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

The earlier periodic table (made by Mendeleev) was grouped by atomic mass(The number of protons + the number of neutrons) rather than atomic number ( just the number of protons). Also, as Mendeleev was not alive during the discovery of the noble gases (and other elements) there were quite a few "holes" in his version of the periodic table. However, Mendeleev was smart enough to predict where some holes could be.

The very first table had much fewer elements and they were ordered by mass, not always grouping according to properties. Mendeleev understood that gaps would need to be left for elements not yet discovered and he predicted what those elements would be like. He also swapped some of the elements, ignoring the mass which was then discovered to be correct when atomic number was later used to order the elements.

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.

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 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 older periodic table derived in the 19th century arranged the known elements (more were discovered after the production of this periodic table) based on their atomic mass. The atomic mass identifies the total number of protons and neutrons in the nucleus of the element and is shown on the element information usually below the element symbol. The modern table arranges element according to their atomic number (number of protons) which is the number usually shown above the element symbol.

In the earlier periodic table, often attributed to Dmitri Mendeleev, elements were arranged in rows in order of increasing atomic mass. Elements in the same column shared similar properties. This enabled him to predict new elements, that had not yet been discovered, and to predict the properties that they would have.

The main difference is that the elements in early versions of the periodic table were sorted by mass number, but in the modern table they are sorted by atomic number.

Some more background on this:

The very first chemists first sorted the elements by mass and looked for patterns in the list. Newlands noticed that patterns seemed to repeat every 8 elements (hence the table he deviced became known as Newland's octaves). But this pattern broke down due to the strict ordering by atomic mass. Some elements were placed in the same groups that didn't have the same properties like iron and oxygen.

Mendeleev came closer with his early version of the periodic table, at the time he had no knowledge of subatomic particles like protons, neutrons and electrons.hen he first sorted the elements it was by atomic mass. Because of this he first sorted by the elements by mass, but critically he made exceptions to this rule and swapped some elements around when the chemical properties didn't fit in with the group they were sorted into. He also left gaps where he felt elements should be. That was his genius insight. Those gaps allowed his table to be tested and proven right. Those elements have since been discovered on the modern periodic table and they fit right in where he predicted they would.

The modern periodic table sorts the elements by their atomic number (which tells you the number of protons in the nucleus) and not by mass.

Early versions of the periodic table were incomplete and elements were placed with others that they had nothing in common with. Dimitri Mendeleev in 1869 developed an early version of the periodic table where he arranged chemical elements by their atomic mass. He predicted that there were other elements out there that were yet to be discovered.

The arrangement of the modern periodic table is in increasing atomic number (proton number in the nucleus of the atom of the element) from left to right in the table.

The original periodic table was created before detailed knowledge of atomic structure was known and was arranged in order of the mass of the atom of the element.

Mendeleev's periodic table orders the elements based on their atomic mass whereas Modern periodic table orders the elements based on their atomic number

In the earlier periodic tables the elements were organised by mass. In modern periodic tables the elements are organised. by atomic number,

The earlier periodic table ordered elements by their atomic mass. Our modern periodic table orders elements by their atomic proton number.

The main difference is that we have discovered so many more elements in the last 200 years!

Early versions of the periodic table, such as Mendeleev's Table, used to organise elements by their atomic mass (number of protons + neutrons), rather than the current table which uses atomic number (strictly number of protons). Furthermore, early versions of the table featured lots of gaps for elements not yet discovered. Over time, these elements are found and added into the gap left for them, for example germanium. Early versions would also attempt to categorise elements by their chemical properties, whilst the current version categorises them based on electron configuration, which therefore determines chemical properties (such as Group 8/0 being inert).