Classification of Elements and Periodicity in Properties - Notes, Topics, Formula, Books, FAQs
Classification of Elements and Periodicity in Properties - Notes, Topics, Formula, Books, FAQs
Edited By Team Careers360 | Updated on Jul 04, 2025 02:17 PM IST
The periodic table arranges chemical elements in a logical framework that reflects recurring trends in their physical and chemical traits. In 1869, Dmitri Mendeleev organized the 63 then-known elements by increasing atomic weight and grouped those with similar properties into vertical columns. This arrangement revealed clear periodic patterns and enabled Mendeleev to leave intentional gaps for undiscovered elements, successfully predicting the properties of gallium, scandium, and germanium.
Classification of Elements and Periodicity in Properties - Notes, Topics, Formula, Books, FAQs
While other chemists, such as Lothar Meyer, proposed similar tables, Mendeleev earned recognition for his foresight and systematic approach. Then, in 1913, Henry Moseley used X-ray spectroscopy to show that atomic number—not atomic weight—was the true organizing principle. By correlating X-ray frequencies with nuclear charge, he established that each element’s identity is defined by its proton count. This insight led to our modern periodic law and the current layout of seven periods and eighteen groups, with elements classified into s‑, p‑, d‑, and f‑blocks based on their electron configurations
Important Topics for Classification Of Elements And Periodicity In Properties:
Development of Modern Periodic Table
The modern periodic table is based on the periodic law proposed by Moseley, which states that the physical and chemical properties of elements are periodic functions of their atomic numbers. Development of modern periodic table has been a long process and it is a refined version of previous ones. Modern periodic table organises elements into periods and groups to reflect various trends in properties.
Mendeleev’s Periodic Table
In Mendeleev's periodic table, elements are arranged by increasing atomic weight reflecting similarities in chemical properties. Mendeleev’s Periodic Table successfully predicted properties of undiscovered elements, certain anomalies, such as the placement of isotopes, led to the development of the modern periodic table.
Modern Periodic Table
The Modern periodic table is arranged by increasing atomic numbers, removing the Mendeleev’s anomalies. Modern periodic table has 18 groups and 7 periods, and elements are organized based on their electronic configurations. This structure provides insights into element properties and trends.
Electronic Configuration In Periods And Groups Electronic configuration of elements within periods and groups determines their chemical behaviour, with periods showing progressive filling of orbitals and groups exhibiting similar valence electron patterns.
Nomenclature Of Elements With Atomic Number Nomenclature of elements with atomic numbers follows systematic rules set by IUPAC to provide unique and universal names for elements, especially those with higher atomic numbers. Questions from this topic is often asked in NEET examination.
Classification of Elements and Periodicity in Properties
Elements are categorized as metals, non-metals, and metalloids based on physical and chemical properties. Classification of Elements and Periodicity in Properties arises from the regular repetition of these properties with increasing atomic numbers.
Metals, Non-Metals, and Metalloids
Metals are primarily found on the left side of the periodic table and are good conductors with malleable properties. Non-metals are found on the right side and are poor conductors with brittle in nature .Metalloids are located along the zig-zag line shows mixed properties. Various questions based on Metals, Non-Metals, and Metalloids are asked in JEE Main and NEET examination.
Atomic Size & Atomic Radius Atomic size decreases across a period due to increased nuclear charge and increases down a group as more shells are added. Trends in atomic radius influence reactivity and bonding.
Ionization Enthalpy
Ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom. It depends on factors like atomic size, nuclear charge, and electron shielding. Trends of ionisation enthalpy in period and groups is often asked in JEE-Main and NEET examination.
Electron Gain Enthalpy
Electron gain enthalpy is the energy change when an electron is added to a gaseous atom, and electron affinity measures an atom’s tendency to accept electrons, reflecting its ability to form anions.
Electronegativity
Electronegativity is the tendency of an atom to attract shared electrons in a bond, while electro-positivity describes an atom’s tendency to donate electrons, both influencing chemical bonding and reactivity.
Physical and Chemical Properties of Elements
The periodic table organizes elements with similar physical and chemical properties of elements into groups. Trends such as metallic and non-metallic character, melting and boiling points, and reactivity are systematically observed across periods and groups.
Overview of the Chapter
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.
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)d1-10ns0-2. 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)f1-14(n-1)d1-1ns2. 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.
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Table 1: Atomic radii (in pm) across the periods and groups
Period/Group
IA
IIA
IIIA
IVA
VA
VIA
VIIA
2
Li(152)
Be(111)
B(88)
C(77)
N(74)
O(66)
F(64)
3
Na(186)
Mg(160)
Al(143)
Si(117)
P(110)
S(104)
Cl(99)
(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
Period/Group
IA
IIA
IIIA
IVA
VA
VIA
VIIA
VIIIA
2
Li(520)
Be(899)
B(801)
C(1086)
N(1402)
O(1314)
F(1681)
Ne(2080)
(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
Period/Group
IA
IIA
IIIA
IVA
VA
VIA
VIIA
2
Li(1.0)
Be(1.5)
B(2.0)
C(2.5)
N(3.0)
O(3.5)
F(4.0)
3
Na(0.9)
Mg(1.2)
Al(1.5)
Si(1.8)
P(2.1)
S(2.5)
Cl(3.0)
(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.
Group
1
2
13
14
15
16
17
18
Number of valence electron
1
2
3
4
5
6
7
8
Valence
1
2
3
4
3,5
2,6
1,7
0,8
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):
Importanat PYQS
Question: 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 :
Statement I is correct but Statement II is false
Both Statement I and Statement II are true
Both Statement I and Statement II are false
Statement I is incorrect but Statement II is true.
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: 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 :
Both Statement I and Statement II are true
Both Statement I and Statement II are false
Statement I is false but Statement II is true
Statement I is true but Statement II is false
Solution:
1. Ionization energy data for given elements -
Element
Na
Li
Cl
F
I.E1 in KJ/mol
496
520
1256
1681
2. Electron gain enthalpy energy data for given elements -
Element
Na
Li
F
Cl
ΔHeg in KJ/mol
- 53
- 60
- 328
- 349
Hence, the answer is the option (1).
Practice more questions from the link given below
For more questions to practice, the following MCQs will help in the preparation for competitive examinations
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.
Frequently Asked Questions (FAQs)
1.What is the periodic law?
The periodic law states that the physical and chemical properties of elements are a periodic function of their atomic numbers. This means elements exhibit recurring properties when arranged in order of increasing atomic number.
2.How are elements classified in the modern periodic table?
In the modern periodic table, elements are classified into groups (vertical columns) and periods (horizontal rows) based on their atomic number, electronic configuration, and recurring chemical properties.
3.What are the trends in atomic radii across a period and down a group?
Across a period: Atomic radii decrease due to increased nuclear charge, which pulls electrons closer to the nucleus.
Down a group: Atomic radii increase due to the addition of new electron shells, which outweighs the effect of nuclear charge.
4.What is electronegativity, and how does it change across a period and down a group?
Electronegativity is an atom's ability to attract shared electrons in a chemical bond.
Across a period: Electronegativity increases due to a stronger nuclear charge.
Down a group: Electronegativity decreases due to larger atomic size and electron shielding.
5.What are isoelectronic species?
Isoelectronic species are atoms, ions, or molecules with the same number of electrons but different atomic numbers or charges (e.g., O2−,F−,and Ne).
