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

Important Topics for Classification Of Elements And Periodicity In Properties

This chapter deals with the systematic classification of elements and periodic trends in their properties, helping in understanding the behaviour of elements in Chemistry.

As of 2001, the periodic table featured 112 recognized elements: 88 natural and 24 synthetic. Most are metals on the table’s left side; under 20 non‑metals appear on the right. Between them lie metalloids, with properties overlapping metals and non‑metals. Nearly all elements are solid at room temperature only 11 are gases (including noble gases, hydrogen, oxygen, nitrogen, fluorine, chlorine), and just two are liquids (mercury and bromine). In the human body, the most abundant elements by mass are oxygen (~65 %), carbon (~18 %), and hydrogen (~10 %). These three, along with nitrogen, calcium, and phosphorus, make up nearly 99 % of body mass.

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.
• 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. These elements mostly form coloured complexes. These elements are also known as "Transition elements".
• The f-block elements: 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:

$\mathrm{H}_3 \mathrm{BO}_3<\mathrm{H}_2 \mathrm{CO}_3<\mathrm{HNO}_3$

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 form. 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.

Recommended Video on ( Classification of Elements and Periodicity in Properties):

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.

Classification of Elements and Periodicity in Properties in Different Exams

This chapter is frequently asked in board and competitive examinations with emphasis on periodic trends, modern periodic law, and the relationship between electronic configuration and properties of elements.

Study Links for Classification of Elements and Periodicity in Properties

Subject Wise Resources of NCERT

This section provides organised study materials and useful links for each subject to support effective learning and exam preparation.

Previous Year Questions Of Classification Of Elements And Periodicity Of Properties

Question 1: Given below are two statements :
Statement I : The metallic radius of "Na is 1.86 Å, and the ionic radius of Na⁺ is less than 1.86 Å."
Statement II: Ions are always smaller in size than the corresponding elements.
In the light of the above statements, choose the correct answer from the options given below :

1. Statement I is correct but Statement II is false

2. Both Statement I and Statement II are true

3. Both Statement I and Statement II are false

4. Statement I is incorrect but Statement II is true.

Answer:

$\mathrm{r}_{\mathrm{Na}}>\mathrm{r}_{\mathrm{Na}^{+}}$

So, Statement (I) is correct but the size of anions is greater than the size of neutral atoms.
So statement (II) is incorrect.

Hence, the answer is the option (1).

Question 2: Given below are two statements:
Statement I : The correct order of first ionization enthalpy values of Li, Na, F and Cl is Na < Li < Cl < F.
Statement II : The correct order of negative electron gain enthalpy values of Li, Na, F and Cl is Na < Li < F < Cl
In the light of the above statements, choose the correct answer from the options given below :

1. Both Statement I and Statement II are true

2. Both Statement I and Statement II are false

3. Statement I is false but Statement II is true

4. Statement I is true but Statement II is false

Answer:

1. Ionization energy data for given elements -

2. Electron gain enthalpy energy data for given elements -

Hence, the answer is the option (1).

Question 3: An element belongs to the 3^rd period and group-13 of the periodic table. Which of the following properties will be shown by the element?

(i) Good conductor of electricity

(ii) Liquid, metallic

(iii) Solid, metallic

(iv) Solid, non-metallic

1) (i) and (iv)

2) (iii) and (iv)

3) (i) and (iii)

4) None of above

Answer:

Melting and boiling points
The elements of this group do not show a regular change in their melting points with increase in atomic number. The melting point decreases from B to Ga and then increases. The high melting point of boron is due to the fact that it exists as a giant covalent polymer in both solid and liquid states. The elements Al, In and Tl all have close-packed metal structures. Gallium has an unusual structure. It consists on only Ga₂ molecules. It has thus low melting point. It exists as liquid upto 2000^oC and hence used in high temperature thermometry.

Explanation: Group-13 third period element is Aluminum which is a metal which is solid, metallic and a good conductor of electricity.

Hence, the answer is Option (3)

Practice more questions from the link given below

For more questions to practice, the following MCQs will help in the preparation for competitive examinations

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.