The square planar molecular geometry in chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 describes the stereochemistry (spatial arrangement of atoms) that is adopted by certain chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s. As the name suggests, molecules of this geometry have their atoms positioned at the corners of a square on the same plane about a central atom.
The addition of two ligands to linear compounds, ML₂, can afford square planar complexes. For example, [AuCl₂]⁻ adds chlorine to give square planar [AuCl₄]⁻.
In principle, square planar geometry can be achieved by flattening a tetrahedron.