Based on the electronegativity values given, in which of the following are the bonds most polar? And we are given a table listing elements and their electronegativity values. Electronegativity for boron is 2.0, for hydrogen 2.2, nitrogen 3.0, and fluorine 4.0. And we’re given possible answers: (A) BF3, (B) BH3, (C) NH3, (D) NF3, or (E) HF.
When nonmetal atoms and sometimes metalloids, such as boron, bond together, they each donate a valency electron into the bond between them which joins them together. The positively charged nuclei of the two atoms are held together electrostatically by the negatively charged pair of electrons in the bonding pair between them. We call this type of bond a covalent bond. Sometimes the bonding pair of electrons are equally shared or equally attracted towards each nucleus. And sometimes a pair of electrons are unequally shared and are not pulled equally.
The strength of this pull depends on the ability of each nucleus to pull electrons towards itself. And this pull is called electronegativity. Electronegativity can be defined as the tendency of an atom to attract a pair of shared electrons in a bond towards itself. Every element has been assigned an electronegativity value. These values are unitless. And the higher the value, the more strongly that element can pull a bonding pair of electrons towards itself. Using only two atoms at a time, we can calculate the electronegativity difference to determine what type of bond exists between the two atoms.
When the difference is less than 0.4, we say that the difference is so minor that each atom pulls the bonding pair of electrons equally or almost equally. And we call this type of bond nonpolar covalent. When the difference is somewhere between 0.4 and 1.8, this tells us that the bonding pair of electrons is pulled much more strongly towards one of the nuclei. And we call this type of bond polar covalent. When the difference is larger than 1.8, we usually refer to this as an ionic bond. Note that ionic bonds generally exist between metals and nonmetals, but not always.
In the case of metals with nonmetals, a positively charged metal cation and a negatively charged nonmetal anion forms. We must remember that these values given are just guidelines to help us figure out what type of bond exists in a particular compound. There are exceptions to the rule. For example, if the electronegativity difference is larger than 1.8, but it is nonmetals or metalloid atoms which are bonded together. This is not considered an ionic bond but will instead be considered an extremely polar covalent bond because electrons are still shared between the two atoms, although not very equally.
The question asked, based on the electronegativity values given, in which of the following are the bonds most polar? So for each substance, let’s determine what type of bonds exist. So in BF3 — taking two atoms at a time, the electronegativity difference between a boron and a fluorine — taking the bigger value minus the smaller value, we get 4.0 minus 2.0, which is 2.0 as the difference. Boron is a metalloid and fluorine a nonmetal. So we know we’re looking at a covalent bond, and this is a polar covalent bond. And in fact, this is one of those exceptions to the rule. It’s an extremely polar polar covalent bond.
Doing the same calculation for each answer, we get the types of bonds as follows. In BF3, we get polar covalent, BH3 nonpolar covalent, NH3 polar covalent, NF3 polar covalent, and HF polar covalent. The largest electronegativity difference exists between boron and fluorine in BF3. This is the most polar polar covalent compound. Finally, based on the electronegativity values given, the compound which contains the bonds which are most polar is BF3.