The formula of the higher oxide formed by a group 3 element. The electronic formula of the outer level is 4s2, 3d5.

If the outer configuration level starts at 4s2, this means that the item is located in the 4th period of the table.
The total number of valence electrons for corresponds to the group number in the periodic table.
Valence electrons of this element: 4s2, 3d5, their total number = 7, which corresponds to the number of group (VII).
look at the table and find an element of the 4th period of the VII group – this is manganese.

You can also write a complete electronic formula and count the number of electrons, this will be the ordinal number of the element:
1s2 2s2 2p6 3s23p6 4s23d5.
Let’s sum up the number of electrons:
2 + 2 + 6 + 2 + 6 + 2 + 5 = 25, we check against the table, the element with the ordinal number 25 is manganese, then everything is correct.

According to the periodic table, we look, for the elements of group VII, the formula of the higher oxide R2O7, which means for our element, the formula of the higher oxide Mn2O7.

Answer: Mn2O7.