Ch 3Periodic Classification of Elements — Class 11 Chemistry, concept-first.

Chemistry deals with an overwhelming variety of matter: millions of known compounds, all built from fewer than a hundred naturally occurring elements.

During the 19th century, chemists isolated many more elements and the known list grew steadily -- today 118 elements are known, of which 92 (atomic numbers 1 to 92) occur naturally.

Lothar Meyer's contribution (1868). Around the same time as Mendeleev, Julius Lothar Meyer built a table of the elements that closely resembles the modern one.

Mendeleev's table, built on atomic weight, showed two kinds of internal contradiction that pointed to a deeper ordering principle waiting to be found.

In 1913 Henry Moseley bombarded a series of elements with high-energy electrons and studied the characteristic X-rays each element emitted in response.

The physical and chemical properties of an element are closely tied to the arrangement of electrons in its outermost (valence) shell.

When a new element is discovered, its discoverer proposes a name that must go through IUPAC's approval process, including a period of public comment.

The modern periodic table organises its 118 known elements into 7 periods and 18 groups, guided throughout by the modern periodic law of Section 3.2.

Every period starts with an element of general outer configuration and ends with one of configuration (Section 3.2.1), and the length of each period is fixed by exactly which orbitals get filled, in w…

Elements within the same group share a similar outer-shell electronic configuration (Table 3.11), and this is really the deeper reason a group's elements share similar properties.

Just as electronic configuration varies periodically with atomic number, so do many physical properties of the elements built from that configuration.

Definition. The atomic radius of an atom is the distance from the centre of its nucleus to the outermost shell that contains a valence electron (Figure 3.1a).

Definition. The ionic radius of an ion is the distance from the centre of its nucleus out to which it exerts influence on the surrounding electron cloud.

Definition. Ionisation energy (or ionisation enthalpy) is the minimum energy needed to remove the most loosely bound electron from the valence shell of an isolated, neutral, gaseous atom in its ground…

Definition. Electron affinity (electron gain enthalpy) is the energy released -- or, for the noble gases, required -- when an electron is added to the valence shell of an isolated, neutral, gaseous at…

Definition. Electronegativity is the relative tendency of an atom, within a covalently bonded molecule, to attract the shared pair of electrons toward itself.

The five properties studied so far -- atomic radius, ionic radius, ionisation enthalpy, electron gain enthalpy and electronegativity -- are all physical properties.

Why the first member of a group is different. Elements in the same group generally share similar physical and chemical properties, but the very first element of a group consistently differs somewhat f…

Reactivity across the table. Elements on the left side of the periodic table have low ionisation energy and so readily lose their valence electrons; elements on the right side have high electron affin…

Sample papers and previous-year board questions for this subject.