The atomic number of an element indicates the number of protons in the nucleus of an atom. In this question, the atomic number of the element is given as 8. This means that the element is oxygen since oxygen has 8 protons.
To form a compound with sodium, which has an atomic number of 11, the element with 8 electrons will gain electrons. Since oxygen already has 8 electrons, it needs to gain 2 more electrons to complete its outermost energy level.
Therefore, the correct answer is option 2, which states that the element will gain two electrons to form a compound with sodium. This is because the element wants to achieve a stable electron configuration by filling its outermost energy level with a total of 8 electrons, which is the same as the electron configuration of a noble gas.
By gaining two electrons, the element will have a total of 10 electrons, completing its outermost energy level. This will allow it to form a compound with sodium and achieve a stable electron configuration.