The electronic configuration can be written two ways.
One shows sub-levels where the other only shows the shells ( levels )
Lithium ( Li ) has 3 electrons ( 3 e- )
Method 1 :
Atoms have levels ( shells ) at which electrons orbit the nucleus ( theory )
At each level there can only be a fixed amount of electrons and electrons have a fixed amount of energy.
The first level can hold 2 electrons , the second can hold 8 electrons and the third fourth and so forth.
The Lithium electron configuration in this following method is ( 2 , 1 ) since there are only 3 electrons that can fill in the levels.
Ok the other method.
Method 2 :
Levels are divided into sub-levels ( theory ).
Sub-levels also are made up of orbitals.
Sub-levels are divisions of the main level which have the same energy.
Orbitals are regions in space where there is a high probability of finding an electron.
There are three main orbitals ( more if you learn in a university ) s orbital p orbital and d orbital. In this case we only need s orbitals.
S orbitals can hold two electrons ( which have opposite spins ).
The first level has a 1 sub-level which has 1 s orbital and the second level has two sub-levels which has 1 s orbital and 1 p orbital.
These how they are marked
1sy 2sy 2py - the number infront represents the level, of course the s and p represent sub-levels and y here represents the number of electrons ( y doesn't equal another y it's just an example here ).
Ok the answer is 1s2 2s1 since Li has 3 electrons it fills in the first sub-level fully and it has only one electron left so it occupies the second sub-level.
Hope this helps :).