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

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).

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,

The main difference between modern and earlier periodic tables is the organisation. In Mendeleev's periodic table, elements were predominately arranged by atomic mass. Mendeleev also identified patterns in element properties and put elements with similar chemical properties in the same groups. Elements in the modern periodic table are organised by atomic number (number of protons). This corrected inconsistencies in Mendeleev's periodic table and better aligned with periodic trends.

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.

I would argue that the main difference between the current periodic table and the earlier periodic table (apart from the new elements discovered) is the way they order the elements. They used to order them by their mass, until Mendeleev started to group elements together that had similar chemical properties - indeed, a periodic table. This led to him leaving gaps to fit elements into their appropriate groups, allowing for the prediction of new elements and their properties. The discovery of the proton allowed for a neat way to arrange the elements, giving us the modern periodic table, and confirming Mendeleev’s genius.

The earlier periodic table is also called as Mendeleev periodic table. The key difference between Mendeleev and Modern periodic table is that it is based on the atomic numbers, whereas, the basis in Mendeleev periodic table was atomic mass of the elements.

Mendeleev periodic table has 65 known elements.

There are 103 elements in modern periodic table with the newly discovered elements.

There are gaps for undiscovered elements in the Mendeleev periodic table.

In modern periodic table the elements are distributed unifromly.

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.