Hey everyone. So here we're going to take a look at the periodic trend when it comes to the density of main Group elements. Here we're going to say periodic trends are used when asked to compare elements of the same phase or when they're in the same group. Now the periodic trend is density tends to decrease going up a group, but across a period there is no uniform trend.
So if we take a look here at this periodic table, again the 7th row, the seventh period is eliminated because it's 2 variables. A lot of those elements are too heavy or man-made, so they're very unstable. So we're paying attention to rows one to six. Now here going up a group, our density decreases generally speaking. But as we go from left to right either direction, it's too much variability. So we see that there's no uniform trend when we're looking across a period.
Now here, you might notice we have purple for calcium and potassium here because they also represent some type of difference in the overall trend. Here we're going to say they represent an exception. We're going to say that potassium and calcium have lower densities than sodium and magnesium because they have higher volumes than expected. So here as we're going up, we should expect our densities to decrease. So we should have expected potassium and calcium to have a higher density than sodium and magnesium, but that's not the case.
All you need to know on your part is that potassium and calcium just have larger than normal volumes. This affects their overall density because remember, density equals massvolume. Anything beyond that is beyond the scope of this course. So don't worry about that, all right. So just keep in mind when we're looking up a group, for the most part our density is decreasing and remember the exception that we have here.