To determine which atom has a greater electron share in a chemical bond, you need to assess the electronegativity of the atoms involved. The atom with a higher electronegativity will attract the shared electrons more strongly, thus having a greater share of those electrons.
Understanding Electronegativity
Electronegativity is a fundamental chemical property that quantifies an atom's ability to attract shared electrons towards itself in a covalent bond. This attraction is influenced by factors such as the atom's nuclear charge and its atomic radius. Atoms with a stronger pull on electrons are considered more electronegative.
Electronegativity and Electron Sharing
The distribution of shared electrons in a bond is directly determined by the electronegativity difference between the bonded atoms. As stated in chemical principles:
"If atoms bonded together have the same electronegativity, the shared electrons will be equally shared. If the electrons of a bond are more attracted to one of the atoms (because it is more electronegative), the electrons will be unequally shared."
When electrons are unequally shared, the more electronegative atom pulls the electron density closer to its nucleus. This results in that atom acquiring a partial negative charge (represented as δ-), while the less electronegative atom takes on a partial positive charge (δ+). This unequal distribution of charge creates a polar covalent bond.
Distinguishing Between Bond Types
The difference in electronegativity (ΔEN) between two bonded atoms helps classify the type of bond formed and the extent of electron sharing:
| Bond Type | Electronegativity Difference (ΔEN) | Electron Sharing | Example |
|---|---|---|---|
| Nonpolar Covalent | ΔEN ≈ 0 | Equal | O₂ (Oxygen gas) |
| Polar Covalent | 0 < ΔEN < ~1.7 | Unequal | HCl (Hydrogen Chloride) |
| Ionic | ΔEN > ~1.7 | Electron transfer | NaCl (Sodium Chloride) |
In a nonpolar covalent bond, like in a diatomic oxygen molecule (O₂), both oxygen atoms have identical electronegativity values. Therefore, the shared electrons are equally distributed, and neither atom has a greater share.
However, in a polar covalent bond, the difference in electronegativity leads to unequal sharing.
Examples of Unequal Electron Sharing:
- Water (H₂O): Oxygen is significantly more electronegative (3.44) than hydrogen (2.20). In the O-H bonds, oxygen attracts the shared electrons more strongly. Consequently, the oxygen atom has a greater electron share and carries a partial negative charge (δ-), while each hydrogen atom carries a partial positive charge (δ+).
- Hydrogen Chloride (HCl): Chlorine (3.16) is more electronegative than hydrogen (2.20). The electrons in the H-Cl bond are pulled closer to the chlorine atom, giving chlorine a greater electron share and a partial negative charge, while hydrogen has a partial positive charge.
Practical Tips for Determining Electron Share
- Consult a Periodic Table: Electronegativity generally increases as you move from left to right across a period and decreases as you move down a group. Fluorine is the most electronegative element, while francium is the least. By comparing the positions of atoms on the periodic table, you can often infer which is more electronegative.
- Calculate the Electronegativity Difference (ΔEN): Find the electronegativity values for each atom (typically provided on charts or advanced periodic tables) and subtract the smaller value from the larger one. A non-zero ΔEN indicates unequal sharing, with the more electronegative atom holding the greater share.
- Identify Partial Charges (δ+ / δ-): In a polar bond, the atom with the greater electron share will be indicated by a partial negative charge (δ-), while the atom with the lesser share will have a partial positive charge (δ+).
