[Chemistry Class Notes] on Classification of Elements and Periodicity in Properties Pdf for Exam

Chemists discovered that all the substances are made up of atoms or elements. Slowly they discovered many elements. In 1789, Antoine Lavoisier published a list of 33 elements. With the discovery of many elements, chemists felt the need for classification of elements for their easy understanding and comparison. At present 118 elements are known. Efforts to synthesize new elements are 

Genesis of Periodic Classification 

Classification by Johann Dobereiner – German chemist Johann Dobereiner classified certain elements on the basis of their similar properties in the groups of continuing. It is very difficult to study the properties of such a huge number of chemical elements individually. Scientists were trying to classify elements in a periodic manner on the basis of their various properties.

Three elements each. He called these groups triads. In each triad, the atomic weight of the middle element was equal to the average of the atomic weights of the first and third element. 

Newlands Law of Octaves – English chemist John Alexander Newlands profounded the Law of Octaves in 1865. He arranged the elements in increasing order of their atomic weights and found that every 8th element shows similarity with the 1st element. 

Mendeleev’s Periodic Table – In 1869, Dmitri Mendeleev arranged all 63 elements in rows or columns in order of their atomic weight. He left space for corresponding elements in his periodic table which were not even discovered then. Although he was able to predict the properties of those elements through his periodic classification of elements. 

Periodic law given by Mendeleev – The properties of the elements are periodic function of their atomic weights. 

• In Mendeleev’s periodic table, vertical rows were called groups while horizontal rows were called periods. 

• There were nine groups (I, II, III, IV, V, VI, VII, VIII and zero group). 

• Group VIII had nine elements which were arranged in triads. 

• Zero group had noble gasses with 0 valency. 

• There were seven periods. 

Modern Periodic Law and the Present Form of the Periodic Table 

English physicist Henry Moseley showed through his experiments that the atomic number of the element is its more fundamental property than its atomic mass. So, accordingly, the periodic law was also changed. 

Modern Periodic Law – The properties of the elements are periodic functions of their atomic numbers.

Elements were rearranged in the periodic table according to the modern periodic law. Thus, the modern periodic table was formed. Presently, Modern periodic table or Long form of the periodic table is widely used by chemists. It helps in the study of physical and chemical properties of elements. 

• In modern periodic table elements have been arranged according to their increasing atomic numbers. 

• It has 18 groups and 7 periods. 

• Elements of the same group have a similar outer electronic configuration.

• The period number corresponds to the highest principal quantum number (n) of the elements in the period. 

• In the modern periodic table, 14 elements of both the 6th and 7th period are placed in the separate panels at the bottom. These are known as lanthanides and actinides respectively. 

Nomenclature of Elements with Atomic Number > 100

To avoid the confusion and conflicts between scientists IUPAC decided that until new elements discovery is proved, and its name is officially recognized, a systematic nomenclature will be followed. Systematic nomenclature is derived directly from the atomic number of the element. Numerical roots for 0 – 9 are used and the suffix ‘ium’ is used in the end. 

 

Electronic Configuration of Elements and the Periodic Table 

Electronic Configurations in Periods – The period number indicates the value of n for the outermost or valence shell. An element placed in 2nd period will have its outermost electrons in 2s or 2p orbitals. Ne is placed in 2nd period and has electronic configuration – 1s2 2s2 2p6. This period has 8 elements (Ne – 2s2 2p6). 

Electronic Configurations in Groups – Elements of the same group have similar valence shell electronic configurations. They have the same number of electrons in the outer orbitals. 

Types of Elements: s, p, d, f – Blocks

Classification of the Elements

• s-block elements: s-block consists of group 1 & 2 elements. They are situated on the extreme left of the periodic table.
• p-block elements: p-block consists of group 13 to 18 elements
• d-block elements: d-block consists of group 3 to 12 elements
• f-block elements: All f-block elements are part of the 3rd  group. General electronic configuration of the f-block is (n−2)f1−14(n−1)d0−1ns2.

The elements of f-blocks are categorized into two series.

1.  4 f-series (Ist inner transition). It comprises 14 elements from 58 Ce to 71 Lu.

2. 5 f-series (IInd inner transition). It comprises 14 elements from 90 Th to 103 Lr.

 

 

Periodic Trends in Properties of Elements 

Following properties of elements show a very clear periodic trends in periodic table –

• Atomic Radius 

• Ionization energy 

• Electron affinity 

• Electronegativity 

• Valence electrons 

• Valency 

• Metallic character of the elements 

• Non – metallic character of the elements 

• Reactivity of elements 

• Melting and boiling points of elements 

Atomic Radius 

Periodic Trend of Atomic Radius Across a Period – As we move from left to right in a period, atomic radius gradually decreases.

Reason – As we move left to right in a period, the atomic number of the elements increases so nuclear charge increases while the number of shells in elements remain the same. 

Example – 

Exceptional Behavior – Noble gases show exceptional behavior. The atomic radii of inter gases suddenly increase as compared to its predecessor halogen atom. The reason for this type of exceptional behavior is that atomic radius refers to van der Waals radius in case of noble gases while in case of other elements it refers to covalent radius. 

Across a Group – On moving top to bottom in a group, atomic radii gradually increase as nuclear charge and number of shells also increase. 

Ionization Energy 

Periodic trend of ionization energy across a period – As we move from left to right in a period, ionization energy gradually increases.

Reason – As we move left to right in a period atomic size or atomic radius decreases while nuclear charge increases. 

Example – 

Exceptional Behavior – Beryllium possesses more first ionization energy than Boron. Because beryllium has half – filled s – orbital and more energy is required to remove an electron from half or completely filled orbitals. That is why noble gases also show exceptionally high ionization energies. 

Across a Group – On moving top to bottom in a group, ionization energy gradually decreases as atomic radius increases.

Electron Affinity 

Periodic Trend of Electron Affinity Across a Period – As we move from left to right in a period, electron affinity gradually increases.

Reason – As we move left to right in a period atomic size or atomic radius decreases while nuclear charge increases. 

Exceptional Behavior – Beryllium does not form a stable anion, so it releases less energy than boron on adding an electron. While nitrogen neither releases nor requires a significant amount of energy on adding an electron so it has electron affinity almost equal to zero. 

Across a Group – On moving top to bottom in a group, electron affinity gradually decreases. 

Electronegativity 

Across a Period – As we move left to right across a period, electronegativity increases in the periodic table. Fluorine is the most electronegative element. 

Reason – As the nuclear charge increases of an atom, its electron loving character also increases. 

Example –

Across a Group – As we move top to bottom in a group, electronegativity decreases. 

Valence Electrons 

Across a Period – As we move left to right across a period in the periodic table, the number of valence electrons increases.

Example –

Across a Group – Across a group, valence electrons remain constant. It means elements present in the same group have the same number of valence electrons. For example, hydrogen, lithium, and sodium elements are present in the 1st group and have the same number of valence electrons which is one. 

Valency 

Valency is the combining capacity of an atom.

Across a Period – On moving left to right across a period in the periodic table, first valency increases then decreases. 

Example –

Across a Group – There is no change in valency across a group. Elements of the same groups show the same valency.

Metallic Character of the Elements 

Across a Period – As we move left to right across a period in the periodic table, metallic character of elements decreases. 

Example –

Across a Group – As we move top to bottom in a group of the periodic table the metallic character increases. 

Non – Metallic Character of the Elements 

Across a Period – As we move left to right across a period in the periodic table, nonmetallic character of elements increases. 

Example –

Across a Group – As we move top to bottom in a group of periodic table nonmetallic character decreases.

Example –

Reactivity of Elements 

Reactivity of metals depends on its electropositive character. So, more is the metallic character, more is the electropositive nature of the element and more is its reactivity. As metallic character decreases across a period left to right, so reactivity also decreases. Although reactivity of nonmetals increases on moving left to right across a period. Thus, we can conclude, as we move left to right in a period, the reactivity of elements gradually decreases up to the group thirteen and then starts increasing. 

Melting and Boiling Points of Elements 

Melting and boiling points of metals decrease gradually from top to bottom in a group. While melting and boiling points of nonmetals increase on moving from top to bottom in a group of the periodic table.  

This ends our coverage on the topic “Classification of Elements and Periodicity in Properties”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this article such as ‘Mendeleev’s periodic table’, ‘Newlands’ octaves law’ etc. on website. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to website or download Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.