A polar covalent bond results when electrons are shared unequally (ex. HCl). In this molecule, a pair of electrons is shared between the H and Cl atoms, but the sharing is not equal. The Cl atom has a stronger attraction for the shared pair of electrons than does the H atom. This makes the Cl end of the molecule slightly negative and the H end slightly positive. This is designated by the symbol delta (ẟ).
ẟ+ H - Cl ẟ-
Electronegativity: Electronegativity is the quantitative measure of an atom’s electron attracting ability. These values range from 0.9 to 4.1. The higher the electronegativity value, the greater the attraction for electrons.
Two elements with very different electronegativities such as sodium (0.9) and Cl (2.9) are expected to form ionic bonds. Two elements of slightly different electronegativity such as carbon (2.5) and hydrogen (2.1) are expected to form only slightly polar covalent bonds. A molecule like H2, where each atom has the same electronegativity forms a purely covalent compound. The greater the difference in electronegativity (ΔENeg), the more polar the bond becomes.
Guidelines for Determining Chemical Bond Types
ΔENeg > 1.7 🠊 IONIC BOND
0.5 < ΔENeg < 1.7 🠊 POLAR COVALENT BOND
ΔENeg < 0.5 🠊 PURE COVALENT BOND
*Note that the above information is a guideline. The only value written in stone is the one for pure covalent bonding. Be sure to use your common sense. For instance, if you find the ΔENeg for two non-metals bonding is 1.75, you have a polar covalent bond. Despite the ΔENeg value falling in the ionic range, we know that two non-metals will bond covalently.
TryIt!: Determine the bond type and provide the Lewis diagram and the Structural diagram for silicon tetrafluoride. The answer is below.
ex. SiF4
Homework:
HW 1: (i) Determine the chemical bond type for the following molecules. (ii) Draw the Lewis diagrams of the molecules. (iii) Draw the structural diagrams of the molecules (if the molecule contains a polar covalent bond, indicate which end of the bond is slightly negative (δ-) and which end is slightly positive (δ+)).
(a) MgCl2 (b) BrCl (c) CO (d) N2 (e) HF (f) K2S
Answers:
