Period
Definition and meaning of Period in chemistry.
A period is a horizontal row in the periodic table of elements. All elements in the same period have the same number of electron shells (or principal energy levels) in their ground state configuration.
In more detail
The periodic table is organized into 7 periods (rows). As you move down the periodic table from one period to the next, each element gains one additional electron shell. The period number directly corresponds to the number of electron shells: Period 1 elements have 1 shell, Period 2 elements have 2 shells, and so on. This fundamental organization reflects the way electrons fill atomic orbitals. Trends in atomic properties such as atomic radius, ionization energy, and electronegativity vary predictably across a period, making periods essential for understanding periodic trends.
Key facts
| Field | General Chemistry |
|---|---|
| Number of periods | 7 |
| Relationship | Period number equals number of electron shells |
| Example Period 2 elements | Li, Be, B, C, N, O, F, Ne |
Sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar) all belong to Period 3 because each atom has exactly 3 electron shells in its ground state.
Frequently asked questions
How many periods are in the periodic table?
There are 7 periods in the modern periodic table, corresponding to elements with 1 to 7 electron shells.
What is the difference between a period and a group?
A period is a horizontal row (which determines electron shells), while a group is a vertical column (containing elements with similar chemical properties and the same number of valence electrons).