Periodic Table
Click an element, or search by name, symbol or number.
Why the shape is the shape
The table is not a convenient arrangement. Its geometry follows directly from how electrons fill the space around a nucleus.
- Rows are periods — each one fills a new electron shell, further from the nucleus.
- Columns are groups — elements in a column have the same number of electrons in their outermost shell, which is what determines how they react.
That second point is the whole insight. Sodium and potassium behave alike because each has a single outer electron it is eager to lose. Fluorine and chlorine behave alike because each needs one more to complete a shell. Helium and neon do essentially nothing because their shells are already full.
The rows are different lengths for a reason
Period 1 holds two elements, periods 2 and 3 hold eight, and periods 4 and 5 hold eighteen. That is the capacity of the subshells being filled: s holds 2, p holds 6, d holds 10 and f holds 14.
The block of transition metals in the middle is the d subshell filling up. The two rows floated underneath — the lanthanides and actinides — are the f subshell, pulled out because putting them in place would make the table fourteen columns wider and unprintable.
Mendeleev left gaps, and that is why he is remembered
Several people noticed periodic patterns in the 1860s. Dmitri Mendeleev's 1869 table was the one that mattered because he trusted the pattern over the data: where no known element fitted, he left a gap and predicted the properties of whatever belonged there.
He described "eka-silicon" — its mass, density, colour and the formula of its oxide. Germanium was discovered in 1886 and matched almost exactly. A classification that predicts unknown things is a theory, not a filing system, and that is what settled the argument.
Reading the numbers
- Atomic number is the count of protons. It defines the element absolutely — six protons is carbon, no exceptions.
- Atomic mass is a weighted average across naturally occurring isotopes, which is why it is rarely a whole number. Chlorine's 35.45 reflects a mixture of chlorine-35 and chlorine-37.
- For elements with no stable isotope, the figure is the mass number of the most stable known one.
The end of the table
Everything past uranium is synthetic. Oganesson, element 118, has been made in quantities of a few atoms with half-lives under a millisecond.
Whether a period 8 exists is an open question: as nuclei get larger the forces holding them together lose to electrostatic repulsion. There may be an "island of stability" around element 120 or so where longer-lived isotopes exist, and there may not.
Common uses
- Looking up an atomic mass for a calculation.
- Finding what a symbol stands for — the ones from Latin catch people out.
- Revision and homework.
- Checking which group an element is in.
Questions people ask
Why is the periodic table shaped like that?
Because rows are electron shells and columns are elements with the same number of outer electrons. The row lengths follow subshell capacities: 2, 6, 10 and 14.
Why are the lanthanides and actinides shown separately?
They are the f-block. Placing them in position would make the table fourteen columns wider, so they are floated below.
Why are atomic masses not whole numbers?
Because they are weighted averages across naturally occurring isotopes. Chlorine's 35.45 reflects a mix of chlorine-35 and chlorine-37.
What made Mendeleev's table special?
He left gaps and predicted the properties of elements not yet discovered. Germanium, found in 1886, matched his prediction almost exactly.
How many elements are there?
118 are confirmed and named. Everything past uranium is synthetic, and the heaviest exist for fractions of a second.
Built into the page
The whole chart is part of the page, so it works with no connection and nothing is requested while you use it.