Final answer:
To construct a model of a square planar molecule with a central atom A, place A in the center and arrange four identical atoms at the corners of a square. This reflects the octahedral electron-pair geometry with two lone pairs on opposite sides of A, which minimize repulsion and are not shown in the model.
Step-by-step explanation:
The question concerns constructing a model of a molecule with a square planar arrangement. In such a molecule, the central atom A is surrounded by four identical atoms in a plane. The electron-pair geometry for such a molecule is typically octahedral, with six regions of electron density around the central atom. When there are two lone electron pairs in addition to the four bonding regions, a square planar shape occurs. These lone pairs reside on opposite sides of the central atom (180° apart), reducing the repulsion between them.
To construct a model of a square planar molecule, one would place the central atom in the center and arrange the four identical atoms at the corners of a square surrounding the central atom. This would reflect the octahedral electron-pair geometry where the lone pairs are not visible in the molecular model but are considered to be present at 180° to each other, orthogonal to the plane of the square.
For instance, in a tetrahedral arrangement, if there are four atoms attached to the electron groups, the shape of the molecule would also be tetrahedral, like methane (CH4). However, this is not the desired shape for a square planar molecular structure.