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A horizontal row in the periodic table is called a period.
- There are seven periods numbered from 1 to 7.
- The number of the period an element is in, in general, is the same number of the energy level of the elements valence electrons.
- Periods 1,2,and 3 are known as short periods.
- Atoms of these elements have only s and p electrons
- Periods 4 and 5 are long
- Contain Transition Elements which is any element with an atom (or a cation) that has an incomplete d subshell
- Periods 6 and 7 contain series of transition elements called lanthanoids and actinoids respectively
- Both of these are characterized as filling the f sublevel
[Another Periodic Table]