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

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 difference is:

While the modern periodic table is based on atomic number, the earlier was based on atomic mass.

The main difference between the modern periodic table and Mendeleev's periodic table (the earlier periodic table) is that we order elements by atomic number whereas we used to order them by atomic mass.

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

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

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

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

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.

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 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.

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 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.

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).

Modern periodic table is arranged on increasing atomic number whereas old periodic table was arranged on increasing atomic weight.

Elements are arranged in increasing atomic number on the modern periodic table while they were arranged in atomic mass/weight in the earlier periodic table.