The next block describes the interaction between the nucleus and valence electrons of an atom. The first ionization potential is the amount of energy it requires to take away one electron in units of kJ/mol. Zeff is the effective nuclear charge that the outer electrons experience, considering electron shielding. Most of the block is dedicated to electronegativity scales. We did not include all the electronegativity scales that we could find, since most of the exotic EN scales were developed with a specific problem in mind and list only a limited number of values (e.g., Allen scale). In such cases, the scale was not included. The cutoff criterion was set as 80% of the values in a particular scale had to be present. Electronegativity was defined by Linus Pauling as "the power of an atom in a molecule to attract electrons to itself." Published in 1932, the Pauling scale has been one of the most used electronegativity scales. Later, various approaches (e.g., electrostatic force) were used to determine the effective charge experienced on valence electrons by the nucleus. Size scales in the next block of properties are the most important features in chemistry ML. There might be multiple ways to identify atomic radii ranging from experimental to theoretical methods. In addition, the radii depend on the coordination geometry and the predominant bonding in the structure. Identifying the radius in elements (consisting of one type of atoms) is as simple as dividing the bond distance by two. The sizes of elements in covalent compounds could be calculated to produce consistent values. However, in ionic compounds, the charge of the ion is the main factor contributing to the sizes, while coordination number plays an important role in compounds with metallic bonding. Selecting the right scale for each problem should be done through analysis of the sample composition and collecting element occurrence statistics. While it does not hurt to include as many scales as possible, mixing different scales in one mathematical formula to get a descriptor is not recommended, since it leads only to more noise in the x-vector.