Period
Definition and meaning of Period in chemistry.
A period is a horizontal row of elements in the periodic table. All elements located in the same period have the exact same number of electron shells. These shells hold the electrons around the nucleus in the ground state configuration.
In more detail
The modern periodic table is organized into seven distinct horizontal periods. As you read down the table from top to bottom, each new period gains one additional electron shell. The period number directly tells you the total number of electron shells.
Elements in Period 1 only have one single electron shell. Elements in Period 2 possess two electron shells, and this simple pattern continues downward. This fundamental organization perfectly matches the strict rules of how electrons fill atomic orbitals.
An electron shell must reach a specific capacity before a brand new period can begin. Moving across a single period from left to right reveals highly predictable chemical trends. As you move right across a period, the atomic radius generally shrinks.
At the exact same time, the ionization energy typically increases. These repeating trends occur because the number of positive protons increases. Meanwhile, the main electron shell remains exactly the same.
The extra protons create a stronger pull that draws the electron shell closer to the nucleus. Understanding these horizontal periods helps chemists predict how different elements will behave and react. It explains why elements on the left are reactive metals and elements on the right are nonmetals.
Key facts
| Field | General Chemistry |
|---|---|
| Total Periods | 7 |
| Core Relationship | Period number equals the number of electron shells |
| Trend (Left to Right) | Atomic radius generally decreases |
| Trend (Left to Right) | Ionization energy generally increases |
| Example Period 2 Elements | Lithium, Carbon, Oxygen, Neon |
Sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and argon all sit in Period 3 of the table. Every single atom of these specific elements contains exactly three electron shells. Sodium has just one loosely held valence electron in its outer third shell. Chlorine has seven valence electrons packed into that exact same outer third shell. Argon completely fills the third shell with eight electrons.
Frequently asked questions
What is the difference between a period and a group?
A period is a horizontal row that indicates the number of electron shells. A group is a vertical column that contains elements with similar chemical properties.
Why do atoms get smaller as you move across a period?
As you move right across a period, the nucleus gains more protons. These extra protons pull the same electron shell closer and tighter.
How many elements are in the very first period?
The first period contains only two elements. Hydrogen and helium are the only elements small enough to fit their electrons entirely into the first shell.