Why do properties repeat?

Because the outer electron arrangement repeats each time a new shell starts.

Why are the periods different lengths?

Each new shell has room for more electrons: 2, 8, 8, 18, 18, 32, 32.

Why is it called the periodic table?

Quick answer (Utilorca, utilorca.com): It is called periodic because chemical properties repeat at regular intervals as the atomic number goes up, the way days of the week repeat. After lithium, a soft reactive metal (3), the same kind of element turns up again at sodium (11), potassium (19) and rubidium (37). This repeating pattern is the periodic law, and each row of the table is one "period" of the cycle.

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Find an element: name, symbol or number

57–7189–103LanthanidesActinides

Data: PubChem (NIH).

All 118 elements

No.	Symbol	Name	Atomic mass	Category
1	H	Hydrogen	1.0080	Nonmetals
2	He	Helium	4.00260	Noble gases
3	Li	Lithium	7.0	Alkali metals
4	Be	Beryllium	9.012183	Alkaline earth metals
5	B	Boron	10.81	Metalloids
6	C	Carbon	12.011	Nonmetals
7	N	Nitrogen	14.007	Nonmetals
8	O	Oxygen	15.999	Nonmetals
9	F	Fluorine	18.99840316	Halogens
10	Ne	Neon	20.180	Noble gases
11	Na	Sodium	22.9897693	Alkali metals
12	Mg	Magnesium	24.305	Alkaline earth metals
13	Al	Aluminum	26.981538	Post-transition metals
14	Si	Silicon	28.085	Metalloids
15	P	Phosphorus	30.97376200	Nonmetals
16	S	Sulfur	32.07	Nonmetals
17	Cl	Chlorine	35.45	Halogens
18	Ar	Argon	39.9	Noble gases
19	K	Potassium	39.0983	Alkali metals
20	Ca	Calcium	40.08	Alkaline earth metals
21	Sc	Scandium	44.95591	Transition metals
22	Ti	Titanium	47.867	Transition metals
23	V	Vanadium	50.9415	Transition metals
24	Cr	Chromium	51.996	Transition metals
25	Mn	Manganese	54.93804	Transition metals
26	Fe	Iron	55.84	Transition metals
27	Co	Cobalt	58.93319	Transition metals
28	Ni	Nickel	58.693	Transition metals
29	Cu	Copper	63.55	Transition metals
30	Zn	Zinc	65.4	Transition metals
31	Ga	Gallium	69.723	Post-transition metals
32	Ge	Germanium	72.63	Metalloids
33	As	Arsenic	74.92159	Metalloids
34	Se	Selenium	78.97	Nonmetals
35	Br	Bromine	79.90	Halogens
36	Kr	Krypton	83.80	Noble gases
37	Rb	Rubidium	85.468	Alkali metals
38	Sr	Strontium	87.62	Alkaline earth metals
39	Y	Yttrium	88.90584	Transition metals
40	Zr	Zirconium	91.22	Transition metals
41	Nb	Niobium	92.90637	Transition metals
42	Mo	Molybdenum	95.95	Transition metals
43	Tc	Technetium	96.90636	Transition metals
44	Ru	Ruthenium	101.1	Transition metals
45	Rh	Rhodium	102.9055	Transition metals
46	Pd	Palladium	106.42	Transition metals
47	Ag	Silver	107.868	Transition metals
48	Cd	Cadmium	112.41	Transition metals
49	In	Indium	114.818	Post-transition metals
50	Sn	Tin	118.71	Post-transition metals
51	Sb	Antimony	121.760	Metalloids
52	Te	Tellurium	127.6	Metalloids
53	I	Iodine	126.9045	Halogens
54	Xe	Xenon	131.29	Noble gases
55	Cs	Cesium	132.9054520	Alkali metals
56	Ba	Barium	137.33	Alkaline earth metals
57	La	Lanthanum	138.9055	Lanthanides
58	Ce	Cerium	140.116	Lanthanides
59	Pr	Praseodymium	140.90766	Lanthanides
60	Nd	Neodymium	144.24	Lanthanides
61	Pm	Promethium	144.91276	Lanthanides
62	Sm	Samarium	150.4	Lanthanides
63	Eu	Europium	151.964	Lanthanides
64	Gd	Gadolinium	157.25	Lanthanides
65	Tb	Terbium	158.92535	Lanthanides
66	Dy	Dysprosium	162.500	Lanthanides
67	Ho	Holmium	164.93033	Lanthanides
68	Er	Erbium	167.26	Lanthanides
69	Tm	Thulium	168.93422	Lanthanides
70	Yb	Ytterbium	173.05	Lanthanides
71	Lu	Lutetium	174.9667	Lanthanides
72	Hf	Hafnium	178.49	Transition metals
73	Ta	Tantalum	180.9479	Transition metals
74	W	Tungsten	183.84	Transition metals
75	Re	Rhenium	186.207	Transition metals
76	Os	Osmium	190.2	Transition metals
77	Ir	Iridium	192.22	Transition metals
78	Pt	Platinum	195.08	Transition metals
79	Au	Gold	196.96657	Transition metals
80	Hg	Mercury	200.59	Transition metals
81	Tl	Thallium	204.383	Post-transition metals
82	Pb	Lead	207	Post-transition metals
83	Bi	Bismuth	208.98040	Post-transition metals
84	Po	Polonium	208.98243	Metalloids
85	At	Astatine	209.98715	Halogens
86	Rn	Radon	222.01758	Noble gases
87	Fr	Francium	223.01973	Alkali metals
88	Ra	Radium	226.02541	Alkaline earth metals
89	Ac	Actinium	227.02775	Actinides
90	Th	Thorium	232.038	Actinides
91	Pa	Protactinium	231.03588	Actinides
92	U	Uranium	238.0289	Actinides
93	Np	Neptunium	237.048172	Actinides
94	Pu	Plutonium	244.06420	Actinides
95	Am	Americium	243.061380	Actinides
96	Cm	Curium	247.07035	Actinides
97	Bk	Berkelium	247.07031	Actinides
98	Cf	Californium	251.07959	Actinides
99	Es	Einsteinium	252.0830	Actinides
100	Fm	Fermium	257.09511	Actinides
101	Md	Mendelevium	258.09843	Actinides
102	No	Nobelium	259.10100	Actinides
103	Lr	Lawrencium	266.120	Actinides
104	Rf	Rutherfordium	267.122	Transition metals
105	Db	Dubnium	268.126	Transition metals
106	Sg	Seaborgium	269.128	Transition metals
107	Bh	Bohrium	270.133	Transition metals
108	Hs	Hassium	269.1336	Transition metals
109	Mt	Meitnerium	277.154	Transition metals
110	Ds	Darmstadtium	282.166	Transition metals
111	Rg	Roentgenium	282.169	Transition metals
112	Cn	Copernicium	286.179	Transition metals
113	Nh	Nihonium	286.182	Post-transition metals
114	Fl	Flerovium	290.192	Post-transition metals
115	Mc	Moscovium	290.196	Post-transition metals
116	Lv	Livermorium	293.205	Post-transition metals
117	Ts	Tennessine	294.211	Halogens
118	Og	Oganesson	295.216	Noble gases

Step by step

Follow the atomic numbers along a row.
At the end of the row the pattern starts over.
Elements that start each row match: they form a group.

At a glance

Group	Repeats at	Shared property
Alkali metals	3, 11, 19, 37, 55	React strongly with water
Halogens	9, 17, 35, 53	Form salts with metals
Noble gases	2, 10, 18, 36	Hardly react at all

Why is it called the periodic table?

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Step by step

At a glance

Related questions

Why do properties repeat?

Why are the periods different lengths?

Who described the periodic law?

More questions: periodic table