Next seven columns describe the position of elements in the periodic table with atomic number, period, group number, family number, metal/nonmetal classification, and l quantum number. Atomic number and weight are excellent sources for features, where structure or property is sensitive to what normally is described as light and heavy elements or early and late elements. For instance, polymorphism or phase transition is common for series, where even tiny changes in atomic weight (like in rare-earth series) coincides with gradual change in atomic sizes. Period and group numbers are the most typical properties that describe the position of an element in the periodic table, basically acting like x and y coordinates. The group number is the quintessential sorting of elements according to their properties, defining their chemistry, therefore would be critical for most ML studies with chemical compositions. For this block of properties, a typical data processing routine would be estimating if a given compound is ionic, covalent or metallic, based on the position of the constituent elements in the periodic table. The columns in the property table (and summary in Table ) are color coded, which is helpful for identifying the origin of this data or proximity of the properties to each other. A helpful approach in generating more features is exploring additional relationships and not limiting yourself to calculations only within the property (e.g., group number difference, group number sum, etc.) but also trying to mix the properties in the same color-coded block. For example, the period number divided by the group number. This gives a new dimension to the features, converting a position in the periodic table in a 2D coordinate system (previously mentioned x and y coordinates) summarized in one number, which also carries information about diagonal similarities of the elements. There is no limit to creativity in this approach. There are multiple ways elements could be ordered or grouped. For example, Mendeleev number, ordering where elements with similar properties follow each other, or the family number, introduced by Oliynyk et al, where numbers are assigned to clusters of elements with similar properties in the periodic table. There are a total of 9 family numbers, each number with the corresponding group as follows: (1) alkali metals, (2) alkaline-earth metals, (3) f-block metals, (4) d-block metals, (5) p-block metals, (6) p-block metalloids, (7) p-block nonmetals, (8) chalcogens, and (9) noble gasses. Family numbers are new to chemical properties and were introduced in the early stages of the development of the list of properties. It was included to describe the similarities and differences of the elements from a solid state synthetic perspective. Including new properties developed by the researchers is a great way to make the feature list specific to your study. There is no such thing as adding a "wrong" property. As long as one can justify the number associated with each element, the property will work, even if the rationale behind the inclusion is based on chemical intuition. Next thirteen columns represent various numbers of electrons or valence. Classifying compounds based on the total of valence electrons is a common approach (e.g., 18-electron rule or octet rule). These numbers happen to be the most helpful in property predictions, where properties gradually change with element substitutions (e.g., band gap engineering or hardness optimization). Metallic valence refers to the total number of electrons which can form covalent bonds and thus steadily increase the total strength of the bonds. While there might be multiple ways to count valence electrons, we simply summed up the outer shell of electron configuration, everything that occurs after the inert gas notation in the second column of our table. Similarly, valence s, p, d, and f electrons were counted. The lacking electrons to fill the orbitals completely are summarized in no. of unfilled s, p,d, and f respectively. The outer shell electron numbers are the valence electrons only from the highest energy orbital. For non-transition metals, Gilman considered the number of valence electrons as equal to the group number. For transition-metals, however, Gilman considered that the electrons in the s-state remain less vital to bond formation than the electrons in the d-states. Therefore, the s-state electrons are neglected in counting the number of valence electrons. For example, Lutetium (Lu), Tantalum (Ta), and Rhenium (Re) have valence electrons of 1, 3, and 5 respectively. Second, Gilman et al. considered that the next d-electron is added after 5 electrons; the newly added electron enters an anti-bonding orbital. Therefore, the effective valence of d-electrons are known to decrease. For example, Osmium (Os), Pt (platinum), and Hg (mercury) are known to have effective valence electrons of 5, 3, and 1 respectively.