What's a Row in the Periodic Table? A Clear Explainer

If you look at a standard periodic table, you see a grid of squares. The horizontal lines that run from left to right are called periods. So the direct answer to “whats a row in the periodic table” is that it is a period. There are seven periods in the standard table. Elements in the same period share the same number of electron shells, which is the main reason they are lined up together.

How Periods Are Organized

Each period corresponds to a principal energy level, or shell, that electrons occupy around an atom’s nucleus. As you move from left to right across a period, each element adds one more proton and one more electron to the atom. The electrons fill the same outer shell, but the number of protons increases, pulling the electrons closer to the nucleus. This makes the atoms smaller across the period.

The first period is the shortest, with only two elements: hydrogen and helium. They both have one electron shell. The second and third periods each have eight elements. The fourth and fifth periods have 18 elements. The sixth period has 32 elements, but a row of 14 elements (the lanthanides) is usually pulled out and placed below the main table to keep the table from being too wide. The seventh period also has 32 elements, with the actinides placed below as well.

Why Periods Matter for Chemical Behavior

The period of an element tells you the number of electron shells it has. This is a basic property that influences how the element reacts with others. For example, elements in period 2 (lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon) all have two electron shells. Their chemical properties change predictably across the period.

A key pattern is that metallic character decreases from left to right across a period. On the left side, elements like sodium and magnesium are metals. In the middle, elements like silicon are metalloids. On the right side, elements like chlorine and argon are nonmetals. This trend happens because the increasing nuclear charge holds electrons more tightly, making it harder for the atom to lose electrons and behave like a metal.

A Concrete Example: Period 3

Take period 3 as an example. It starts with sodium (Na) and ends with argon (Ar). Sodium has one electron in its outer shell and readily gives it away, making it a highly reactive metal. Magnesium (Mg) has two outer electrons and is also a metal, but less reactive than sodium. Aluminum (Al) is a metal, silicon (Si) is a metalloid, and phosphorus (P), sulfur (S), chlorine (Cl), and argon (Ar) are nonmetals. Chlorine is a very reactive nonmetal that wants to gain one electron. Argon is a noble gas with a full outer shell, so it rarely reacts.

How Periods Differ from Groups

People often confuse periods with groups. Groups are the vertical columns. While a period tells you the number of electron shells, a group tells you the number of electrons in the outer shell (for main group elements). Elements in the same group have similar chemical properties. For example, all elements in group 1 (the alkali metals) are highly reactive metals that have one outer electron. Elements in the same period do not have similar chemical properties; they show a gradual change from metal to nonmetal.

The Royal Society of Chemistry provides a clear interactive periodic table that shows both periods and groups Royal Society of Chemistry. You can click on any element to see its period and group.

Key Takeaways

• A row in the periodic table is called a period. There are seven periods.
• Elements in the same period have the same number of electron shells.
• Moving left to right across a period, atoms get smaller and metallic character decreases.
• The lanthanides and actinides belong to periods 6 and 7 but are placed below the main table.
• Periods are different from groups, which are vertical columns and indicate similar chemical properties.