Classification of Elements and Periodicity in Properties of Elements

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Mandeleev’s Periodic Law:- The elements ' properties are the periodic function of their atomic masses.

Moseley, the English physicist has shown that an element's atomic numbers are more fundamental properties than its atomic mass. The position of an element in the periodic table therefore depends on its atomic number as compared with its atomic mass.

Modern Periodic Law: Elements  physical and chemical properties are the periodic features of their atomic numbers.

Elements are Classified into: s-, p-, d- and f- blocks.

Main Group Elements/ Representative Elements:

The elements of the s-and p-block are called major group elements, or representative elements.

s- block Elements: Group-1 (Alkali metals) and Group-2 (Alkaline earth metals) elements with outermost electronic configurations of ns1 and ns2, respectively.

p- Block Elements: They are part of group-13 to 18. The most electronic parameter outer is ns2 np1-6. He (1s2) is a s-block element but is positioned with the group 18 elements (ns2 np6) because it has a fully filled valence shell and thus exhibits characteristic properties of other noble gasses.

d- block Elements (Transition Elements): They are community 3 to 12 elements having external electronic configuration(n-1) d1-10 ns1-2. There are four transformation sequences 3d, 4d, 5d, and 6d. The 6d-series is incomplete. Atomic radius usually decreases over a period of time, and increases as we descend the group.

f-Block Elements (Inner- Transition Series)

Characterized by the filling of 4 f-orbitals, the lanthanum-following elements are from 58Ce to 71Lu. Actinoids distinguished by 5f-orbitals filling are the elements from Th 103Lr following actinium. The outer electronic configuration is characteristic of (n-2) f1-14 (n-1) d0-1 ns2.

Noble Gases: Group 18 gaseous elements are called noble gasses. The general electronic outermost configuration of noble gases (except for He) is ns2 np6. He 's beautifully made to have 1s2. So the outermost shell of noble gasses is filled up entirely.

Periodicity: The occurrence of similar properties after regular intervals is called periodicity.

Cause of Periodicity: The properties of the elements are that similar electronic product configuration occurs regularly as the number of atoms increases.

Atomic Properties: The physical traits of an element's atom are called atomic properties. Properties like atomic radius, ionic radius, ionization energy, electro-negativity, affinity and valence of electrons, etc., called atomic properties.

Atomic Radius- The size in the atom of any product from the middle of the nucleus to the electrons ' outermost shell is called its atomic radius.


(a) In Periods- The atomic radius of elements decreases in a period from left to right.

In Group-Atomic element radius increases when moving from top to bottom in a group.

Covalent Radius- Half the inter-nuclear distance between two similar atoms is called covalent radius of any element that is covalently bonded to each other by a single covalent bond.

Van Der Waals’ Radius: Half the inter-nuclear separation of two similar adjacent atoms in solid state belonging to the two adjacent molecules of the same substance is called the van der waals ' radius of that atom.

Metallic Radius: Half the distance between the nuclei of the two adjacent metal atoms in the tightly packed metal lattice is called its metal radius.

Ionic Radius: The effective distance from the center of an ion 's nucleus to which it impacts its electron cloud is called its ionic radius.

A cation is smaller than the parent atom but the anion is larger. Among iso-electronic material, the cation with a higher positive charge has a smaller radius, but anion with a higher negative charge has the larger radius.

Ionisation Enthalpy: The enthalpy of ionisation is the change in molar enthalpy that accompanies the removal of an electron from a gaseous phase atom or ion in its ground state. So change in enthalpy for reaction.

Is the ionisation enthalpy of element M. Like the ionization energies for successive ionization, the successive ionization enthalpy can also be called the 2nd ionization enthalpy (i.e., rH2), the third ionization enthalpy (i.e., rH3), etc. The term ionization enthalpy is used for the initial ionization enthalpy (both in kg mol and in eV).


The ionisation enthalpies generally follow the order (there are few exceptions):

(∆rH1) < (∆rH2)< (∆rH3)

The enthalpy of ionisation decreases when moving in a group from top to bottom.

The enthalpy of ionization increases when traveling in a time from left to right.


In Period-The enthalpy contribution from the electron decreases in a time from left to right.

In Group- The enthalpy of the electron production decreases from top to bottom in a group.

Electronegativity: "In a molecule the electro-negtivity is called the relative tendency of an atom to draw the mutual pair of electrons towards itself.


Over Time- The electro-negtivity increases in a period from left to right.

In Group-The electro-negative diminishes in a group from top to bottom.

Valence Electrons: The electrons in outermost shell are referred to as valence electron. Because in the outermost shell the electrons decide the valence of an element.

Valency Of An Element: The number of hydrogen or halogen atom, or double the amount of oxygen atom, which is taken as its valence when paired with one atom of the element. According to the electronic valence principle, "the number of electrons that an atom loses or receives or shares with other atoms in order to achieve the noble gas configuration is called its valence."


In Period- The valence increases first, and then decreases in a time from left to right.

In Group- The valence remains constant within a group from top to bottom.

Electropositive or Metallic Character: The propensity of an element to lose electrons is called electropositive or metallic character and forms positive ions (cations). Elements with lower ionization energies have a greater tendency to lose electrons, so in their behavior they are electropositive or metallic.

The most strongly electropositive elements are the alkali metals.

Periodicity: In Period-Electropositive or metallic characters diminish in a time from left to right.

In Group- The electropositive or metallic characters in a group are rising from top to bottom.

Electro-Negative or Non- Metallic Characters: 

An element's propensity to admit electrons into forming anion is called its non-metallic or electronegative character. The high electro-negativity elements have a higher tendency to gain anion forms and electrons. In the upper right portion of the periodic table, therefore, the elements are electro-negative or non-metallic in nature.


In period-The electro-negative or non - metallic characters increase in a certain time from left to right.

In Group- The electro-negative or non - metallic characters in a group diminish from top to bottom.

Reactivity of Metals:


In period- The tendency of an element to lose electrons decreases in period. So, the reactivity of metals decreases from left to right in a period.

In group-In a period, an element 's tendency to lose electrons increases. Thus the reactivity of metals in a group increases from top to bottom.

Reactivity of Non- Metals:

In Period- An element 's tendency to gain electrons increases over time. So the non - metals reactivity increases in a time from left to right.

In Group- The tendency of an element to gain electrons decreases in a group. So, the reactivity of non-metals increases from top to bottom in a group.

Solubility of Alkali Metals Carbonates And Bicarbonates:

Periodicity in Group: Alkali metal carbonates and bicarbonates solubility in water increases down the group (from Lithium to Caesium).

Solubility of Alkaline Earth Metal Hydroxides And Sulphates:

Periodicity in Group: Alkaline earth metal hydroxide and sulphates solubility in water is increasing down the group (from Beryllium to Barium).

Basic Strength of Alkaline Earth Metal Hydroxides:

Periodicity in Group: The basic strength in water of alkaline earth metal hydroxide increases the group (from Beryllium to Barium), i.e.,

Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2

Basic strength increases…………………………………………………………………………………….>>>>>>>

Thermal Stability Of Carbonates Of Alkali And Alkaline Earth Metals:

With the exception of lithium carbonate (LiCO3), all other alkali metals ' carbonates are heat stable, i.e. alkali metal carbonates (except LiCO3) do not decompose when heated. LiCO3 decomposes to give lithium oxide (LiCO3) when heated.

The alkaline earth metal carbonates are relatively lower in stability. They decompose on heating to give corresponding oxide and carbon dioxide. The temperature of decomposition for alkaline earth metal carbonates increases as we descend the group.

Anomalous Properties of Second Period Elements

A anomalous behavior is due in a valence shell to their small size, large charge / radius ratio, high electro negativity, non - availability of d-orbitals. The first member of each group of p - Block elements shows greater ability to form several pp-pp bonds to itself (e.g. C= C, CBC O= O, NB) and other period elements (e.g. C= O, CBC, N= O) compared to subsequent group members.

FAQ (Frequently Asked Questions)

1. What Properties are Used to Classify the Elements?

Colour, density, melting point, boiling point, and thermal and electrical conductivity are among these properties. While some of these properties are primarily due to the element's electronic structure, others are similar to nucleus properties, e.g. mass size.

2. Why do we Classify the Elements?

The number of discovered elements has increased, making it difficult to recall certain elements ' behaviors and properties. Hence, classification of elements according to their properties is essential. It helps us to understand how different compounds make up different elements.

3. How do You Classify Metals?

Metals, nonmetals, or metalloids can be classified as the elements. Metals are good heat and electricity conductors, and are maleable (can be hammered into sheets) and ductile (can be drawn into wire).