Lithium and Beryllium are small atoms and when in the form of ions, have higher charge density (charge/volume ratio). Thus they have a very high tendency to distort their counterpart anion's electron cloud. According to Fajan's rules, in a molecule, smaller the size of the cation and smaller it's size(i.e high charge density), greater is the covalency of the bonds they form with the anions. So due to small size and high charge density Li and Be form predominately covalent compounds.
Further since charge on Be ion is more than Li ion and size of Be(2+) ion is smaller, Be(2+) forms more covalent compounds than Li+.

Introduction:
Lithium (Li) and beryllium (Be) are both elements in the periodic table. Although they are in the same group (Group 1) and have similar properties, they are not predominantly ionic. In this response, we will explain in detail why lithium and beryllium are not predominantly ionic.

Atomic Structure:
- Lithium and beryllium both have small atomic radii, which means that their valence electrons are relatively close to the nucleus.
- Lithium has three electrons, and beryllium has four electrons in their outermost energy level.
- The valence electrons of lithium and beryllium are shielded by the inner electrons, reducing the attraction between the valence electrons and the nucleus.

Ionization Energy:
- Ionization energy is the energy required to remove an electron from an atom or ion.
- Lithium and beryllium have relatively low ionization energies compared to other elements.
- This low ionization energy indicates that it is easier to remove their valence electrons, making them less likely to form stable ions.

Electronegativity:
- Electronegativity is a measure of the ability of an atom to attract electrons in a chemical bond.
- Lithium and beryllium have low electronegativity values, indicating that they have a weaker ability to attract electrons.
- This weak electronegativity makes it less likely for lithium and beryllium to form predominantly ionic compounds.

Covalent Character:
- Lithium and beryllium have a tendency to form covalent bonds due to their low electronegativity and small atomic size.
- In a covalent bond, electrons are shared between atoms rather than transferred completely.
- This sharing of electrons in covalent compounds leads to a partial ionic character, but not predominantly ionic.

Conclusion:
In conclusion, lithium and beryllium are not predominantly ionic due to their atomic structure, low ionization energy, low electronegativity, and tendency to form covalent bonds. These factors contribute to the formation of compounds with a partial ionic character but not predominantly ionic.