The periodic table
118 elements arranged so that families and trends become visible. Find an element, compare it with its neighbours, then use its position to understand what it shares with other elements — and what sets it apart.
Hydrogen
Nonmetal · Gas
Empty spaces for elements that had yet to be discovered.
In 1869, Dmitri Mendeleev tried to organise the known elements while taking their chemical properties into account. When he arranged them this way, he saw that some positions ought to be occupied even though no known element fitted them.
He chose to leave those positions empty. For several of them, he even estimated the properties that the still-unknown elements should have. The later discoveries of gallium, scandium and germanium showed the power of this approach.
The table used today is no longer ordered in exactly the same way. Elements are arranged by atomic number — the number of protons in the nucleus — and electronic structure then helps explain why certain chemical properties recur periodically.
Why are Li, Na and K placed one below another?
Lithium, sodium and potassium belong to group 1. Their position in the same column reflects a similarity in electronic structure that also appears in their chemistry.
The numbers below the symbols show a simplified distribution of electrons by shell: lithium has 2, then 1; sodium 2, 8, 1; and potassium 2, 8, 8, 1.
In all three cases, the outer shell contains one electron. This helps explain their shared properties: alkali metals very often form ions with a +1 charge.
Moving down the column adds an occupied electron shell while the outer pattern repeats. It is one of the simplest examples of the periodicity visible in the table.
Understand electron arrangement →Understand the table, step by step
Four ideas already explain much of what we observe in the periodic table.
Science is already all around us.
Start with a familiar object, action or question, then work back to the elements that explain it.
What is the world around us made of?
Phone, coffee, tap water, bicycle: ordinary objects are often the best doorway into chemistry.
What is really inside a smartphone?
Follow an object part by part and discover the elements that make its screen, battery, chips and connections possible.
Elements that resemble one another
Why do noble gases react so little? Why are alkali metals so reactive? Families give the table meaning.
Move from one unit to another
Temperature, pressure, energy, mass and much more: practical tools with explanations and examples.