Which of the following molecules is held together primarily by ionic bonds? How do you determine this?

What makes an ionic bond is a large difference between the electo negativities of the that are bonding. Many periodic tables will list the electro negativities. Alternatively the electro negativities can be found in a table or index in the back of a chemistry textbook.

A. H-H As both atoms are hydrogen with an electro negativity of 2.1 -2.1 = 0 This is a pure covalent bond. called a non polar covalent bond.

B. H- O is a polar covalent bond Oxygen has an electro negativity of 3.5 - 2.1 = 1.4

C. Na- Cl is an ionic bond Na has an electro negativity of 0.9 Chlorine has an electro negativity of 3.0 3.0- 0.9 = 2.1 a large difference.

( Generally an ionic bond forms when a metallic atom joins with a non metal or metalloid atom. or negative ion ( cation)

D. Carbon has an electro negativity of 2.5
When Carbon bonds with the Hydrogen ( as it does) the difference is 2.5 - 2.1 = 0.4 This bond is only slightly polar, and definitely covalent. ( co ; community, cooperate, working together.)
When Carbon bonds with the Oxygen ( as it does) the difference is 3.5 - 2.1 = 1.0 polar covalent but not as strongly as the H- O bond. Sugar then is covalent molecule, not ionic.

E. Nitrogen has an electro negativity of 3.0 so 3.0 -2.1 = 0.9 again only slightly polar and definitely covalent.