken these interactions.
Crystal lattice structures or alignment in their solid phases, oppositely-charged ends link in a way that maximizes attraction while minimizing repulsion.
Higher solubility in polar solvents due to the presence of charges or dipoles.
More viscosity and surface tension, particles are more strongly attracted to each other than nonpolar compounds.
Drawing Bond Dipoles
Dipole arrows point towards the more electronegative atom.
The H-Cl bond in HCl is polar. Therefore, the molecule itself is polar.
Both dipoles point from H to O in H₂O. This creates a net dipole across the molecule pointing in the direction of O.
Both dipoles of the C=O bond go towards the O atoms. Because the O atoms face opposite ways, the dipoles cancel out.
Ionic vs. Covalent... Not a Clear Answer
Ionic and covalent bonds exist on the same spectrum, as electronegativity difference influences bond type.
There are no clear boundaries between these differences. As a result, some compounds can exhibit both ionic and covalent properties.
Example: Acids are covalent compounds, yet they can dissolve in water as H⁺ and a conjugate base (anion).
Fajan's Rule
This rule is used to predict covalent character in ionic compounds.
This and Coulomb's Law (lattice energy) both influence how soluble ionic compounds are.
Even in ionic compounds, cations can pull on the electrons of anions. This distorts the anion's electron cloud by pulling the electron distribution toward the cation.
More distortion means more covalent character.
Factors that Increase Covalent Character:
Cations:
Higher Positive Charge: More electrostatic attraction towards anion electrons.
Smaller Radius: Less electron shielding, higher effective nuclear charge.
Higher electronegativity: Stronger attraction towards anion electrons.
Anions:
Higher Negative Charge: More electrons, larger electron cloud due to repulsion.
Larger Radius: More electron shielding, resulting in looser valence electrons.
Lower electronegativity: Looser pull towards its own electrons.
Example of Fajan's Rule:
NaCl vs BeCl₂, which is more ionic?
NaCl is more ionic because it has an electronegativity difference of 2.1. Na⁺ has a 1+ charge and is larger than Be²⁺ in radius.
It shows what ionic compounds are expected to do, such as: Na⁺ and Cl⁻ stay discretely as ions and are arranged in a lattice.
There is a debate on if BeCl₂ is ionic or covalent. It has an electronegativity difference of 1.5. Be²⁺ has a 2+ charge and is smaller than Na⁺ in radius.
BeCl₂ exists in polymers instead of lattices. It also experiences a lot of electron-sharing.
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