Why the table is that shape
The layout is not decorative. Each row is a period — one more electron shell than the row above. Each column is a group, and elements in a group share their outer electron arrangement, which is why they behave alike: the alkali metals down the left are all violently reactive, the noble gases down the right are all inert.
The lanthanides and actinides are pulled out underneath purely to keep the page a sensible width. Chemically they belong in the gap you can see in periods 6 and 7.
Masses in brackets
Most atomic masses are weighted averages across an element's naturally occurring isotopes — which is why chlorine is 35.45 rather than a whole number. Elements with no stable isotope have no such average, so the figure quoted is the mass number of the longest-lived isotope instead. Those are marked here with an asterisk.
Colouring by mass or period
Switching the colour scheme makes different patterns visible. By mass, the steady climb from top-left to bottom-right shows up as a gradient — along with the handful of places where it reverses, at potassium, nickel and iodine. By period, the shells stack as clean bands.
Frequently asked questions
Why is hydrogen above the alkali metals when it is a gas?
Because group position follows electron structure, and hydrogen has one outer electron like the alkali metals do. Its behaviour is genuinely unlike theirs, which is why many tables float it above the table or place it twice.
Is anything uploaded?
No. The element data is bundled with the page, so it works with no connection at all.
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