A chemist arranged known elements so repeating properties lined up, even if it meant leaving blank spaces for elements not yet found. Which scientist is described?

Which scientist used X-ray spectra to show that each element has a unique whole-number nuclear charge?

A scientist predicted that the blank space called eka-aluminum would be filled by a metal with specific properties. Who made that prediction?

Who provided evidence that the periodic table should follow increasing atomic number rather than strictly increasing atomic mass?

In an early periodic table, an element could be placed where its chemical family fit even if its measured mass seemed slightly out of order. Which scientist used that approach?

A scientist's nuclear charge measurements showed that elements were still missing at certain atomic numbers, such as 43 and 61. Which scientist made that advance?

A blank space was left under aluminum in an early periodic table. What later discovery supported that choice?

Why were some positions left empty instead of forcing known elements into every spot?

A predicted element was later discovered with properties close to those expected for eka-silicon. Why was that a strong test of the table?

An early table sometimes challenged accepted atomic masses when an element's properties fit a different family. What evidence was being emphasized?

When argon and the other noble gases were discovered in the 1890s, how were they added to the periodic table?

Tellurium has a slightly greater atomic mass than iodine, but it belongs before iodine so each element stays with similar properties. What ordering rule explains this?

In the modern periodic table, what value increases by one from each element to the next?

Why do isotopes of the same element occupy one position in the modern periodic table?

If two elements appear out of order by average atomic mass, why can the modern table still place them correctly?