Showing the occupancy of the shells for a particular atom

It is possible to deduce the electron shell occupancy for a particular atom from its position in the periodic table.

This is represented by a sequence of numbers separated by commas.
The numbers give, in order of increasing energy, the number of electrons in the various shells (energy levels).

To deduce electron shell occupancy for an atom:
Find the atomic number for the element. This is the total number of electrons available.

The atomic number for lithium is 3. Thus the total number of electrons available is 3.

Fill the energy levels (shells) in order of increasing energy, taking into account the maximum occupancies given below.

Energy level 1 (shell 1): 2 electrons
Energy level 2 (shell 2): 8 electrons
Energy level 3 (shell 3): 8 electrons*

*For elements of atomic number > 20, energy level 3 (shell 3) may contain up to 18 electrons.

Use the position of the element in the periodic table to deduce the number of electrons in partially filled energy levels (shells).

To do this count from the beginning of the last row occupied, stopping at the element in question.
For lithium the electron configuration is 2,1
Thus the first energy level is full, and there is one electron in the second shell.

1	2
							1H										2He
3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
11 Na	12 Mg											13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
55 Cs	56 Ba	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At

To check that your representation of the electron shell occupancy is correct, confirm that

the number of energy levels (shells) occupied is equal to the row number in which that element appears in the periodic table;
the sum of the numbers is equal to the atomic number.