An atom of an element A has three electrons in its outermost orbit and that of B has six electrons in its outermost orbit. The formula of the compound between these two will be

The correct option is A₂B₃.

To determine the chemical formula of the compound formed between element A and element B, we can analyze their valencies based on the number of electrons in their outermost orbits (valence shells):

1. Valency of Element A:
Element A has three electrons in its outermost orbit. To achieve a stable octet configuration, it can easily lose these 3 valence electrons. Therefore, the valency of element A is 3 (it forms $A^{3+}$ ions).

2. Valency of Element B:
Element B has six electrons in its outermost orbit. To achieve a stable octet configuration (8 electrons), it needs to gain 2 electrons. Therefore, the valency of element B is 8 - 6 = 2 (it forms $B^{2\−}$ ions).

3. Deriving the Formula:
Using the crossover method (criss-cross method) to combine the valencies:
- Write the symbols of the elements side by side with their valencies: A (valency 3) and B (valency 2).
- Cross over the valency numbers to write them as subscripts for the opposite elements.
- This gives 2 as the subscript for A, and 3 as the subscript for B.

Thus, the chemical formula of the compound formed is A₂B₃.