What is the main difference between the modern and earlier periodic table?

The first periodic table discovered had many gaps in in. Dmitri Mendeleev discovered many of the first elements but did not discover all of them. The more modern periodic table, has over 100 elements in it and Mendeleev predicted that some elements would slot in to certain gaps (due to similarities in physical and chemical properties).

The earlier periodic table involved elements being ordered in atomic mass, whereas the modern day periodic table includes elements being ordered in their atomic number. This is due to the presence of neutrons in each element, which was unknown during the earlier periodic table, as well as the presence of isotopes.

Also, the elements not being strict ordered in their atomic number (in the modern day periodic table) if the element fits the properties of one group better.

The main difference between the Earlier (Mendeleev) and the Modern Periodic Table is that Mendeleev's periodic table orders the elements based on their atomic mass whereas Modern Periodic table orders the elements based on their atomic number. This was because the earlier periodic tables were incomplete and some of the elements were placed in inappropriate groups if the strict order of atomic mass was followed. Mendeleev overcame some of the problems by leaving gaps for elements that had not yet been discovered. Later on the Modern Periodic Table became more convenient and elements were placed in appropriate groups and arranged in order of their atomic (proton) numbers.

earlier mode; based on atomic mass ( Mendeleev ) .Current model based on Atomic number

Hi Annalise - the main difference between the modern Periodic Table (the one you see in every textbook) and earlier Periodic Tables (such as those by Dalton, Dobereiner and Newlands) is that our current periodic table shows not only a list of every known element, but also shows which elements have similar properties by putting them in the same group or period (column or row).

Mendeleev, the designer of what we consider the "modern periodic table" grouped elements by their properties, and noticed that the pattern repeated every few elements. Because of this, he placed them in repeating row, or 'periods' - this meant that reactive metals such as Lithium, Sodium and Potassium end up in the first position (or group 1), the halogens (fluorine, chlorine and bromine) ended up above one another in group 7. However, the pattern didn't always work, and some elements seemed to be in the wrong position - so instead of forcing his ideas on the elements, Mendeleev left gaps so that any elements he may not have known about at the time. Later on, these gaps were able to be filled perfectly by new elements with similar properties, which proved to the scientific community that Mendeleev's table worked!

I hope this helped!