If you’ve been inside a science classroom, chances are you’ve seen the periodic table of elements. 

It’s also a popular nerdy symbol — you might find it on a mug or a t-shirt, too! 

The periodic table of elements with each group of elements having a different background color. Image by Offnfopt via Wikimedia Commons.

Click here for an accessible version of the period table. 👈

But have you ever wondered why the periodic table looks the way it does?  

It’s not a typical table with even rows and columns — it has breaks and gaps, and looks like a castle. 

A castle made of yellowed brick with tall sections on the left and middle right and lower, boxy parts. Photo by Nabih El Boustani on Unsplash

What if I told you that weird shape is intentional, and even helpful when you are studying chemistry?

Organizing the Elements

A lot of the periodic table's shape comes from its construction.

Dmitri Mendeleev was a Russian scientist who created the first periodic table of elements.

Alack and white photo of Mendeleev. He has long, white hair and a beard and is wearing a blazer, sitting, looking to the left Image from Brady Haran, CC0, via Wikimedia Commons

What's special about his work is how he organized the elements into a meaningful pattern:

  • Elements are ordered by number of protons and increasing mass.

  • Rows and columns line up elements with similar properties.

Icon of the periodic table with rainbow colors and no text. Once you know how to read the periodic table's layout, you, too, can understand the huge amount of information it contains!

Reading the Periodic Table

The periodic table is read left to right, top to bottom. Some rows have gaps (like row 1) to line up elements into columns and rows based on their properties.

Though it's not a perfect grid, the periodic table can be read like a map. That means we can use periods and rows like coordinates to locate elements.

Icon of a mostly unfolded map with a red location pin pointing to the middle.


A period is a row from left to right. There are 7 periods on the periodic table.

Ex: Lithium (symbol = Li) and Carbon (symbol = C) are both in period 2, or the second row.


A group is a column from top to bottom. There are 18 groups on the periodic table.

Ex: Lithium (Li) and sodium (Na) are both in group 1, the first column.

Locating an Element

If you know what period and group an element is in, you can locate it exactly on the periodic table.

For example, lithium is the only element in both period 2 and group 1!

Icon of Lithium's box on the periodic table. Li in the center, 3 in the top right corner.

The simplest definitions of periods and groups: they're rows and columns on the periodic table.

But there's so much information you can tell about an element based on which period and group it's a part of. Let's take a closer look!


Review the periodic table above. What element is located in period 4, group 6?

Periods = Rows

Rows on the table are called periods — that's where the name "periodic table of elements" comes from. Periodic here means "occurring at regular intervals."

Many elements in the same period have similar physical properties, and patterns of those properties occur at regular intervals — they're periodic!

Icon of a rainbow colored periodic table with no letters or words on it. Examples of Properties:

From left to right, across a period:

  • Metallic character (being shiny and formable) decreases

  • Number of protons in the nucleus increases

  • Atomic radius (how big the atom is) decreases

Icon of a dark-skinned female-presenting scientist with dark hair wearing a lab coat with a flask of green liquid.

Example: Sashary wants to know more about the properties of nickel (Ni). She looks at the periodic table and sees that iron (Fe) is in the same period as nickel (Ni). She knows that iron is shiny, silvery metal and (correctly) concludes that nickel is similar because they are both in period 4.

A close-up image of a pile of silver dollar coins.

Groups = Columns

A group of elements are in the same column, reading from top to bottom, and are labeled 1-18.

Elements in the same group have similar reactivity — they react with other elements in similar ways.

Icon of a male-presenting scientist with tan skin and dark hair wearing goggles and a lab coat. Example: Rafael wants to know more about the element bromine (Br). He looks at the periodic table and sees that chlorine (Cl) is in the same group as bromine (Br). He knows chlorine is very reactive because he's a lifeguard and adds chlorine to the pool to kill germs. He correctly assumes that bromine is also very reactive since it's in group 17 with chlorine.

Photo of a colorful beach ball floating on the blue, reflective water of a swimming pool.

Periods vs. Groups

Now that you have a deeper understanding of periods and groups on the periodic table, let's summarize:

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  • Horizontal rows

  • 7 total

  • Elements have similar physical properties like: metal/nonmetal, size, number of protons

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  • Vertical columns

  • 18 total

  • Elements have similar reactivity —chance they'll have a chemical reaction


Shiloh wants to know more about element 27, cobalt (Co). Which of these elements will cobalt be similar to, and why?

Take Action

Nice work! Now you know that periods are rows, and groups are columns that organize elements into the periodic table based on their properties.

A teacher stands in front of periodic table poster and says That means you're ready to learn even more about chemistry:


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