Chapter 3 — Classification of Elements and Periodicity in Properties
Why 118 elements fit into one table of rows and columns — from Döbereiner's triads and Mendeleev's daring gaps to the modern periodic law, the s, p, d and f blocks, and the trends in size, ionisation enthalpy, electron gain enthalpy and electronegativity that let you predict how an element behaves.
In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran found a new metal in a zinc ore and named it gallium. He measured its density as $4.7\ \mathrm{g\,cm^{-3}}$. Within weeks a letter arrived from Dmitri Mendeleev in St Petersburg, who had never seen the metal: this was the element he had predicted in 1871 and called eka-aluminium, he wrote, and its density should be close to $5.9$. Lecoq de Boisbaudran purified his sample, measured again — and got $5.9$. A chemist had described an unknown element, correctly, from its place in a table.
That table is the single most useful tool in chemistry. More than a hundred elements are known, each with its own physical and chemical properties; nobody can learn them one by one. The periodic table arranges them so that elements with similar properties fall into the same column, and properties change smoothly along each row. In this chapter you will see how the table grew from early attempts at classification, why it is now built on atomic number rather than atomic mass, and how its shape — periods of 2, 8, 8, 18, 18, 32 and 32 elements, and the s, p, d and f blocks — follows directly from the electronic configurations of Chapter 2. Then you will use it to explain and predict the trends in atomic and ionic size, ionisation enthalpy, electron gain enthalpy, electronegativity, valence and the nature of oxides. These trends are among the most frequently examined ideas in Board, JEE and NEET papers, and they are the language of all the chemistry of the elements in Class 12.
- explain why elements were classified, and describe Döbereiner's triads, Newlands' law of octaves, Lothar Meyer's curve and Mendeleev's periodic table with its successes and limitations;
- state the modern periodic law and describe the long form of the periodic table: its 7 periods, 18 groups, lanthanoids and actinoids;
- write the IUPAC systematic name and symbol of any element with $Z\gt100$;
- find the period, group and block of an element from its electronic configuration, and the configuration from its position;
- classify elements as s-, p-, d- or f-block, and as metals, non-metals or metalloids;
- explain and predict the trends in atomic radius, ionic radius (including isoelectronic species), ionisation enthalpy, electron gain enthalpy and electronegativity, with their anomalies;
- relate valence, the anomalous behaviour of second-period elements, diagonal relationships and the acid–base nature of oxides to position in the table.
Sections in this chapter
Development of the Periodic Table and the Modern Periodic Law
Why classify; Döbereiner's triads, Newlands' octaves, Lothar Meyer and Mendeleev; the modern periodic law and the long form of the table; IUPAC names for elements with $Z\gt100$.
Electronic Configuration and the Blocks of the Periodic Table
Periods and groups explained by configuration; the s, p, d and f blocks and their general configurations; locating an element from its configuration; metals, non-metals and metalloids.
Periodic Trends in Properties
Atomic and ionic radii, isoelectronic species, ionisation enthalpy and its anomalies, electron gain enthalpy, electronegativity, valence, second-period anomalies, diagonal relationships and the nature of oxides.