You must have noticed in a retail shop, to make it easier to find related items, the shopkeeper arranges them together in one section. Different brands of soaps, shampoos, detergents, and other household items, for example, are stored in distinct locations in a specific order. Similarly, different scientists ordered elements in order to explore the properties of different elements in a more simple manner. The genesis of the periodic table, Law of triads, Law of octaves, Mendeleev’s periodic table, and the Modern periodic table, as well as its properties, advantages, and limitations, will all be covered in this article, Periodic Classification of Elements. Apart from that, you will compare periodic trends like atomic size, valency, ionisation energy, and so on, across periods and groups.

Students usually find Physics and Chemistry as very tough subjects. But at the same time, it is very important for students to learn these subjects as it is part of the curriculum. With regular practice, they can learn and master the subjects. Continue reading this article to know more about the periodic classification of elements, periodic trends, Mendeleev’s table, etc

Why do we Need the Classification of Elements?

The elements are the building blocks of all substances. There are \(118\) elements known today. It is not easy to study the properties of all these \(118\) elements separately.
Therefore, scientists felt the necessity to group the elements of similar characteristics together so that if the properties of one of them are known, the others could be correlated. This led to the arrangement of the known elements in certain groups in such a way so that the elements with similar properties are grouped together. This arrangement of the element is called the periodic classification of elements.

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How did Lavoisier classify the elements?

Initially, Lavoisier classified the elements as metals and non-metals but this classification failed with the discovery of some elements that could neither be classified as metal nor as non-metal.

What are Dobereiner’s Triads?

A German chemist, Johann Wolfgang Dobereiner, made one of the earliest attempts to classify the elements discovered at that time. He noticed that some elements formed groups of three that showed similar properties. These groups of three elements each were termed as a triad. Some triads classified by him are:

Triad \(1\)	Triad \(2\)	Triad \(3\)
Lithium	Calcium	Chlorine
Sodium	Strontium	Bromine
Potassium	Barium	Iodine

According to Dobereiner’s s law of triads “When three elements of any particular triad were arranged in order of their increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of other two elements”.

Example: The atomic mass of the middle element, sodium is the arithmetic mean of the atomic weight of the lithium and potassium, as shown below.

Element	Lithium \(\left({{\text{Li}}} \right)\)	Sodium \(\left({{\text{Na}}} \right)\)	Potassium \(\left({{\text{K}}} \right)\)	Arithmetic means
Atomic mass	\(6.941\,{\text{u}}\)	\(22.989\,{\text{u}}\)	\(39.098\,{\text{u}}\)	\(23.019\,{\text{u}}\)

Arithmetic mean of atomic masses of Lithium and Potassium \( = \frac{{6.941 + 39.098}}{2} = 23.019\,{\text{u}} = \) Atomic mass of sodium.

Limitations of Dobereiner’s Triads

Dobereiner could identify only a few such triads from the elements known at that time, and he could not arrange all the elements known at that time into triads.

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Newland’s Law of Octave

According to Newland’s law of octaves, “When elements are arranged in the order of increasing atomic masses, the properties of the eighth element are a repetition of the properties of the first element”. Since there is repetition in the properties of elements, just like the repetition of the eighth note in an octave of music, so he named his law as “law of octaves”.

sa (do)	re (re)	ga (mi)	ma (fa)	pa (so)	da (la)	ni (ti)
\({\text{H}}\)	\({\text{Li}}\)	\({\text{Be}}\)	\({\text{B}}\)	\({\text{C}}\)	\({\text{N}}\)	\({\text{O}}\)
\({\text{F}}\)	\({\text{Na}}\)	\({\text{Mg}}\)	\({\text{Al}}\)	\({\text{Si}}\)	\({\text{P}}\)	\({\text{S}}\)
\({\text{Cl}}\)	\({\text{K}}\)	\({\text{Ca}}\)	\({\text{Cr}}\)	\({\text{Ti}}\)	\({\text{Mn}}\)	\({\text{Fe}}\)
\({\text{Co}}\) and \({\text{Ni}}\)	\({\text{Cu}}\)	\({\text{Zn}}\)	\({\text{Y}}\)	\({\text{ln}}\)	\({\text{As}}\)	\({\text{Se}}\)
\({\text{Br}}\)	\({\text{Rb}}\)	\({\text{Sr}}\)	\({\text{Ce}}\) and \({\text{La}}\)	\({\text{Zr}}\)	–	–

The elements lying under a particular note have similar properties. This means every \({8^{{\text{th}}}}\) element has similar chemical and physical properties. For example, sodium is the \({8^{{\text{th}}}}\) element from lithium, and both have similar properties.

Limitations of Newland’s Law of Octaves

1. Newland’s law of octaves was applicable only up to the element calcium, i.e., lighter elements. After that, the properties of \({8^{{\text{th}}}}\) element were different from the first one.
2. He placed two elements in the same place and also placed unlike elements in the same column in order to fit all \(56\) elements into his table.
3. Newland considered that only \(56\) elements will be existing in nature and that no more elements will likely be discovered in future. But several new elements were discovered later whose properties did not fit into the law of octaves.
4. When noble gases were discovered later, the properties of the first element and eighth element were not similar, and it became the first element and ninth element. Thereby, the entire arrangement was disturbed. Thus, Newlands’ law of octaves was also discarded.

What is Mendeleev’s Periodic Table?

Russian chemist Dmitri I. Mendeleev in the year \(1869,\) made the first meaningful and remarkable contribution in the field of classification of elements, and he is called as the Father of the Periodic Table. He arranged all the \(63\) elements known at that time in the increasing order of their atomic weight in the form of a table called Mendeleev’s periodic table.

What is Mendeleev’s Periodic Law?

Mendeleev’s periodic law states that “The physical and chemical properties of the elements are the periodic function of their atomic weights”.

The Properties of Mendeleev’s Periodic Table:

Mendeleev arranged the elements having similar properties in the same group in the increasing order of atomic masses. With the discovery of new elements, the Mendeleev periodic table was revised, and the new elements were added at appropriate places. This table is called the modified form of Mendeleev’s periodic table.

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The main properties of Mendeleev’s Periodic Table are:

1. The \(9\) groups indicated by Roman numbers as \({\text{I}},{\text{II}},{\text{III}},{\text{IV}},{\text{V}},{\text{VI}},{\text{VII}},{\text{VIII}}\) and zero. The elements belonging to the first \(7\) groups have been divided into subgroups designated as \({\text{A}}\) and \({\text{B}}\) based on similarity in characters. Group \({\text{VIII}}\) consists of \(9\) elements which are arranged in three triads. Inert gases are placed in the zero group.
2. There are seven periods, and they are numbered from \(1\) to \(7.\) The \(4,5,6\) and \(7\) periods are divided into two halves to accommodate more elements.

What are the Advantages of Mendeleev’s Periodic Table?

1. For the first time in a symmetric way, the Mendeleev periodic table classified the elements. This greatly contributed to the study of the elements and their compounds.
2. Based on its position, Mendeleev predicts the features of some unknown elements.
Gallium and germanium, for example, have not been known. These Eka-aluminum and Eka Silicon were named by Mendeleev because he believed that their properties respectively were consistent with aluminium and silicon.

Demerits of Mendeleev’s Periodic Table

1. Hydrogen resembles the elements of the \({\text{IA}}\) group (alkaline metals) as well as those of the \({\text{VIIA}}\) group (halogens) in its properties. Therefore, it should have been placed in both \({\text{IA}}\) and \({\text{VIIA}}\) groups in the periodic table. It is placed in the \({\text{IA}}\) group in this periodic table.
2. According to atomic masses, isotopes of an element should be placed at different places. But this will be anomalous because isotopes possess similar chemical properties.
3. The periodic table of Mendeleev only shows one valence for a faulty element. Many elements display more than one valence.
4. Similar properties are shown by elements such as silver and thallium, barium and plum, copper and mercury but in various groups in the regular Mendeleev table.

Modern Periodic Law

In \(1913,\) Henry Moseley, from his studies on \({\text{X}}\)-ray, found that the atomic number is the fundamental property of an element but not atomic mass. Thus, he gave the new periodic law known as Modern Periodic law. According to modern periodic law, the physical and chemical properties of elements are the periodic functions of their atomic number. This means if the elements are arranged in tabular form in the increasing order of their atomic numbers, then the properties of the elements are repeated after definite regular intervals.

Modern Periodic Table

The modern periodic table is a systematic arrangement of elements into groups and periods based on similarities in chemical properties and in the order of their increasing atomic number. This arrangement is known as the long form of the periodic table. This table consists of seven horizontal rows called periods and eighteen vertical columns called groups numbered as \(1\) to \(18.\)

Periods: In the modern periodic table, there are seven horizontal rows called periods. Elements in a period have the same number of shells.

1. First period is called shortest and contains only two elements.
2. Second and third periods are called short periods containing eight elements each.
3.\({4^{{\text{th}}}}\) and \({5^{{\text{th}}}}\) periods are long periods containing \(18\) elements each.
4. \({6^{{\text{th}}}}\) and \({7^{{\text{th}}}}\) periods are the longest periods with \(32\) elements each.
5. There is a periodicity occurring at a regular interval of \(2,8,8,18,18\) and \(32.\)

Groups: The vertical columns in a periodic table are called groups. There are \(18\) groups in the modern periodic table.

1. The elements in a particular group exhibit the same valency and similar chemical properties. The physical properties of the elements in the group vary gradually.
2. The number of the shell increases down the group.
3. The elements of group-\(1\) are called alkali metals, and group-\(2\) are called alkaline earth metals.
4. Elements of group-\(16\) are called chalcogens, group-\(17\) are called halogens, and group-\(18\) are called noble gases.

Merits of Modern Periodic Table

1. This classification is based on the atomic number that is a more fundamental property of an element than atomic mass.
2. The position of isotopes at one place is justified on the basis of atomic number.
3. The systematic grouping of elements has made the study of elements simpler.
4. The position of elements is governed by electronic configuration and is useful in studying their properties.
5. The position of those elements, which was not justified on the basis of atomic mass, is now justified on the basis of atomic number.
6. The whole table is easy to remember and reproduce in terms of the electronic configuration and properties of the elements.

Defects in the Modern Periodic Table

1. The position of hydrogen is not clear because its electronic configuration is similar to that of alkali metals, and thus, it can be placed in Group \({\text{I}}\) with the alkali metals. Its electronic configuration is also similar to that of halogens, so it can be placed with halogens.
2. The lanthanides and actinides have not been placed in the main body of the table.

Types of Elements Present in Modern Periodic Table

1. Main group elements: The elements present in groups \(1\) and \(2\) (\({\text{s}}\)-block elements) on the left-hand side and Groups \(13\) to \(17\) (\({\text{p}}\)-block elements) on the right-hand side of the periodic table are called main group or representative elements.
2. Noble gases (Inert gases): Group \(18\) elements contain a completely filled outermost shell. Therefore, they are non-reactive or inert in nature.
3. Transition elements (\({\text{d}}\)-block elements): These elements are present in the middle block of the periodic table (Group \(3\) to \(12\)). These elements contain incomplete two outermost shells; due to this, they change from electropositive elements to electronegative elements. Therefore, they have named transition elements.
4. Inner transition elements (\({\text{f}}\)-block elements): These are placed in two separate rows as lanthanides and actinides at the bottom of the periodic table to avoid the expansion of the periodic table. Each row has \(14\) elements.
5. Metals: In the periodic table, metals are placed on the left-hand side. Group \({\text{1}}\) and \({\text{2}}\) are called alkali metals, and alkaline earth metals, respectively.
6. Non-metals: The left side of the periodic table is occupied by non-metals.
7. Metalloids: Metalloids are the elements that show the properties of both metals and non-metals. They are boron \(\left({\text{B}} \right),\) silicon \(\left({\text{Si}} \right),\) germanium \(\left({\text{Ge}} \right),\) arsenic \(\left({\text{As}} \right),\) antimony \(\left({\text{Sb}} \right),\) tellurium \(\left({\text{Te}} \right),\) and astatine \(\left({\text{At}} \right).\)

Trends in the Modern Periodic Table

There are many observable patterns in the properties of elements as we move down a group or across a period in the modern periodic table. This regular graduation of properties across the period or moving down the group is called periodicity in properties.

The chemical properties of the elements depend upon their electronic configuration. In a period from left to right, there is a regular change in the electronic configuration of elements. Whereas in a group, from top to bottom, the outermost shell electronic configuration is similar. Therefore, there is a regular change in chemical properties in a period while in a group elements have similar chemical properties. Some of the periodic trends are as follows:

1. Valence electrons: The electrons present in the valence shell (outermost shell) of an atom are called valence electrons. On moving from left to right across the period, the number of valence electrons increases from \(1\) to \(8.\) On moving down a group, the number of valence electrons remains the same. For example, the number of valence electrons in all the elements of Group \(1\) is one.
2. Valency: It is defined as the combining capacity of an atom of an element to acquire inert gas configuration. The valency of an atom is determined by the number of valence electrons present in the atom of an element.
If the number of valence electrons is \(1,2\) or \(3,\) then valency is equal to the number of valence electrons. But if the number of valence electrons is \(4,5,6,7\) or \(8\) then, Valency \( = 8 – \) number of valence electrons.
On moving from left to right across a period the valence first increases and then decreases. Example:

Group Number	\(1\)	\(2\)	\(13\)	\(14\)	\(15\)	\(16\)	\(17\)	\(18\)
Elements of period \(2\)	\({\text{Li}}\)	\({\text{Be}}\)	\({\text{B}}\)	\({\text{C}}\)	\({\text{N}}\)	\({\text{O}}\)	\({\text{F}}\)	\({\text{Ne}}\)
Atomic Number	\(3\)	\(4\)	\(5\)	\(6\)	\(7\)	\(8\)	\(9\)	\(10\)
Electronic coniguration	\(2,1\)	\(2,2\)	\(2,3\)	\(2,4\)	\(2,5\)	\(2,6\)	\(2,7\)	\(2,8\)
Valency	\(1\)	\(2\)	\(3\)	\(4\)	\(3\)	\(2\)	\(1\)	\(0\)

On moving down a group, valency remains the same as all the elements of the group have the same number of valence electrons.

For example, the valency of all the elements of Group-\(1\) is \(1.\) Since all elements have one valence electron.

3. Atomic size: Atomic size is measured in terms of atomic radius that is defined as the distance between the centre of the atomic nucleus and the outermost shell of an isolated atom. It is generally expressed in picometer, pm. \(\left({1\,{\text{pm}} = {{10}^{ – 12}}~{\text{m}}} \right).\)
On moving from left to right across a period, the atomic radius decreases. This is due to an increase in nuclear charge that tends to pull the electrons closer to the nucleus and reduces the size of the atom.

\(48\) Group Number	\(1\)	\(2\)	\(13\)	\(14\)	\(15\)	\(16\)	\(17\)
elements of period \(2\)	\({\text{Li}}\)	\({\text{Be}}\)	\({\text{B}}\)	\({\text{C}}\)	\({\text{N}}\)	\({\text{O}}\)	\({\text{F}}\)
Atomic radius (in pm)	\(167\)	\(112\)	\(87\)	\(67\)	\(56\)	\(48\)	\(42\)

On moving down a group, the atomic size increases in spite of the increase in nuclear charge. This is due to an increase in the number of shells. As a result, the distance between the outermost electrons and the nucleus increases, and the atomic size increases.

For example, the atomic size of group \(1\) elements increases down the group.

Period Number	Elements of Group \(1\)	Atomic size (pm)
\(2\)	\({\text{Li}}\)	\(167\)
\(3\)	\({\text{Na}}\)	\(190\)
\(4\)	\({\text{K}}\)	\(243\)
\(5\)	\({\text{Rb}}\)	\(265\)
\(6\)	\({\text{Cs}}\)	\(298\)

4. Metallic and Non-metallic Character: Metallic character (electropositive character) refers to the tendency to lose electrons, while non-metallic character (electronegative character) refers to the tendency to gain electrons.
On moving from left to right in each period, the atomic size decreases and nuclear charge increases, due to which the loss of electrons becomes difficult. As a result, the tendency to lose electrons decreases, and metallic character also decreases, and non-metallic character increases.
On moving down the group, the atomic size increases. Due to an increase in atomic size, nuclear attraction for the valence electron decreases, and the loss of electrons becomes easier. Thus, the tendency to lose electrons increases, i.e., the metallic character increases down the group, and non-metallic character decreases down the group.

5. Ionization energy: It is defined as the energy required to remove an electron from a gaseous atom or ion to form a positive ion. 
Across a period, atomic size decreases; therefore, the ionization energy increases. Down a group, atomic size increases, hence the ionization energy decreases.

6. Electronegativity: The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is called its electronegativity.
As we godown the group, the electronegativity decreases, and along a period, it increases.

7. Electron affinity: The quantity of energy released when transformed into a negatively charged gaseous ion (anion) by the addition of electrons is defined as the amount of energy.
Moving from the top to the bottom in a group, electron affinity decreases and across a period, electron affinity increases.
In a group from top to bottom, the affinity of the electron is declining and electron affinity is increasing over a period.

Periodic Classification of Elements Important Questions

Q.1. State the modern periodic law.
Ans: Modern periodic law states that the physical and chemical properties of elements are the periodic functions of their atomic number.

Q.2. How atomic size varies across the period from left to right in the modern periodic table?
Ans: On moving from left to right in a period, electrons are added to the same shell. Therefore, nuclear charge increases and atomic size decreases.

Q.3. What is the number of the groups and periods are present in the modern periodic table?
Ans: The modern periodic table consists of \(18\) groups and \(7\) periods.

Summary

Frequently Asked Questions

We have provided some frequently asked questions on the periodic classification of elements here:

Q.1. What was the need for the classification of elements?
Ans: There are 118 elements known today. It is not easy to study the properties of all these 118 elements separately. Therefore, it is necessary to group the elements of similar characteristics together so that if the properties of one of them are known, the others could be correlated.

Q.2. What are the advantages of the classification of elements?
Ans: Classification of elements makes the study of elements easier. During the classification of elements, elements with the same number of valence electrons are placed in the same group. Hence, if the properties of one of them are known, the others can be correlated. This also helps in the comparative study of the physical and chemical properties of elements.

Q.3. What elements are in the main groups?
Ans: Elements belong to group \(1\) (alkali metals), group \(2\) (alkaline earth metals) and elements of group \(13 – 18\) are called main group elements. In other words, elements of \({\text{s}}\)-block (group \(1\) and \(2\)) and \({\text{p}}\)-block (group \(13 – 18\) ) are called main group elements.

Q.4. What is meant by periodic classification of elements?
Ans: The arrangement of the known elements in certain groups in such a way so that the elements with similar properties are grouped together is called periodic classification of elements.

Q.5. What is the classification of elements?
Ans: Elements are classified as metals, non-metals, and metalloids (semimetals) based on their physical and chemical properties.

Q.6. What are the main features of the periodic classification of the elements?
Ans: Elements with the same number of valence electrons are placed in the same group since they show similar properties.

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