Beryllium is the 4th element on the periodic table because it has 4 protons in its nucleus. It therefore has 4 electrons. The first two occupy an inner core shell, which is the 1s2 di-electron, just like helium’s. This first shell is surrounded by two more electrons in a second shell, forming a 2s2 di-electron. This makes beryllium less willing to lose an electron than lithium, giving beryllium a higher ionization energy and making it less reactive than lithium.
Another view of the spherical beryllium atom. The 2nd shell introduces a significantly larger volume to the atom, which is why it can accommodate four times as many electrons as the 1st shell.
While beryllium can become an ion, it does so less readily than magnesium (which has the same electron configuration but is one shell larger). Beryllium can be convinced to lose its two valence electrons to more electronegative atoms in an ionic interaction, and it will lose both at the same time in order to reach the stability of the 1s2 di-electron state. That is why beryllium forms a 2+ ionic state. The positive ion is much smaller than its neutral version because protons now outnumber electrons by two. This results in a much higher effective nuclear charge — a higher average attraction by the nucleus on each electron, which shrinks the size of the electron shell as it is attracted inward with more force.
Beryllium is also the only metal in Group II that is diamagnetic. This may be due to the fact that the 2s2 di-electron state is small and well-bound, rendering the valence electrons less susceptible to aligning with an external magnetic field.
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