Hydrogen (H) and helium (He) are two elements with things in common beyond their initial h. They are both gas at room temperature and nonmetals. They are the elements with the lowest atomic number and the most abundant elements in the universe. However, they do have differences. Hydrogen can be combined with most elements of the periodic table, whereas helium is a noble gas and forms no compounds.
But one of their most fascinating shared trait is that they defy the systematic classification of the periodic table. In the periodic table, elements are ordered by increasing atomic number and each column is a group containing elements with similar chemical properties. They are similar because the most external electrons of the elements in the same group have the same configuration. Hydrogen and helium are peculiar because they don’t fit in the system.
Hydrogen is so different from any other element that the most ordered option is to place it at the top of group 1, the first column, because of its electronic configuration, in spite of not sharing the typical properties of said group. On the other hand, according to its electronic configuration, Helium should be placed at the top of group 2. Helium shares the properties of noble gases, though, justifying its placement in group 18, the last column, where noble gases are.