Chemistry Exam Review

Question

What type of bond would hold each of the following pairs of atoms together

A) N-S

B) Si-l

C) N-Br

D) K-F

Answer 1

A: (on the weaker end of) polar Covalent bond

B: same as above

C: Polar Covalent

D: Ionic

Explanation Below

At the simplest level, there are two types of bonds: ionic and covalent. Ionic bonds involve atoms of one element taking electrons from the atoms of the other element. Covalent bonds involve the sharing of electrons between the atoms of the elements. A really general way of thinking about this is that if you have a metal (leftish portion of the periodic table) reacting with a non-metal (rightish portion) an ionic bond will form. Two non-metals will form covalent bonds.

Diving a little more deeply, the reason ionic bonds form is that one of the elements has a stronger pull (electronegativity) for electrons than the other. Non-metals tend to have higher electronegativity than metals and can pull electrons from the metals. Because non-metals tend to have electronegativity values that are relatively close, neither one of a pair "wins."

The Si-I question requires getting down to the mathematical details. Looking at the numerical values for electronegativity, Si (1.9) and I (2.66) are pretty close to N-S (about the same absolute value of the difference). The generally accepted difference value for an ionic bond is 1.6. The Si-I difference is nowhere near that.

Covalent bonds are subdivided into polar and non-polar depending on how equal the sharing of electrons is. Think of a tug of war and how different the strengths are between participants. Differences less than or equal to .4 are generally non-polar. Differences greater than .4 are considered polar.