Concept 01 / 05Classification of Elements and Periodicity
The Modern Periodic Table: Periodic Law, Blocks and Locating Elements
From Dobereiner, Newlands and Mendeleev to the modern periodic law based on atomic number, the long form of the table with 18 groups and 7 periods, s, p, d and f blocks and their general configurations, predicting period, group and block from a configuration, and IUPAC names for new elements.
Early classifications. Dobereiner's triads grouped elements in threes where the middle element's atomic mass was roughly the average of the other two (Li, Na, K). Newlands' law of octaves noticed that properties repeated every eighth element, but it failed beyond calcium. Mendeleev (1869) arranged elements by increasing atomic mass in rows and columns so that similar elements fell in the same column; he left gaps for undiscovered elements and predicted their properties (eka-aluminium, later gallium, and eka-silicon, later germanium), and he placed some elements out of mass order when their properties demanded it. Anomalies remained, such as Ar () before K ().
Modern periodic law (Moseley, 1913). Moseley showed that the frequency of characteristic X-rays satisfies , so atomic number, not atomic mass, is the fundamental property. The modern law: the physical and chemical properties of elements are periodic functions of their atomic numbers. The reason is that properties depend on electronic configuration, which repeats periodically as shells fill.
The long form of the table has vertical groups and horizontal periods. The period number equals the principal quantum number of the outermost shell. Period lengths are , matching the number of electrons that can fill the , , and subshells in that period.
Blocks, named after the subshell receiving the last electron (see figure):
s-block (groups , ): ; reactive metals (alkali and alkaline earth metals).
p-block (groups to ): ; metals, metalloids and non-metals, ending with the noble gases. Helium () is placed in group for its properties although it is an s-element.
d-block (groups to ): ; transition metals.
f-block: ; lanthanoids (Ce to Lu) and actinoids (Th to Lr), placed separately below the table.
Hydrogen is unique: its configuration resembles the alkali metals, but it also forms like the halogens.
Locating an element from its configuration.
Period highest .
Block subshell receiving the last electron.
Group: s-block, number of electrons; p-block, number of outer and electrons; d-block, number of plus electrons.
Example: is period , d-block, group (iron). is period , p-block, group (sulfur).
IUPAC names for elements with (before official names are assigned) use numerical roots: nil, un, bi, tri, quad, pent, hex, sept, oct, enn, followed by '-ium'. Element would be unbinilium (Ubn).
Key Takeaways:
- Modern periodic law: properties are periodic functions of atomic number.
- Period of the valence shell; period lengths .
- s: groups 1-2; p: 13-18; d: 3-12; f: lanthanoids and actinoids.
- p-block group (outer electrons); d-block group electrons.
- IUPAC provisional names from digit roots plus '-ium'.
- Pitfall: placing an element in group electrons only, forgetting the electrons.
Brain-Teaser 1: Identify the period, group and block of the element with , and of .
Solution: : : period , p-block, group (bromine). : (molybdenum, like Cr, prefers the half-filled d set): period , d-block, group .
Brain-Teaser 2: Why does the fourth period contain elements?
Solution: In period the subshells filled are ( electrons), () and (), in that order by the Aufbau principle. . The and subshells are higher in energy and fill in later periods. Period lengths are therefore determined by energy order, not by the capacity of the shell.
Brain-Teaser 3: What would be the IUPAC provisional name and symbol of element , and in which group would it fall?
Solution: Digits , , : un-un-enn-ium, ununennium, symbol Uue. Its configuration would be , placing it in group (period ) as the next alkali metal.
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