Modern Periodic Law and the Present Form of the Periodic Table

Moseley's modern periodic law states: the physical and chemical properties of the elements are periodic functions of their atomic number. Replacing atomic mass with atomic number as the ordering property fixed every anomaly in Mendeleev's table at a stroke — cobalt (atomic number 27) now correctly precedes nickel (atomic number 28) even though cobalt's atomic mass is slightly higher, and tellurium (atomic number 52) correctly precedes iodine (atomic number 53) for the same reason. The noble gases, discovered soon after Mendeleev's table was published, slotted in naturally as a new group once atomic number became the ordering principle.

The long form of the periodic table, built directly on the modern periodic law and on the Aufbau filling order of atomic orbitals, is the version used today. It has:

• 7 periods (horizontal rows), numbered 1 to 7. An element's period number equals the highest principal quantum number ($n$) of any electron shell it uses — Period 1 elements ($H$, $He$) use only the $n=1$ shell, Period 3 elements use shells up to $n=3$, and so on. Moving left to right along a period, electrons are added one at a time to the same outermost shell.
• 18 groups (vertical columns), numbered 1 to 18 in the current IUPAC convention. Elements in the same group generally have the same number of valence electrons (electrons in the outermost shell) and hence similar chemical behaviour — e.g. every Group 1 element (except hydrogen, which behaves atypically) has one valence electron and forms a $+1$ ion.

Blocks. Elements are further divided into four blocks according to which subshell receives the last (differentiating) electron in their ground-state electronic configuration:

• $s$-block — Groups 1 and 2 (plus helium, placed in Group 18 for chemical reasons despite its $1s^2$ configuration). The last electron enters an $s$ subshell.
• $p$-block — Groups 13 to 18. The last electron enters a $p$ subshell.
• $d$-block (the transition elements) — Groups 3 to 12, appearing only from Period 4 onward once the $3d$ subshell begins filling. The last electron enters a $(n-1)d$ subshell. …