How to tell from the formula if a compound is ionic: QUESTION: Which of the following is a molecular compound? A. Na2S, B. NH3, C. (NH4)2SO4, D. N2 How to tell from the formula if a compound is ionic: there is a metallic element (typically written first) there is an ammonium (NH4+) ion; look for “NH4” grouping (also written first). SCRIPT: Which of the following compounds is molecular? A. Na2S, B. NH3, C. (NH4)2SO4, or D. N2 PAUSE CLICK Immediately, we can rule out choice D. It’s not a compound. It’s made up of only one element, nitrogen. CROSS OUT Choice D CLICK We can easily tell from the formula of a compound if it is ionic Most cations are derived from metallic elements. So, the first thing we need to look for is the symbol for a metallic element. It’s usually the first one written. If there is one, then we can say that the compound is ionic. Let’s look at our choices. In choice A, Na is the symbol for sodium, which is a metallic element. So we know that the compound in choice A, sodium sulfide, is ionic, not molecular. CROSS OUT A If we don’t see a metallic element, the compound can still be ionic if we see an ammonium ion. HIGHLIGHT We look for an NH4 grouping in the formula. We can see that there is one in choice C. HIGHLIHGT NH4 in (NH4)2SO4 Therefore, choice C, ammonium sulfate, is ionic, not molecular. CROSS OUT C Therefore the correct answer is B. NH3 is a molecular compound. This compound is called ammonia.

Here’s a quick review of where to find metallic elements in the periodic table. The metallic elements are shown here highlighted in red. And here’s sodium… CLICK Sodium is a metal. The other elements we saw earlier are nitrogen CLICK hydrogen CLICK and oxygen CLICK All of these are nonmetals. PAUSE END RECORDING

Video ID: 1-23-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08