Question

A. 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 both to be trigonal planar? Answer in American notation B. From what group must the terminal atoms come in an ABx molecule where the central atom is from group 7A, for the electron-domain geometry to be Octahedral and the molecular geometry to be square pyramidal? Answer in American notation

Answer 1

The group 6A is also known as the oxygen family (called the chalcogens), consists of the elements found in Group 16 of the periodic table and is considered among the main group elements. It consists of the elements oxygen, sulfur, selenium, tellurium and polonium. These can be found in nature in both free and combined states.

For a molecule to have a trigonal planar geometry, The molecule is made up of 3 equally spaced sp2 hybrid orbitals arranged at 120o angles. The shape of the orbitals is planar triangular. Since there is an atom at the end of each orbital, the shape of the molecule is also planar triangular.

In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120° . Such species belong to the point group D_3h.

For example the H₃O⁺, This molecule is electron deficient and does not follow the octet rule because it has only 6 valence electrons and the oxygen miss an electron. The hydrogen atoms are as far apart as possible at 120°. This is trigonal planar geometry. The molecule all in a plane and is two dimensional.

So, for a molecule in the ABx form where the central atom belongs to the group 6A, it's terminal atoms would have to be from the group 1A to do this (Hydrogen in this case).

Now I have a doubt with part B, so, post it in a new question thread. Hope this helps.