To understand how Mendeleev created the table in the first place, you have to go back in time and erase what you’ve learned about the table. Imagine for a moment that the distinctive shape of the present-day periodic table with its neat columns and rows doesn’t exist. You don’t yet know about protons and therefore atomic numbers, which, for the most part, conveniently run in order from one to 118 from left to right and top to bottom of the table.
All you know about the elements identified at the time is how they interact with each other, their physical properties, and their relative atomic weights. And you want to categorize them.
Before Mendeleev came along with his approach, other scientists were attempting to organize the elements. As early as 1789, French chemist Antoine Lavoisier had categorized elements into metals, nonmetals, “earths,” and gases, based on their physical and chemical characteristics. By 1829, German chemist Johann Döbereiner had noticed patterns among triplets of elements. In 1865, British chemist John Newlands noticed the periodicity of chemical properties and likened the phenomenon to musical octaves, in which the same tone repeats after an increase or decrease of eight notes. In Germany, chemist Julius Lothar Meyer was developing his own periodic table that was published in 1870. But Mendeleev beat Meyer to the punch a year earlier.
What sort of chemical properties did Mendeleev have in mind when he developed his table? To get a better idea of the patterns he noticed, let’s start with the metal lithium (Li). Mendeleev knew the hydride—a compound of hydrogen with another element—that Li formed had the formula LiH.
2 Li(s) + H2(g) → 2 LiH(s)
In contrast, the next element by weight, beryllium (Be), formed the hydride BeH2.
Be(s) + H2(g) →BeH2(s)
Each successively heavier element formed different kinds of hydrides until he got to sodium (Na). Sodium behaved like lithium in its reactions with hydrogen, forming NaH.
2 Na(s) + H2(g) → 2 NaH(s)
Thus, a pattern started to emerge.