development of periodic table

Before the discovery of sub-atomic particles, how were elements classified?

By their physical and chemical properties, and by their atomic weight.

What is the atomic number of an element?

The number of protons in its nucleus (modern basis for periodic table).

Why were early periodic tables incomplete?

Not all elements had been discovered, so there were gaps and errors.

What problem arose when elements were arranged strictly by atomic weight?

Some elements ended up in the wrong group with others that had very different properties.

What repeating pattern did scientists notice when arranging elements by atomic weight?

Properties of elements repeated at regular intervals — called periodicity.

Who created the first widely recognized periodic table and when?

Dmitri Mendeleev in 1869.

How many elements did Mendeleev arrange in his table, and what did he do about missing elements?

He arranged 57 known elements and left gaps for undiscovered ones, predicting their properties.

Which elements were later discovered that confirmed Mendeleev’s predictions?

Gallium, scandium, and germanium — their properties matched his predictions.

How did Mendeleev’s table improve on earlier versions?

He prioritized chemical properties over strict order by weight and left gaps for undiscovered elements.

How did the discovery of isotopes support Mendeleev’s periodic table?

It explained why ordering by atomic weight sometimes gave “wrong” results — isotopes have different masses but the same properties.

Why was the modern periodic table arranged by atomic number instead of atomic weight?

Because atomic number (protons) gives the true order of elements, fixing misplacements caused by using weight alone.