Any collection of letters that does not begin with a capital letter is one of the most important exclusions, but a complete answer is simply any collection of letters that is not one of the approximately 118 recognized atomic symbols.
ClO4- Note that this is not an "element" and hence doesn't have an "element symbol."
The symbol of helium is he
There is no element with symbol a or A even
It is an imaginary element not found in periodic table, Ca is the symbol for Calcium, Cd is the symbol for Cadmium so the symbol of Candium may be Cn.
Take the element "Iron" then the symbol for the element is "Fe" and "Fe" is also the symbol for a single atom of the element in a chemical equation.
O is the the symbol of the element oxygen.
B is the symbol for which element?Boron
The element symbol for barium is Ba.
The symbol for the element Terbium is Tb
If you meant "element" as in a chemical element, there is no element with the symbol "ef" in the periodic table.
The symbol for Aluminum (Al) could be nicknamed "Alan" as a fun and informal way to reference the element.
Potassium, with an Atomic Number of 19, is designated by the symbol 'K'.