What Are Rows in the Periodic Table Called? Understanding Periods
The rows in the periodic table are called periods. There are seven periods, numbered 1 through 7 from top to bottom IUPAC.
A period is a horizontal row of elements arranged by increasing atomic number. Its number indicates the highest principal energy level occupied by electrons in the atoms of its elements. In introductory chemistry, this is often described as the number of occupied electron shells.
The word period comes from the repeating patterns in the properties of elements. As the table moves from one element to the next, chemical and physical properties change. Similar patterns recur in later rows, which is why the table is called periodic Britannica.
One example: period 3
Period 3 contains sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, and argon. These eight elements have very different properties. Sodium is a metal, silicon is a metalloid, chlorine is a nonmetal, and argon is a noble gas.
Their shared place in one period does not mean they behave alike. It means they have electrons occupying the same highest principal energy level. As atomic number increases from sodium to argon, the number of protons in each nucleus increases, and electrons are added to the atom’s electron structure. In this period, the third principal energy level is being filled.
The details involve subshells, so it is not quite right to say that every added electron simply goes into the same shell in the same way. The period number gives a useful guide to the highest occupied energy level, while the specific arrangement of electrons varies across the row.
Why periods have different lengths
The seven periods do not all contain the same number of elements. Their lengths are:
| Period | Number of elements |
|---|---|
| 1 | 2 |
| 2 | 8 |
| 3 | 8 |
| 4 | 18 |
| 5 | 18 |
| 6 | 32 |
| 7 | 32 |
These differences reflect how electrons fill energy levels and subshells. Period 1 contains hydrogen and helium. Periods 2 and 3 each contain eight elements. Periods 4 and 5 include the transition metals, so they each contain 18 elements. Periods 6 and 7 include the f-block elements, which are usually shown in two separate rows below the main body of the table.
The detached rows are a layout choice. The lanthanides belong to period 6, and the actinides belong to period 7. They are part of those periods even though many periodic tables print them below the main grid to keep the table from becoming too wide Britannica.
What a period tells you
The period number helps describe an element’s electron structure. For a neutral atom in its ground state, it corresponds to the highest principal energy level occupied by electrons. This is commonly taught as the number of occupied electron shells.
That makes the period useful when reading the table: it gives a quick structural clue about an element. It does not, on its own, tell you everything about how that element behaves. Elements in the same period can have very different chemical properties because their outer-electron arrangements differ.
A period also shows how properties change across a row. In broad terms, elements on the left tend to be metallic, while elements toward the right tend to be nonmetals. The row ends with a noble gas, except in period 1, which ends with helium. The exact properties vary by element, and trends across a period have exceptions.
Atomic radius generally decreases from left to right across a period. As proton number increases across the row, the nucleus has a stronger positive charge, while added electrons enter the same principal energy level. The increased attraction tends to draw the electron cloud closer to the nucleus.
Other properties also follow periodic patterns, though not all in a simple, uninterrupted way. Ionization energy generally increases across a period, but there are exceptions. Chemical behavior also varies across the row as the electron arrangements change. The recurring patterns across successive periods are the reason for the table’s name Royal Society of Chemistry.
Periods and groups
Periods are the horizontal rows. The vertical columns are called groups. In the modern IUPAC layout, groups are numbered 1 through 18 IUPAC.
The two directions give different information. A period number relates to the highest occupied principal energy level. A group helps locate an element among others with related electron arrangements and chemical properties. For example, elements in group 18 are noble gases. The period and group together give an element’s position in the table, but neither number alone describes all of its chemistry.
Key Takeaways
- The horizontal rows of the periodic table are called periods.
- There are seven periods, numbered from top to bottom.
- A period’s number indicates the highest principal energy level occupied by electrons in its elements.
- Periods contain different numbers of elements: 2, 8, 8, 18, 18, 32, and 32.
- The lanthanides and actinides belong to periods 6 and 7, even when printed below the main table.
