Question

The periodic table has undergone some changes since it was first introduced. List three ways the modern periodic table differs from the one first published by Mendeleev in 1869.
I have two of the three ways:
1: updating the atomic weight
2: adding newly discovered elements

However, I cannot come up with a last way the modern periodic table differs from Mendeleev's. Please help (:

Answer 1

One of the differences I can think of is that hydrogen is no longer listed as a group I element.

According to the mendeleev tables that I looked up, hydrogen is catorgrized as a group I element, along with Lithium, sodium, Potassium etc. However, nowadays, hydrogen does not belong to any groups in the periodic table. This is because there are arguments about whether hydrogen belongs to group I. Group I elements are all alkali metals, while hydrogen is not. However, some people says that hydrogen only have one outer shell electron so it should be in group I. Some people even say hydrogen should belong to group VII because it only needs one more electron in order to achieve the duplet of electrons.

Therefore as you may notice, hydrogen in modern periodic tables are put in the center of the periodic table on the top.