Why do some chemical symbols for elements consist of 2 letters and others of only one

Since there are only 26 letters in the alphabet and there are (currently) 111 known elements, therefore not every element can be represented by only 1 letter. Also some elements date back to antiquity and already have symbol names assigned by Alchemists, for instance, rather than being G, gold is Au for the Latin word aurum  for shining dawn.

describe matter a earth

they it depends on if one believes about radioactivity, they make a geiger counter beep but maybe it was going to beep anyway electronics do wierd things all the time or it was programmed to beep near certain metals.