CBSE Class 10 Science Periodic Classification Of Elements Notes Set 02

Dobereiner’s Triads: This classification is based on the atomic mass. According to this, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained. The atomic mass of middle element of the triad being nearly equal to the average of the atomic masses of the other two elements.
For Example Li (6.9), Na (23), K (39).

Limitation: It fails to arrange all the known elements in the form of triads, even having similar properties.
Newland’s Law of Octaves: According to this ‘when elements are placed in order of increasing atomic masses, the physical and chemical properties of every 8th element are a repetition of the properties of the first element.’

Limitations :
* Law of octaves was applicable only up to calcium (only for lighter elements).
* Newland adjusted two elements in the same slot (e.g. Co and Ni), having different properties. For example; Co and Ni with Fluorine, Chlorine, Bromine and Iodine.
* According to Newland, only 56 elements existed in nature and no more elements would be discovered in future.

Mendeleev’s Periodic Table: Mendeleev’s periodic table is based on the physical and chemical properties of elements and their atomic masses.
Merits of Mendeleev’s Periodic Table
* Mendeleev’s left vacant places in his table which provided an idea for the discovery of new elements. Example: Eka-boron, Eka-aluminum and Eka-silicon.
* Mendeleev’s periodic table was predicted properties of several undiscovered elements on the basis of their position in Mendeleev’s periodic table.
* It is useful in correcting the doubtful atomic masses of some elements.

Limitations of Mendeleev’s Periodic Table
(a) Noble gases could accommodate in the Mendeleev’s periodic table without (a) No fixed position for hydrogen: No correct position of the hydrogen atom was in Mendeleev’s periodic table.
Example: Position of hydrogen with alkali metals and halogens (17th group).
(b) No place for isotopes: Position of isotopes were not decided. Example: Cl-35 and Cl-37.
(c) No regular trend in atomic mass: Position of some elements with lower atomic masses before with higher atomic mass. Example: Ni-58.7 before Co-58.
* disturbing the periodic table after discovery.
* The Modern Periodic Table: In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
* Modern Period Law: The physical and chemical properties of elements are the periodic function of their atomic number.

Trends in Modern Periodic Table: Valency, Atomic size, metallic and non-metallic characters, and Electronegativity.
(i) Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom (i.e. the combining capacity of an element is known as its valency).
In Period: On moving from left to right in a period, the valency first increases from 1 to 4 and then decreases to zero (0).

In Groups: On moving from top to bottom in a group, the valency remains same because the number of valence electrons remains the same.

ii) Atomic size: Atomic size refers to radius of an atom. It is a distance between the centre of the nucleus and the outermost shell of an isolated atom.
In Period: On moving from left to right in a period, atomic size decreases because nuclear charge increases.
Example: Size of second period elements: Li > Be > B > C > N > O > F
In Group: Atomic size increases down the group because new shells are being added in spite of the increase in nuclear charge.
Example ; Atomic size of first group element : Li < Na < K < Rb < Cs < Fr

(iii) Metallic character: It is the tendency of an atom to lose electrons
In Period: Along the period from left to right, metallic characters decreases because a tendency to lose electron decreases due to the increase in nuclear charge.
Example: Metallic character of second period elements: Li > Be > B > C >> N > O > F
In Group: Metallic character, when moving from top to bottom increases because the atomic size and tendency to lose electrons increases.
Example: First group element: Li < Na < K < Rb < Cs

(iv) Non-metallic character: It is tendency of an atom to gain electrons
In Period: Along the period from left to right, non-metallic character increases because tendency to gain electrons increases due to increase in nucleus charge.
Example; Non-metallic character of 2nd period elements: Li < Be < B < C < N < O < F
In Group: On moving from top to bottom in a group, non-metallic character decreases because atomic size increases and tendency to gain electrons decreases. Ex. Non-metallic character of 17th period element: F > Cl > Br > I

(v) Chemical Reactivity
In metals: Chemical reactivity of metals increases down the group because tendency to lose electrons increases. Example; Li < Na < K < Rb < Cs (1st group)
In non-metals: Chemical reactivity of non-metals decreases down the group because tendency to gain electrons decreases. Example: F > Cl > Br > I (17th group)

(vi) Electronegativity: It is tendency of an element to attract the shared pair of electrons towards it in a covalently bonded molecule. It increases with increase of nuclear charge or decrease in atomic size.
Along the period electro negativity increases. Example; Li < Be < B < C < N < O < F. Down the group electronegativity decreases. Example; Li > Na > K > Rb > Cs
F > Cl > Br > I