Finding materials doesn't work like that in any way.
You're describing how to find POIs which are not what contain arsenic. You need to scan a planet from SC so you get it's materials composition in the details on the system map - that will tell you what %age chance of them there is. Arsenic is one of the rare types so when you find a planet with some you'll want to search for Metallic Meteorites to get the best chance of finding the rares though any outcrop type can have them.
The wavescanner guide above will help you identify the meteorites.
This guy is right. Arsenic is not hard to find, it just takes a little time to gather alot of it. You need to scan a planet with a detalied surface scanner (or drop in at a nav beacon and scan the beacon; doing it will give you surface info on every planet in that system). In the system map you can select planets and see what materials you can mine form their surface.Alternatively, you can buy surface exploration data form the galaxy maps; this is only possible for some systems inside "the bubble". In my experience most systems with more than 15 planets have at least one planet where you can find Arsenic. Planets that have Arsenic, usually have 3-4% of it. You can find it anywhere on the planet. I've found it in rocky areas, plains and craters. I find looking for it in plains the easiest, beacouse it's easier to drive around.
So it's not that hard. I gatherd 50 units of Arsenic last week. Took me 10 minutes to find and travel to a planet with 4 % Arsenic, and 2-3 hours of SRV mining to hit my target of 50 units. I'm not the best SRV driver, and I wasn't rushing from outcrop to outcrop, so you could probably do it alot faster than me. But, it's more fun if you don't rush it
(Fun-tip: While you're driving around on the surface, jump around and se how many consecutive backflips you can do)
(Suicide-tip: jump of the edge of a deep crater and se how many consecutive backflips you can do. Preferably on 1g (or above) planet)