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PERIODIC TABLE PERIOD

In the periodic table of the elements, a period is a horizontal row of the table.

The number of electron shells an atom has determines what period it belongs to. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order:

1s 
2s           2p  
3s           3p  
4s        3d 4p  
5s        4d 5p
6s     4f 5d 6p  
7s     5f 6d 7p  
8s  5g 6f 7d 8p  
...

Hence the structure of the periodic table. Since the outermost electrons determine chemical properties, those tend to be similar within periodic table groups.

Elements adjacent to one another within a group have similar physical properties, despite their significant differences in mass. Elements adjacent to one another within a period have similar mass but different properties.

See also:

Periodic tables
Standard table | Vertical table | Table with names | Names and atomic masses (large) | Names and atomic masses (small) | Names and atomic masses (text only) | Inline F-block | Elements to 218 | Electron configurations | Metals and nonmetals | Table by blocks | Alternatives

Lists of elements
Name | Atomic symbol | Atomic number | Boiling point | Melting point | Density | Atomic mass
Groups: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12 - 13 - 14 - 15 - 16 - 17 - 18
Periods: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9
Series: Alkalis - Alkaline earths - Lanthanides - Actinides - Transition metals - Poor metals - Metalloids - Nonmetals - Halogens - Noble gases
Blocks: s-block - p-block - d-block - f-block - g-block