Electronic Configurations Of Elements
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The arrangement of elements in the Modern Periodic is based on electronic configurations.

The long form of periodic table is the most widely-used modern periodic table. It arranges all the 117 known elements into 7 periods and 18 groups based on their electronic configuration.

The term electronic configuration refers to the distribution of electrons in the shells and orbitals of an atom.

The principal quantum number of a shell (n) defines the energy level of an electrons and gives its the distance from the nucleus.

Shell is further divided into four sub shells called orbitals. These are s, p, d and f.

The electronic configuration of an atom defines its physical and chemical properties.

Ex: Moving from left to right in a period, the atomic size keeps on decreasing which happens because of increase in atomic number, more electrons get added to the same principal energy level.

Due to the same effective distance of electrons from a more positive nucleus (due to additional protons), electrons get more strongly attracted to the nucleus, thereby, decreasing the atomic size.

And if principal quantum number increases as we go down a group as the outer shells are more shielded from the nuclear attraction due to additional layer of electrons and distance from the nucleus as compared to the previous level. This results in increasing atomic size with increase in atomic number.

The chemical properties of elements depend on their electronic configuration for the same reason.

The elements with higher principal quantum number more readily take part in chemical reactions due to weakly bound outer electrons.

The chemical reactivity decreases with increase in atomic number for the set of elements having the same principal quantum number.

The other physical and chemical properties of elements such as density, boiling and melting points, ionization energy, electron gain enthalpy and electronegativity also depend on their electronic configuration. These physical and chemical properties repeat periodically for elements having the same electronic configuration in their outermost shell, a trend that can be seen across the elements belonging to the same group in the long form of periodic table.

The long form of periodic table arranges elements with the same principal quantum number (n) across a period such that their electronic configuration changes only at the outermost shell therefore, depends on the maximum number of electrons that principal energy level or shell can accommodate

Ex: The first period corresponds to the filling of electrons in the first energy shell or K shell, where n equals 1

Therefore, the first period has only two elements - Hydrogen 1s¹ and Helium 1s².

The second period (n=2) corresponds to the filling of electrons in the second energy or L shell.

It has a total of four orbitals - one 2s and three 2p, that can collectively accommodate eight electrons.

The second period starts with Lithium with atomic number 3 and electronic configuration 1s² 2s¹ and ends with neon with atomic number 10. Neon has completely filled p shell and electronic configuration of 1s² 2s²2p⁶.

The 3rd period (n=3) corresponds to the filling of electrons in the third energy shell (or M shell).

Although this shell has nine orbitals, one 3s, three 3p and five 3d orbitals and filling electrons in 3d orbitals is not energetically favorable as per Aufbau principle. Therefore, only four orbitals are available for filling electrons, which allow a maximum of 8 electrons in the outermost shell.

As a result, there are 8 elements in the third period starting from sodium whose electronic configuration is 3s¹ and ending with argon whose electronic configuration is 3s² 3p⁶.

The 4th period corresponds to the partial filling of electrons in the fourth energy level, where n equals 4.

This is because, for this period, filling the 3d orbitals after the 4s orbitals and before the 4p orbitals becomes energetically favorable in accordance with the Aufbau principle.

Therefore, this period contains 3d transition series of elements placed after potassium and calcium in the fourth period.

The first element in the 3d series, Scandium has an electronic configuration 3d¹ 4s². The last element in this series is Zinc having an electronic configuration 3d¹⁰4s², where the 3d orbital is completely filled.

The 4th period ends with the element krypton with atomic number 36 and completely filled 4p orbitals.

The 5th period corresponds to the filling of electrons in the fifth energy (n=5) shell.

This period contains 4d transition element series which starts with yttrium that has an atomic number 39 and ends with cadmium, with atomic number 48.

The first element in the 4d series, yttrium has an electronic configuration 4d¹5s². The last element in this series is cadmium having an electronic configuration 4d¹⁰5s², where the 4d orbital is completely filled.

In the 5th period the electrons enter into the 5s, 4d and 5p orbitals successively.

The 5th period ends with xenon with atomic number 54 and completely filled 5p orbitals.

The 6th period (n=6) corresponds to the filling of electrons in the sixth energy shell. It contains a total of 32 elements and follows the orbital sequence of 6s, 4f, 5d and 6p.

This period contains the 4f inner transition series of elements after cesium and barium called the lanthanide series.

The lanthanide series starts at cerium and ends at lutetium.

The 7th period consists of most of the man-made radioactive elements that have 7s, 5f, 6d and 7p orbitals filled.

This period also contains 32 elements including the 5f-inner transition series called the actinide series. The actinide series starts at thorium and ends at lawrencium.

   Period

   (n)

Orbitals filled

    Number of Orbitals filled

Number of Elements

First

1

1s

1s-orbital = 1 orbital

               2

Second

2

2s2p

1s-orbital +3p-orbitals = 4-orbitals

          2+6=8

Third

3

3s3p

1s-orbital +3p-orbitals = 4-orbitals

          2+6=8

Fourth

4

4s3d4p

1s-orbital +5d-orbitals+3p-orbitals = 9-orbitals

         2+10+6=18

Fifth

5

5s4d5p

1s-orbital +5d-orbitals+3p-orbitals = 9-orbitals

         2+10+6=18

Sixth

6

6s4f5d6p

1s-orbital +7f-orbitals+5d-orbitals+3p-orbitals = 16-orbitals

     2+14+10+6=32

Seventh

7

7 7s5f6d7p

1s-orbital +7f-orbitals+5d-orbitals+3p-orbitals = 16-orbitals

      2+14+10+6=32


It is important to note that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled. For example, in the 1st period only one atomic orbital is filled hence the number of elements is two, in the second and third periods, four atomic orbitals can be filled therefore the number of elements is eight, in the fourth and fifth periods nine orbitals can be filled hence the number of elements is 18 and in the sixth and seventh periods, 16 orbitals can be filled and hence the number of elements is thirty two.

Elements were classified across periods results in vertical columns of elements with similar outer electronic configuration, same number of valence electrons and therefore posses similar properties. So, they belong to the same family or group.

Ex: Elements in Group 1 of the periodic table have ns¹ valence shell electronic configuration.

Due to the presence of same number of valence electrons, all elements (alkali metals) of this group show similar properties.

The only group that could have distorted this characteristic of all elements in a group having similar properties is Group 3 that contains the lanthanide and actinide series of elements in periods 6 and 7.

In order to avoid this discrepancy and to maintain the structure of the periodic table, these series are placed separately below the periodic table to preserve the principle of classification.

Summary

The arrangement of elements in the Modern Periodic is based on electronic configurations.

The long form of periodic table is the most widely-used modern periodic table. It arranges all the 117 known elements into 7 periods and 18 groups based on their electronic configuration.

The term electronic configuration refers to the distribution of electrons in the shells and orbitals of an atom.

The principal quantum number of a shell (n) defines the energy level of an electrons and gives its the distance from the nucleus.

Shell is further divided into four sub shells called orbitals. These are s, p, d and f.

The electronic configuration of an atom defines its physical and chemical properties.

Ex: Moving from left to right in a period, the atomic size keeps on decreasing which happens because of increase in atomic number, more electrons get added to the same principal energy level.

Due to the same effective distance of electrons from a more positive nucleus (due to additional protons), electrons get more strongly attracted to the nucleus, thereby, decreasing the atomic size.

And if principal quantum number increases as we go down a group as the outer shells are more shielded from the nuclear attraction due to additional layer of electrons and distance from the nucleus as compared to the previous level. This results in increasing atomic size with increase in atomic number.

The chemical properties of elements depend on their electronic configuration for the same reason.

The elements with higher principal quantum number more readily take part in chemical reactions due to weakly bound outer electrons.

The chemical reactivity decreases with increase in atomic number for the set of elements having the same principal quantum number.

The other physical and chemical properties of elements such as density, boiling and melting points, ionization energy, electron gain enthalpy and electronegativity also depend on their electronic configuration. These physical and chemical properties repeat periodically for elements having the same electronic configuration in their outermost shell, a trend that can be seen across the elements belonging to the same group in the long form of periodic table.

The long form of periodic table arranges elements with the same principal quantum number (n) across a period such that their electronic configuration changes only at the outermost shell therefore, depends on the maximum number of electrons that principal energy level or shell can accommodate

Ex: The first period corresponds to the filling of electrons in the first energy shell or K shell, where n equals 1

Therefore, the first period has only two elements - Hydrogen 1s¹ and Helium 1s².

The second period (n=2) corresponds to the filling of electrons in the second energy or L shell.

It has a total of four orbitals - one 2s and three 2p, that can collectively accommodate eight electrons.

The second period starts with Lithium with atomic number 3 and electronic configuration 1s² 2s¹ and ends with neon with atomic number 10. Neon has completely filled p shell and electronic configuration of 1s² 2s²2p⁶.

The 3rd period (n=3) corresponds to the filling of electrons in the third energy shell (or M shell).

Although this shell has nine orbitals, one 3s, three 3p and five 3d orbitals and filling electrons in 3d orbitals is not energetically favorable as per Aufbau principle. Therefore, only four orbitals are available for filling electrons, which allow a maximum of 8 electrons in the outermost shell.

As a result, there are 8 elements in the third period starting from sodium whose electronic configuration is 3s¹ and ending with argon whose electronic configuration is 3s² 3p⁶.

The 4th period corresponds to the partial filling of electrons in the fourth energy level, where n equals 4.

This is because, for this period, filling the 3d orbitals after the 4s orbitals and before the 4p orbitals becomes energetically favorable in accordance with the Aufbau principle.

Therefore, this period contains 3d transition series of elements placed after potassium and calcium in the fourth period.

The first element in the 3d series, Scandium has an electronic configuration 3d¹ 4s². The last element in this series is Zinc having an electronic configuration 3d¹⁰4s², where the 3d orbital is completely filled.

The 4th period ends with the element krypton with atomic number 36 and completely filled 4p orbitals.

The 5th period corresponds to the filling of electrons in the fifth energy (n=5) shell.

This period contains 4d transition element series which starts with yttrium that has an atomic number 39 and ends with cadmium, with atomic number 48.

The first element in the 4d series, yttrium has an electronic configuration 4d¹5s². The last element in this series is cadmium having an electronic configuration 4d¹⁰5s², where the 4d orbital is completely filled.

In the 5th period the electrons enter into the 5s, 4d and 5p orbitals successively.

The 5th period ends with xenon with atomic number 54 and completely filled 5p orbitals.

The 6th period (n=6) corresponds to the filling of electrons in the sixth energy shell. It contains a total of 32 elements and follows the orbital sequence of 6s, 4f, 5d and 6p.

This period contains the 4f inner transition series of elements after cesium and barium called the lanthanide series.

The lanthanide series starts at cerium and ends at lutetium.

The 7th period consists of most of the man-made radioactive elements that have 7s, 5f, 6d and 7p orbitals filled.

This period also contains 32 elements including the 5f-inner transition series called the actinide series. The actinide series starts at thorium and ends at lawrencium.

   Period

   (n)

Orbitals filled

    Number of Orbitals filled

Number of Elements

First

1

1s

1s-orbital = 1 orbital

               2

Second

2

2s2p

1s-orbital +3p-orbitals = 4-orbitals

          2+6=8

Third

3

3s3p

1s-orbital +3p-orbitals = 4-orbitals

          2+6=8

Fourth

4

4s3d4p

1s-orbital +5d-orbitals+3p-orbitals = 9-orbitals

         2+10+6=18

Fifth

5

5s4d5p

1s-orbital +5d-orbitals+3p-orbitals = 9-orbitals

         2+10+6=18

Sixth

6

6s4f5d6p

1s-orbital +7f-orbitals+5d-orbitals+3p-orbitals = 16-orbitals

     2+14+10+6=32

Seventh

7

7 7s5f6d7p

1s-orbital +7f-orbitals+5d-orbitals+3p-orbitals = 16-orbitals

      2+14+10+6=32


It is important to note that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled. For example, in the 1st period only one atomic orbital is filled hence the number of elements is two, in the second and third periods, four atomic orbitals can be filled therefore the number of elements is eight, in the fourth and fifth periods nine orbitals can be filled hence the number of elements is 18 and in the sixth and seventh periods, 16 orbitals can be filled and hence the number of elements is thirty two.

Elements were classified across periods results in vertical columns of elements with similar outer electronic configuration, same number of valence electrons and therefore posses similar properties. So, they belong to the same family or group.

Ex: Elements in Group 1 of the periodic table have ns¹ valence shell electronic configuration.

Due to the presence of same number of valence electrons, all elements (alkali metals) of this group show similar properties.

The only group that could have distorted this characteristic of all elements in a group having similar properties is Group 3 that contains the lanthanide and actinide series of elements in periods 6 and 7.

In order to avoid this discrepancy and to maintain the structure of the periodic table, these series are placed separately below the periodic table to preserve the principle of classification.

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