Why is lithium ionic and beryllium covalent

The periodic table of the elements online

What is the oblique relationship?

The oblique relationship in the PSE describes the fact that the starting members of the main group elements from lithium to oxygen behave more like lighter homologues of the group adjacent to the right from the second homologue onwards. In plain language this means that the main groups "offset one place to the right and below" have more similarities than in the direct vertical row. This results in six oblique relationship pairs: Clearly pronounced in lithium in its similarity to Mg instead of Na. Strongly pronounced in Be in its resemblance to Al instead of Mg. Very pronounced in boron in its similarity to silicon, instead of aluminum. Weakly pronounced for carbon and phosphorus, nitrogen and sulfur. Clearly pronounced in the pair of oxygen and chlorine versus fluorine and chlorine, or oxygen and sulfur.

What are the reasons for this phenomenon?

The ratio of ionic radius to charge can serve as a rule of thumb for the chemical behavior of elements in the reactions. When comparing the elements of the second to the third period, it is noticeable that the elements shifted by one group to the right are more similar than their own homologues: