What is the term for the unique abbreviation given to each element?

The term for the unique abbreviation given to each element is the atomic symbol.
The atoms of all the elements are represented by a unique atomic symbol which is a one or two lettered notation used to represent an atom corresponding to an element. If the atomic symbol has two letters, the first letter is capitalized and the second is in lower case. For example hydrogen is represented by the letter H.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts