Classification of Elements and Periodicity in Properties - Notes, Topics, Formula, Books, FAQs

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A periodic table is the study of various properties of the elements. The periodic table is the table of the elements that are found on the earth. Initially, this table was given by a Russian chemist Dmitri Mendeleev in 1869. Some periodic tables existed before but Mendeleev’s periodic table was considered as:
(i) Atomic weight is the fundamental property of elements.
(ii) The physical and chemical properties of elements are a periodic function of their atomic weight.

Then in 1972, Moseley proposed the Modern periodic law, which says “the physical and chemical properties of the elements are periodic functions of their atomic numbers”. Then finally, came the "Long-form" of the periodic table. This was based on Bohr-Bury concept of electronic configuration. This table has 7 periods and 18 groups. The elements existed in the table can be classified on the basis of their electronic configuration such as s-block, p-block, d-block, and f-block elements. Hydrogen with one electron occupies a special position in the periodic table.

As of 2001, there are 112 elements available to us so far. Out of which, 88 elements are found on earth naturally and rest others are synthesized in the laboratory. Most of the elements in the periodic table are metals. These metals exist at the left side of the periodic table. Some of the elements in the table are non-metals and they are less than 20 in number and they exist on the right side of the table. Some of the elements that exist on the border of the metals and non-metals are known as metalloids. Their properties are similar to those of metals as well as to non-metals. Most of these elements are solids, out of them only 11 elements i.e, noble gases, oxygen, nitrogen, fluorine, chlorine, hydrogen exist as gases and two of them are liquids i.e, mercury and bromine. Among all these elements, carbon, oxygen, and hydrogen are the three most important elements for the human body. Carbon consists of 18% in the human body, oxygen is 65% and hydrogen is 10% by mass.

Notes for Classification of Elements and Periodic Table

In this section, you will study about the important topics of the chapter, overview, formulae and some important tips and guidelines for the preparation.

Important Topics:

(i) Atomic Radii, and its trend in periods and groups

(ii) Ionic Radius, isoelectronic species

(iii) Ionization Enthalpy and factors affecting ionization enthalpy

(iv) Electron Gain Enthalpy and Electron Affinity

(v) Electronegativity and Electropositivity

(vi) Acids, Bases and Amphoteric Nature

(vi) Acidic Nature of Non-Metals Hydrides and Oxyacids

Overview of the Chapter

In this chapter, there are various important topics that you must understand completely:

Electronic configurations and types of elements

Based on the filling of electrons in the respective orbitals, elements are classified as s, p, d and f block elements.

• The s-block elements: These are those elements in which the last electron enters into the s orbital. Their general electronic configuration is ns¹ or ns². These all are metals. They have very low ionisation enthalpy and thus they loose electron very soon and hence they are very reactive. These metals being very reactive in nature, do not exist in pure form.
• The p-block elements: These elements have general outermost electronic configuration as ns²np¹ to ns²np⁶. Most of these elements are non-metals. Most of these elements especially group 17 and group 16 elements are very reactive. These elements have very high electron gain enthalpy and thus they gain the electron very easily.
• The d-block elements: These are those elements in which the last electron goes to the d orbital. All of these elements are metals. They have the general outermost electronic configuration as (n-1)d^1-10ns^0-2. These elements mostly form coloured complexes. These elements are also known as "Transition elements".
• The f-block elements: The f-block elements have the general electronic configuration as (n-2)f^1-14(n-1)d^1-1ns². These elements are divided into two categories i.e, Lanthanoids and Actinoids. These elements are also known as "Inner transition elements".

Periodicity in properties

The elements of the periodic table show trends in various properties. These properties are mentioned below:

(i) Atomic radius

• This is basically the size of an atom. Within a given period, the atomic radius decreases from left to right because of an increase in nuclear charge in the same shell.

• In the group, the atomic radius increases from top to bottom due to an increase in the number of shells.

Table 1: Atomic radii (in pm) across the periods and groups

(ii) Ionization Enthalpy:

• It is defined as the amount of energy required for the electron to come out of the atom. It is expressed in kJ/mol.

• It increases in the period from left to right due to the increased nuclear charge and decreases down the group because of the distance of the electron from nucleus increases.

Table 2: Trend of ionization energy (kJ/mol) in second period

(iii) Electron gain enthalpy and electronegativity:

• Electron gain enthalpy is defined as the amount of energy released when a new outside electron enters into an atom. It can be positive or negative that completely depends upon the nature of the element.
  Electronegativity is defined as the ability of an atom to attract the electrons towards itself. It is not a measurable quantity, but according to the Pauling scale, fluorine has given the arbitrary value of 4, and rest other elements are measured on the basis of this.

Table 3: Electronegativity values according to the Pauling scale across the second and third period

(iv) Acidic nature of Hydrides and Oxyacids:

• Acidic nature of non-metal hydrides increases down the group.

H-F < H-Cl < H-Br < H-I

• Acidic nature of oxyacids increases from left to right as follows:

H₃BO₃ < H2CO₃ < HNO₃

Periodic trends in chemical properties

Most of the chemical properties of the elements will be discussed in the later chapters, here we are focusing only on two important properties i.e, periodicity of valence state and anomalous properties of second-period elements.

• Periodicity of valence or oxidation state: Valence may be defined as the number of electrons that an atom can donate or accept or share with other atoms. Mostly oxidation state is also used for valency. Many of the elements of the periodic table have variable oxidation state. For example, fluorides of sulphur can exist in three forms i.e, SF₂, SF₄ and SF₆. These three different compounds exist only because of the variable oxidation states of the sulphur.

• Anomalous properties of second period elements: The properties of the first elements of each group differs in many ways with respect to their respective groups. For example, compounds of lithium have a sufficient amount of covalent character as compared to other elements of group 1 which have ionic character. This anomalous behaviour of these elements is due to their small size, large charge/radius ratio and high electronegativity of the elements.

How to prepare for the Periodic table and its elements?

• This chapter is the part of Inorganic chemistry. It is completely theory based. You are not supposed to memorize any formula and numerical practice for getting the good hold on this chapter.

• First, you must have the complete knowledge of Atomic Structure chapter. For this, you must go through chapter 3 of the NCERT book 11th class part 1 thoroughly.

• You must deeply observe that how and why the properties of elements like atomic radius, ionization enthalpy, electron gain enthalpy, etc. are following some general trends.

• In these properties, there are also some exceptional cases which exist that you must understand, for example, why oxygen atom has bigger size than nitrogen atom or why electron gain enthalpy of chlorine is more than fluorine.

Prescribed Books

For this chapter, first, you need to finish the theory thoroughly from the NCERT book and then simultaneously solve the examples and questions given in the book. Apart from this, if you want to prepare for the advanced level for competitive exams like JEE and NEET, you must read the book - O.P. Tandon. Meanwhile, in the preparation, you must continuously give the mock tests for better understanding. Our platform "entrance360" will help you with the variety of questions for deeper knowledge and it will also provide you with concept videos, articles and mock tests for better understanding.