NCERT Solutions for Class 10 Chemistry Chapter 5 : Periodic Classification of Elements

NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements – Brief Overview

Periodic Table

The periodic table is a tabular representation of chemical elements in which components with comparable properties are found in the same vertical column or group. The periodic table is divided into sections based on atomic weight and valence electrons.

Dobereiner’s Triads

In 1817, Johann Wolfgang Döbereiner, a German scientist, identified certain groups with three elements each. As a result, he dubbed these groups "triads." The atomic mass was used to classify these elements. Döbereiner demonstrated that when the atomic masses of the three elements in a triad were listed in ascending order, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

For example, consider the triad of lithium (Li), sodium (Na), and potassium (K) having atomic masses of 6.9, 23.0, and 39.0, respectively.

Newland’s Law of Octaves

In 1866, John Newlands, an English physicist, organized the known elements in ascending order of atomic mass. When atoms were placed in increasing atomic mass order, he discovered that every eighth element exhibited properties similar to the first. He linked it to the musical octaves. As a result, he dubbed it the "Law of Octaves."

Mendeleev’s Periodic Table

The periodic table of Mendeleev is based on the physical and chemical properties of elements as well as their atomic masses. The elements were ordered according to their fundamental attribute, atomic mass, as well as chemical property similarity. The Periodic Table of Mendeléev has vertical columns called 'groups' and horizontal rows called 'periods.'

The Modern Periodic Table

Henry Moseley demonstrated in 1913 that an element's atomic number is a more fundamental feature than its atomic mass. The atomic number (Z) of an element is equal to the number of protons in its nucleus.

The modern periodic table has 18 vertical columns called groups and 7 horizontal rows called periods. From left to right in the periodic table, the number of valence electrons in the elements present increases from 1 to 8. The number of shells remains constant when travelling from left to right. Each element in a periodic table group has the same number of valence electrons.

Trends in Modern Periodic Table

1. Valency – The number of valence electrons present in an element's outermost shell determines its valency.
2. Atomic size – The radius of an atom is referred to as its atomic size. It is the distance between an isolated atom's nucleus and its outermost shell.
3. Metallic and Non-metallic character – Metal: It is an atom's tendency to lose electrons. Non-metal: It is an atom's tendency to gain electrons.
4. Chemical reactivity – Metals' chemical reactivity increases down the group as their proclivity to lose electrons increases. The chemical reactivity of nonmetals diminishes down the group due to a decrease in the propensity to acquire electrons.
5. Electronegativity – In a covalently bound molecule, it is an element's proclivity to attract the shared pair of electrons.
6. Nature of oxides – Metal oxides are naturally basic and Non-metal oxides are naturally acidic.