From what group must the terminal atoms come in an abx molecule where the central atom is from group 6a, for the electron-domain geometry to both be trigonal planar

First of all, I need to complete some missing parts of the question, so here I copy it.

From what group must the terminal atoms come in an ABₓ molecule where the central atom is from group 6a, for the electron-domain geometry and the molecular geometry to both be trigonal planar

Explanation:

In the molecule ABₓ, x a subscript.

A trigonal planar molecule is a symmetric molecule with one atom in the centre surrounded by three atoms, all in the same plane, forming a triangle with bond angles equal to 120°.

In this case we want the central atom to be oxygen, so our molecules must be of the kind AB₃, where A is the central atom, from group 6a, and B represents the terminal atoms.

Group 6a is the group of O, S, Se, Te, and Po. The atoms of these elements have 6 valence electrons.

So, the central atom will have 6 valence electrons. To be symmetrical, this central atom will have three pairs of electons separated 120°.

So, each of the terminal atoms must be willing to form covalent bonds by with two electrons, which means that, to meet the octet rule, the terminal atoms must have 6 electrons each. Thus, the terminal atoms also must have 6 valence electrons, and you conclude that they belong to the same group 6a (or group 16 as per the modern periodic table).

One example of these kind of compounds is SO₃.

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