A block on the periodic table is a group of elements that all have their electrons in the same atomic orbital. There are four blocks, s-, d-, f, and p-.^[1] The word "block" was first used to describe this by Charles Janet.^[2]

v t e Blocks in the periodic table
Group →	1	2	3		4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
↓ Period
1	1 H																		2 He
2	3 Li	4 Be												5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg												13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	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
5	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
6	55 Cs	56 Ba	57 La		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	86 Rn
7	87 Fr	88 Ra	89 Ac		104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
8	119 Uue	120 Ubn	121 Ubu
					58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
					90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr
					122 Ubb	123 Ubt	124 Ubq	125 Ubp	126 Ubh	127 Ubs

The p-block is on the right side of the periodic table. Elements from groups 13-18 are in the p-block.

The d-block is in the middle of the periodic table. Elements from groups 3 to 12 are in the d-block.

The f-block is shown in green in the illustration above. The elements 57 to 71 are called Lanthanides, elements 89 to 103 are called Actinides.

The s-block is on the left side of the periodic table. Elements from groups 1-2 are in the s-block.