Classification of Elements and Periodicity in Properties
Class 11 Chemistry · Chemistry Part I · Chapter 3
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Summary
This unit traces the development of the periodic law and the modern periodic table. Early attempts — Döbereiner's triads, Newlands' law of octaves and de Chancourtois' telluric helix — preceded the independent work of Mendeleev and Lothar Meyer, who arranged elements by increasing atomic weight so that similar properties recurred periodically. Mendeleev left gaps and successfully predicted eka-aluminium (gallium) and eka-silicon (germanium). Moseley later showed that atomic number, not atomic weight, is the fundamental property, giving the modern periodic law: the properties of elements are periodic functions of their atomic numbers. The long-form table has seven periods and eighteen groups, and elements are classified by the orbital being filled into s-, p-, d- and f-blocks; the f-block lanthanoids and actinoids are inner-transition elements. Over three-quarters of the elements are metals, with metalloids (Si, Ge, As, Sb, Te) bordering the diagonal zig-zag line, and superheavy elements (\(Z>100\)) named by IUPAC numerical roots. Periodic trends arise from the interplay of nuclear charge and shielding: atomic and ionic radii decrease across a period and increase down a group, while ionisation enthalpy and electronegativity increase across and decrease down. Chemical reactivity is greatest at the two extremes of a period and least in the centre, and the nature of oxides changes from basic through amphoteric to acidic across a period.
Genesis of periodic classificationModern periodic law and the long-form tables-, p-, d- and f-block elementsNomenclature of elements with Z greater than 100Atomic and ionic radii trendsIonisation enthalpy and electron gain enthalpyElectronegativityPeriodicity of valence and chemical reactivity
Key terms
Modern periodic law
The physical and chemical properties of elements are periodic functions of their atomic numbers.
Ionisation enthalpy
The energy required to remove an electron from an isolated gaseous atom in its ground state; always positive.
Electron gain enthalpy
The enthalpy change when an electron is added to a neutral gaseous atom to form an anion; may be exothermic or endothermic.
Electronegativity
A qualitative measure of the tendency of an atom in a molecule to attract the shared bonding electrons to itself.
Isoelectronic species
Atoms and ions that have the same number of electrons, such as \(O^{2-}\), \(F^{-}\), \(Na^{+}\) and \(Mg^{2+}\).
Diagonal relationship
The similarity between the first member of a group and the second member of the next group, e.g. Li with Mg and Be with Al.
Extra questions & answers
Try one from this chapter
The modern periodic law states that properties of elements are periodic functions of their: