Period (periodic table)

A period is a single horizontal row of chemical elements in the periodic table.

Each period tells us something important about how an element is built. All elements in a single row have the same number of electron shells. These shells are the layers where electrons move around the center of an atom. As you move from left to right across a period, each new element has one more proton. This change also makes the elements look less like metals as you go.

This specific arrangement helps scientists predict how different elements will behave.

To understand how these rows work, we look at how electrons fill up. The Madelung rule describes the order in which energy levels are filled. As the atomic number of an element grows, the electron shells fill in a specific way. This filling process is what creates the rows we see in the table. Each completed shell corresponds to a new row or period. In some areas, like the d-block, the trends across the rows are very clear. In other areas, like the f-block, elements in different periods can look very similar.

Scientists have discovered and confirmed a total of 118 elements so far. These elements are organized into seven complete periods. The first period is the shortest with only two elements: hydrogen and helium. Hydrogen is the most abundant element, making up about 75% of the universe's mass. Helium is the second-most abundant and was mostly made during the Big Bang. Period 2 contains elements like carbon, nitrogen, and oxygen, which are vital for life.

These elements are the building blocks for almost every living thing.

Different periods contain very different kinds of matter. Period 3 elements, like sodium and magnesium, all occur naturally on Earth. Period 4 is the first to include the d-block of transition metals. This includes iron, which is a major part of our planet. Period 6 is the first to include the f-block, known as the lanthanides. Period 7 is unique because every single element in it is radioactive. This means they are unstable, and many were made by scientists in labs.

Some of these elements are so rare that they only exist for a tiny moment.

Learning about periods helps us see how the whole universe is connected. You can see these elements in your own body or in the stars. For example, oxygen is the main part of the water you drink. Carbon is the basis of every important biological compound in your cells. Even the silicon in sand follows these same rules of the periodic table.

By studying these rows, we learn how everything from a tiny atom to a huge star works. The table is like a map for all the matter in existence.