Elements 31–40 on the periodic table

The period changes after krypton

Gallium, germanium, arsenic, selenium, bromine and krypton complete the relevant part of period four. Rubidium then begins period five, followed by strontium, yttrium and zirconium. Remembering krypton as the noble gas at the boundary is a useful way to divide the ten answers into two smaller groups. It is a structural memory aid grounded in the table, rather than a rule based on how common or commercially valuable the elements are.

Atomic number stays fixed across isotopes

An isotope changes the neutron count while retaining the element’s proton count. Gallium remains number 31 and zirconium number 40 regardless of which isotope is present. Atomic weights, by contrast, can reflect natural isotopic mixtures and need not increase neatly at every step. Bromine and krypton are a useful nearby comparison when distinguishing the two columns on a periodic table. The answer order comes from atomic number, so rounded mass values should never be used to settle a position.

Names and symbols provide different clues

Gallium’s Ga and germanium’s Ge can be distinguished by their second letters, while yttrium uses the single-letter symbol Y. Zirconium’s Zr is another useful anchor at the end. Bromine and krypton may be more familiar as Br and Kr than as full names. The game accepts element-symbol aliases from the independent directory, but it does not treat a symbol as a separate answer. A misspelling that points to a different element should still be resolved carefully rather than guessed from one shared initial.

Why this is not a list of ten metals

The interval includes elements with different chemical classifications and physical states. Bromine is a liquid under ordinary room conditions, while krypton is a gas; several neighbouring entries are solids. Arsenic and germanium also illustrate why classroom category labels can be more nuanced than a simple metal/nonmetal split. The question avoids relying on those labels by specifying a numerical interval. This makes the set reproducible even when a reference uses a different visual colour scheme for chemical families.

Sources & further reading

• IUPAC — Periodic Table
• Royal Society of Chemistry: periodic table

Explanation updated Sunday, November 1, 2026. The puzzle’s original data date is listed below.