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On the left‐hand side of the periodic table the row numbers 1–7 indicate the highest principle quantum level in which electrons occur in the elements in that row. Every element in a given horizontal row has its outermost electrons in the same principle energy region or level. Within each row, the number of electrons increases with the atomic number from left to right. The number of elements in each row varies, and reflects the sequence in which electrons are added to various quantum levels as the atoms are formed. For example, row 1 has only two elements because the first quantum level can contain only two 1s electrons. The two elements are hydrogen (1s₁) and helium (1s₂). Row 2 contains eight elements that reflect the progressive addition of 2s, then 2p electrons during the formation of lithium (helium + 2s₁) through neon (helium + 2s₂, 2p₆). Row 3 contains eight elements that reflect the filling of the 3s and 3p quantum regions respectively during the addition of electrons in sodium (neon + 3s₁) through argon (neon + 3s₂, 3p₆) as indicated in Table 2.3. Row 4 contains 18 elements which reflects the addition of 10 3d‐subshell electrons after the 4s electrons and before the 4p elections. Row 5 contains 32 elements because of the addition of 14 3f electrons after the two 5s electrons prior to the addition 10 4d subshell electrons and the six 5p electrons (Figure 2.5). The process continues through rows 6 and 7 ending with uranium. The rows that contain up to 14 f‐subshell electrons are too long to fit conveniently in a table, so the elements (lanthanides and actinides) to which f‐subshell electrons have been added are shown separately at the bottom of the table. In summary, elements are grouped into rows on the periodic table according to the highest ground state principle quantum level (1–7) occupied by their electrons. Their position within each row depends on the distribution and numbers of electrons within the principle quantum levels.

Table 2.3 Periodic table of the naturally‐occurring elements displaying atomic symbols, atomic number (Z), average mass, ground state electron configuration, common valence states and electronegativity of each element.