The important advantages of the modern periodic table are given below :

1. This classification is based on the atomic number which is a more fundamental property of the elements.
2. Since this classification is based on the atomic number and not on the atomic mass, the position of placing isotopes at one place is fully justified.
3. It is easy to remember and reproduce.
4. The systematic grouping of elements into four blocks; s, p, d and f has made the study of the elements more simple.
5. The position of some elements which were misfit on the basis of atomic mass is now justified on the basis of atomic number. For example, argon precedes potassium because argon has atomic number 18 and potassium has 19.
6. The lanthanoids and actinoids which have properties different from other groups are placed separately at the bottom of the periodic table.

Defects of the Modern Periodic Table

The main defects of this table are :

1. The position of hydrogen is not settled. It resembles with alkali metals as well as halogens. However, it has been placed with alkali metals.
2. Lanthanoids and actinoids have not been accommodated in the main body of the periodic table.

Test your understanding and answer these questions:

1. What are advantages of modern periodic table?
2. What are defects of modern periodic table?

In addition to classifying the elements into s block, p block, d block and f block, the elements can be classified in three categories on the basis of their properties as: (1) Metals (2) Non-metals and (3) Metalloids.

Position of Metals on the Periodic Table

Metals appear on the left hand side of the Periodic Table, They have high melting and boiling points and are generally solid at room temperature (mercury is an exception which is a liquid at room temperature. They are good conductors of heat and electricity. They are malleable (can be flattened into thin sheets) and ductile (can be drawn into wires). They have bright luster.

Position of Nonmetals on the Periodic Table

Nonmetals are present on the right hand side of the periodic table. They are usually solids or gases at room temperature with low melting and boiling points (boron and carbon are exceptions). The only liquid non-metal is bromine. Most non-metals are brittle and are neither malleable nor ductile.

Metalloids or Semimetals

The elements silicon, germanium, arsenic, antimony, tellurium, etc. show properties characteristic of both metals and non-metals. These are called metalloids or semimetals. These are present diagonally on the p block of modern periodic table.

Trends in metallic and non-metallic character

In general, the metallic increases as we go down a group and non-metallic character increases as we move from left to right across a period in the Periodic Table.

Test your understanding and answer these questions:

1. Name a liquid metal.
2. Name a liquid nonmetal.
3. What are metalloids?
4. What is the position of metals on periodic table?
5. What is the position of nonmetals on periodic table?
6. What is the position of metaloids on periodic table?

The elements in which the last electron enters the f-orbitals of their atoms are called f-block elements. These consist of two series of elements placed at the bottom of the periodic table.

Lanthanoids or Lanthanides or rare earth metals: The first series follows lanthanum, La (Z= 57) and the elements present in this series from cerium to lutetium (₅₈Ce – ₇₁Lu) are called lanthaniods or lanthanides. These are also called rare earth elements.

Actinoids or Actinides: The second series follows actinium, Ac (Z = 89) and the elements present in this series from thorium to lawrencium (₉₀Th – ₁₀₃Lr) are called actionoids or actinides. These are of radioactive elements.

The general electronic configuration of f-block elements may be written as: (n-2) f^1-14 (n-1) d^0-2 ns² The elements included in these two series are called inner transition elements, because they form transition series within the transition elements of d-block.

General characteristics of f-block elements.

1. They are heavy metals.
2. They generally have high melting and boiling points.
3. They exhibit variable oxidation states.
4. They form coloured ions.
5. They have the tendency to form complex compounds.
6. Actinoids are radioactive in nature. The elements after uranium are called transuranium elements.

Test your understanding and answer these questions:

1. What are lanthanoids or lanthanides?
2. What are actinoids or actinides?
3. Why are f block elements called inner transition elements?
4. What is the general electronic configuration of f block elements?

The elements in which the last electron enters the d-orbitals of their last but one (called penultimate energy level) constitute d-block elements. This block consists of the elements lying between s and p blocks starting from fourth period and onwards. They constitute groups 3 to 12 in the periodic table. In these elements the outermost shell contains one or two electrons in their s-orbital (ns) but the last electron enters the last but one d-subshell i.e., (n-1)d. The general electronic configuration for the atoms of d-block elements may be written as (n-1) d^1-10 ns^0-2

These elements are also called transition elements because transition elements form a bridge between the chemically active metals of s-block elements and non-metals elements of p block. Therefore, they represent transition (change) in behaviour and take their familiar name “transition elements”.

The d-block comprises three complete rows of ten elements and one incomplete row. These rows are called first, second and third transition series which involve the filling of 3d, 4d and 5d –orbitals respectively. These series are also called transition series.

Elements of First transition series
Scandium (Z = 21) to Zinc (Z = 30)
(4th period : 3d- orbitals are gradually filled)

Elements of Second transition series
Yttrium (Z = 39) to Cadmium (Z = 48)
(5th period : 4d-orbitals are gradually filled)

Elements of Third transition series
Lanthanum (Z = 57), Hafnium (Z = 72) to Mercury (Z = 80)
(6th period : 5d- orbitals are gradually filled)

Elements of Fourth transition series
Actinium (Z = 89), Rutherfordium (Z = 104) to Copernicium (Z = 112)
(7th period : 6d- orbitals are gradually filled)

General characteristics of d-block elements

1. They are metals having high melting and boiling points.
2. Most of them form coloured compounds.
3. They have a good tendency to form complex compounds.
4. Their compounds are generally paramagnetic.
5. They exhibit several oxidation states.
6. Most of the transition elements such as Mn, Ni, Co, Cr, V, Pt and their compounds are used as catalysts.

Test your understanding and answer these questions:

1. What are transuranium elements?
2. Why are d block elements called transition elements?
3. What is the general electronic configuration of d block elements?

The elements in which the last electron enters the p-orbital of their outermost energy level are called p block elements. The elements of groups 13 to 18 involving addition of one (ns² np¹), two (ns² np²), three (ns² np³), four (ns² np⁴), five (ns² np⁵), and six (ns² np⁶) electrons respectively in p-orbitals are called p block elements. s-orbitals are already filled in their atoms. The general electronic configuration for the atoms of p block elements may be written as : ns² np^1-6

The elements of s-and p-block are collectively called representative elements. The elements of last group (18) having ns2 np6 configuration are called noble gases. All the orbitals in the valence shell of the noble gases are completely filled and they have no tendency to lose or gain electrons. Therefore, the noble gases exhibit very low reactivity. Preceding the noble gas family are two chemically important groups of non-metals. These are halogens (group 17) and chalcogens (group 16). These two groups of elements can readily accept one and two electrons respectively to attain noble gas configurations and form univalent and divalent negative ions.

General characteristics of p-block elements

1. They include both metals and non-metals
2. Their ionization enthalpies are relatively high as compared to s-block elements.
3. They form mostly covalent compounds.
4. Some of them show more than one oxidation states in their compounds.

Test your understanding and answer these questions:

1. What are noble gases?
2. What is the general electronic configuration of p block elements?

All the existing elements in the modern periodic table is divided into four main segments or blocks. A block consists of groups of elements having similar electronic configurations and properties. These are called s block, p block, d block and f-block.

S block elements on modern periodic table

The elements in which the last electron enters the s-orbital of their outermost energy level are called s-block elements. It consists of elements of group 1 of modern periodic table and group 2 of modern periodic table having the ground state electronic configurations of outermost shell as ns¹ and ns² respectively (where n stands for outermost energy shell). The elements of first group on modern periodic table corresponding to ns1 configuration are called alkali metals while the elements of group 2 on modern periodic table corresponding to ns² configuration are called alkaline earth metals. Thus the general electronic configuration of s-block elements may be expressed as : ns^{1 – 2}


General characteristics of s-block elements

1. They are soft metals having low melting and boiling points.
2. They have low ionization enthalpies.
3. They are very reactive and readily form univalent ions (alkali metals) or bivalent ions (alkaline earth metals) by losing one or two valence electrons respectively.
4. Most of them impart characteristic colours to the flame.
5. They mostly form ionic compounds except lithium and beryllium.

Test your understanding and answer these questions:

1. What are alkali metals?
2. What are alkaline earth metals?
3. What is the general electronic configuration of s block elements?

The modern periodic table was constructed by the Russian chemist Dmitri Mendeleyev in 1869. The modern periodic table consists of horizontal rows called periodsand vertical columns called groups. These are discussed below:

Groups in Modern Periodic Table

A group may be defined as vertical column in the periodic table. In terms of electronic structure of the atom, a group constitutes a series of elements whose atoms have the same outermost electronic configurations. There are 18 groups in the long form of the periodic table. According to the new recommendations ofinternational Union of Pure and Applied Chemistry (IUPAC), the groups are numbered from 1 to 18.

Each group contains elements that tend to react chemically in similar ways, because they all have atoms in which the arrangement of electrons around the nucleus is similar. Some groups in the periodic table are given special names e.g.

Group 1        the alkali metals

Group 2        the alkaline earth metals

Group 15       the pnicogens

Group 16       the chalcogens

Group 17       the halogens

Group 18       the noble gases

Representative elements in modern periodic table
The first two groups on the extreme left and last six groups on the extreme right involve the filling of s- and p- orbitals, respectively, These groups represent the main groups of the periodic table and are numbered as 1, 2, 13, 14, 15, 16, 17 and 18. The elements present in these groups are known as normal elements or representative elements.

Transition series elements in modern periodic table
The ten groups lie in between first two and last six groups i.e. between group 2 and group 13. These are numbered from 3 to 12. The elements present in these groups are called transition elements. The name is derived from the fact that they represent transition (change) in character from reactive metals (elements of groups 1 and 2) on one side to the non-metals (elements of group 13 to 18) on the other side.

Inner transition series elements in modern periodic table

There are two more rows at the bottom of the periodic table. These rows consist of fourteen elements after lanthanum (Z = 57) and fourteen elements which follow actinium (Z = 89). These are placed separately in the periodic table to save space and avoid undue sidewise expansion of the periodic table. The elements in the first row, starting from cerium are called lanthanoids (or lanthanides) and the elements present in the second row starting from thorium are called actinoids (or actinides). These lanthanoids and actinoids together are called inner transition elements or rare earth metals and these are built up by filling of f-orbitals.

Test your understanding and answer these questions:

1. What are alkali metals?
2. What are alkaline earth metals?
3. What are representative elements?
4. What is transition series of elements?
5. What is inner transition series of elements?
6. How many groups are there in modern periodic table?
7. What is definition of group in modern periodic table?

The modern periodic table was constructed by the Russian chemist Dmitri Mendeleyev in 1869. The modern periodic table consists of horizontal rows called periods and vertical columns called groups. These are discussed below:

Periods in the modern periodic table

A period may be defined as horizontal row in the periodic table. In terms of electronic structure of the atom, a period constitutes a series of elements whose atoms have the same number of electron shells i.e. principal quantum number (n). Each successive period in the periodic table is associated with the filling of the next higher principal energy level (n = 1, n = 2, n = 3, etc.) There are seven periods and each period starts with a different principal quantum number.

Reading across a period from left to right, the number of electrons in the outer shell or subshell of the element increases.

Period 1 Elements

The first period corresponding to n = 1 is unique because it contains only two elements. First energy shell (K) has only one orbital (i.e. 1s) which can accommodate only two electrons. This means that there can be only two elements. Hydrogen (1s¹) and Helium (1s²).

Period 2 Elements

The second period contains 8 elements because for n = 2, there are four orbitals (one 2s and three 2p) in second energy shell (L). In all, these four orbitals have a capacity of eight electrons and, therefore, second period has eight elements in it. It starts with lithium (Z = 3) in which one electron enters the 2s-orbital. The period ends with neon (Z = 10) in which the second shell is complete (2s² 2p⁶).

Period 3 Elements

This period contains eight elements from sodium (Z = 11) to argon (Z = 18).

Period 4 Elements

There are eighteen elements in fourth period from potassium (Z = 19) to krypton (Z = 36). In this period, we come across elements which involve the filling of 3d orbitals. These are known as transition series of elements. This starts from scandium (Z = 21) which has electronic configuration 3d¹ 4s² and ends at zinc (Z = 30) with 3d orbitals completely filled having electronic configuration 3d¹⁰ 4s².

Period 5 Elements

The fifth period like the fourth period also consists of 18 elements. It begins with rubidium (Z = 37) with filling of 5s-orbital and ends at xenon (Z = 54) with the filling up of the 5p-orbitals. This period also contains 10 elements of 4d transition series starting at yttrium (Z = 39).

Period 6 Elements

The sixth period contain 32 elements (Z = 55 to 86). In addition to 10 elements of 5d transition series, this period contains 14 elements which involve filling up of 4f orbitals beginning from Cerium (Z = 58) to Lutetium (Z = 71). The series of elements are called inner transition series or lanthanoid series.

Period 7 Elements

The Seventh period is similar to the sixth period. Though expected to have 32 elements, this period is incomplete and contains only 29 elements at present. This period is expected to end at the element with atomic number 118, which would belong to the noble gas family. This period also contains 14 elements which involve the filling of 5f orbitals starting from actinium (Z = 89). This is also called 5f-inner transition series or actinoid series.

The first three periods containing 2, 8 and 8 elements respectively are called short periods, the next three periods containing 18,18 and 32 elements respectively are called long periods.

Test your understanding and answer these questions:

1. What are lanthanoids?
2. What are actinoids?
3. What are alkali metals?
4. What are alkaline earth metals?
5. What are representative elements?
6. What is transition series of elements?
7. What is inner transition series of elements?
8. How many periods are there in modern periodic table?
9. What is definition of period in modern periodic table?
10. How many elements are present in first period of modern periodic table?
11. How many elements are present in second period of modern periodic table?
12. How many elements are present in third period of modern periodic table?
13. How many elements are present in fouth period of modern periodic table?
14. How many elements are present in fifth period of modern periodic table?
15. How many elements are present in sixth period of modern periodic table?
16. How many elements are present in seventh period of modern periodic table?

In chemistry periodicity of elements means the recurrence of similar properties of the elements after certain regular intervals when they are arranged in the order of increasing atomic numbers.

Cause of Periodicity of Elements

The cause of periodicity of the properties of elements is the repetition of similar electronic configuration of their atoms in the outermost energy shell (or valence shell) after certain regular intervals. To explain the cause of periodicity of elements we are taking two examples.

Electronic configuration of Alkali Metals:

The chemical properties of all alkali metals resemble each other. This is due to the fact that chemical properties of elements depend primarily on the arrangement of electrons in the outermost shell (valence shell). The electronic configuration of the atoms of alkali metal group shows that all the alkali metals have one electron in their outermost s-orbital. So, their electronic configuration may be represented as (noble gas) ns¹.

Table of electronic configurations of Alkali Metals

Electronic configuration of Halogens:

Similarly, all the members of the halogen family have 7 electrons in the valence shell i.e. two electrons in the s-orbital and five electrons in the p-orbital of the valence shell. The general electronic configuration of halogens may be expressed as ns² np⁵. Thus, all the halogens have similar chemical properties due to same outer-shell electronic configuration.

Table of Electronic Configurations of Halogens

Test your understanding and answer these questions:

1. What is Periodicity of elements?
2. What is the cause of periodicity of elements in modern periodic table?

Modern Periodic Law of element may be defined as the Physical and chemical properties of the elements are periodic functions of their atomic numbers. It means, when the elements are arranged in the order of their increasing atomic numbers, it is observed that the elements of similar properties recur at regular intervals or periodically. As a result of this, the elements fall in certain groups and lead to an arrangement called the modern periodic table of elements. It must be noted that elements are arranged in the periodic table in order of atomic numbers because the atomic number is the most fundamental property of an element. The systematic arrangement of elements in the modern periodic table according to their atomic number helps in justification of isotopes of elements at one place.

Test your understanding and answer these questions:

1. What is modern periodic law?