In the Lewis structure shown, X and Y represent different elements and all valence electrons are shown. The molecule is stable and neutral, and Y is an element in the second period of the periodic table. What is the chemical symbol of element X?
First, let’s take a look at this Lewis structure. It has eight dots, which each represent electrons. So our compound has eight valence electrons. When we have two electrons drawn between different elements, that represents a single bond. So we alternatively could’ve drawn our structure like this, with each line representing a single bond.
We want to know the identity of the element X. One thing that we might notice about each X in our structure is that it only has two electrons. This means that it’s not obeying the octet rule, which is a rule of thumb that tells us that elements tend to bond in a way so that they’ll have a full valence shell, which is eight electrons. There are, of course, exceptions to the octet rule, such as hydrogen and helium, which each can only accommodate two valence electrons. This matches our element X in our structure. So one of these must be our unknown element X.
However, helium is a noble gas. Noble gases have full valence shells on their own. So they don’t typically like to bond anything else. This means that hydrogen, which has the chemical symbol H, must be element X. Since we’ve determined that X is hydrogen, I’ve gone ahead and drawn that into our structure.
What is the chemical symbol of element Y?
The question tells us that Y is an element in the second period or row of the periodic table. So we just need to figure out which element in the second period Y is. The number of valence electrons that an element has can be determined by its position in the periodic table. So if we can figure out how many valence electrons Y must have, we can determine its chemical symbol.
Our total structure has eight valence electrons. We have three hydrogens in our structure, which are each contributing one valence electron. So if we subtract three from the total number of valence electrons in our structure, we should be able to figure out the number of valence electrons that Y has. So Y has five valence electrons. This corresponds to nitrogen, which has the chemical symbol N. So our element Y is nitrogen.