What Are Rows in the Periodic Table Called? A Clear Answer

If you have ever looked at a periodic table, you have seen the grid of squares that organizes all known chemical elements. The rows that run horizontally across the table have a specific name. These rows are called periods. There are seven periods in the standard periodic table, and each one tells you something fundamental about the elements it contains.

How Periods Are Organized

A period is a horizontal row. The first period is at the top, and the seventh period is at the bottom. Elements are placed in a period based on the number of electron shells their atoms have. An electron shell is the region around an atom’s nucleus where electrons are most likely to be found.

As you move left to right across a period, the atomic number of each element increases by one. Each element has one more proton in its nucleus and one more electron in its outer shell. The properties of the elements change in a predictable way across a period. You start with a highly reactive metal on the far left, move through less reactive metals and metalloids, and end with a nonmetal and finally a noble gas on the far right.

Period 1: The Shortest Row

Period 1 contains only two elements: hydrogen and helium. The first electron shell can hold a maximum of two electrons. Hydrogen has one electron, and helium has two, which fills the shell completely.

Periods 2 and 3: The Building Blocks

Periods 2 and 3 each contain eight elements. The second and third electron shells can each hold up to eight electrons before the next shell starts to fill. These periods include many elements essential for life and common materials, such as carbon, nitrogen, oxygen, sodium, and chlorine.

Periods 4, 5, and 6: The Transition Metals

These periods are longer, containing 18 elements each. They include the transition metals, which are the large block of elements in the middle of the table. These elements have electrons filling inner d-orbitals, which gives them properties like the ability to form colored compounds and multiple oxidation states. Period 6 also includes the lanthanide series, usually placed below the main table.

Period 7: The Heavyweights

Period 7 contains the heaviest naturally occurring elements, as well as many synthetic elements created in laboratories. This period includes the actinide series, also placed below the main table. All elements in period 7 are radioactive.

Why Periods Matter

The period number is a shortcut for understanding an element’s electron configuration. If you know an element is in period 3, you know its atoms have three electron shells. This is key because the number of shells influences the size of the atom and how strongly it holds onto its outer electrons.

For example, consider sodium (Na) in period 3 and potassium (K) in period 4. Both are alkali metals and have one electron in their outer shell. Potassium has one more electron shell than sodium. The outer electron is farther from the nucleus and held less tightly, so potassium is more reactive than sodium.

The Difference Between Periods and Groups

Periods are horizontal rows and tell you about the number of electron shells. Groups are the vertical columns and tell you about the number of electrons in the outer shell, which is the main factor in determining an element’s chemical behavior. Elements in the same group share similar chemical properties. Group 1 elements (alkali metals) all have one outer electron and are highly reactive. Group 18 elements (noble gases) all have a full outer shell and are very unreactive.

How Periods Reveal Electron Configuration Patterns

Each period corresponds to the filling of a new electron shell. The first period fills the 1s subshell. The second period fills the 2s and 2p subshells. The third period fills the 3s and 3p subshells. The fourth period introduces the 4s subshell before the 3d subshell, which is why the transition metals appear there.

This pattern explains why periods have different lengths. Period 1 has 2 elements because the 1s subshell holds only 2 electrons. Periods 2 and 3 have 8 elements because the s and p subshells together hold 8 electrons. Periods 4 and 5 have 18 elements because they include the d subshell, which holds 10 additional electrons. Period 6 has 32 elements because it includes the f subshell, which holds 14 more electrons. Period 7 is incomplete but would theoretically hold 32 elements.

The Historical Reason for the Period Name

The term “period” was chosen because the properties of elements repeat in a periodic way as you move down the table. When Dmitri Mendeleev first arranged the elements in the 1860s, he noticed that if he lined them up by increasing atomic weight, similar properties appeared at regular intervals. The modern periodic table, refined by Henry Moseley in 1913, uses atomic number instead of atomic weight. The International Union of Pure and Applied Chemistry (IUPAC) is the official body that standardizes the naming and numbering of the periodic table IUPAC. The Royal Society of Chemistry also uses this terminology Royal Society of Chemistry.

Key Takeaways

• The horizontal rows in the periodic table are called periods.
• There are seven periods in the standard periodic table.
• The period number indicates the number of electron shells an atom of an element in that row has.
• As you move left to right across a period, atomic number increases and properties change from metallic to nonmetallic.
• Periods are different from groups, which are vertical columns that indicate the number of outer-shell electrons.