Why is there a diagonal line cutting through some of the nonmetals and metalloids on the periodic table in groups IIIA, IVA, VA, VIA, and VIIA?
Is there any historical background about it?
The red line is the dividing line between metals and non-metals. It is rather arbitrary; as Mendeleev himself wrote: “It is...impossible to draw a strict line of demarcation between metals and nonmetals, there being many intermediate substances.” Most modern periodic tables do not feature it.
The elements with a blue background are the metalloids. Against, it's a rather arbitrary choice, but it corresponds to elements that have some properties of metals. Wikipedia describes them in the following way:
Physically, metalloids usually have a metallic appearance but they are brittle and only fair conductors of electricity; chemically, they mostly behave as (weak) nonmetals. They can, however, form alloys with metals. Ordinarily, most of the other physical and chemical properties of metalloids are intermediate in nature
Those elements are considered to be good semiconductors.
See this periodic table that has them designated as such.
I think they are no longer emphasized on the periodic table because there are so many other semiconducting materials that lie outside of that particular pattern. For example, Gallium is often paired with Arsenic (in Ga-As semiconductors), but only As is along the artificial "metal/non-metal" horizon designated by the line.
Note that from this list aluminum phosphide is listed as a semiconductor, but neither element is situated on that line.
Boron (B), Silicon (Si), Arsenic (As), Tellurium (Te), Germanium (Ge), and Antimony (Sb) are considered to be the semi-metals or more professionally known as metalloids. Metalloids are half-metallic, half-nonmetallic types of elements. The ones on the right side of the steps that I mentioned are more non-metallic than the ones on the left. The stairs also represent the seperation between metals and non-metals so you can see why the ones I name are adjacent to the highlighted line. They don't have a specific placement that's why the put them adjacently to the line and on one side or the other depending on which is more metallic or nonmetallic. Hope this helped you.
Source: I learned this today in Chemistry and I pay attention in class. If I'm wrong somebody can correct me but I'm 100% sure I'm right.