Chemistry Electronic Configurations of Elements and the Periodic Table

Topics Covered :

● Electronic Configuration of Elements and the Periodic Table
● Electronic Configurations in Periods
● Groupwise Electronic Configurations

Electronic Configuration of Elements and the Periodic Table :

`=>` An electron in an atom is characterised by a set of four quantum numbers, and the principal quantum number (`n`) defines the main energy level known as shell.

`=>` We have also studied about the filling of electrons into different `text(subshells)`, also referred to as `text(orbitals)` (`s`, `p`, `d`, `f`) in an atom.

`text(Electronic Configuration :)` The distribution of electrons into orbitals of an atom is called its electronic configuration.

`=>` An element’s location in the Periodic Table reflects the quantum numbers of the last orbital filled.

`=>` Here, we will see a direct connection between the electronic configurations of the elements and the long form of the Periodic Table.

Electronic Configurations in Periods :

`=>` The period indicates the value of `n` for the outermost or valence shell.

● In other words, successive period in the Periodic Table is associated with the filling of the next higher principal energy level (`n = 1`, `n = 2`, etc.).

● It can be readily seen that the number of elements in each period is twice the number of atomic orbitals available in the energy level that is being filled.

`=>` The first period (`n = 1`) starts with the filling of the lowest level (`1s`) and therefore has two elements — hydrogen `(1s^1)` and helium `(1s^2)` when the first shell `(K)` is completed.

`=>` The second period `(n = 2)` starts with lithium and the third electron enters the `2s` orbital.

● The next element, beryllium has four electrons and has the electronic configuration `1s^2 , 2s^2`.

● Starting from the next element boron, the `2p` orbitals are filled with electrons when the `L` shell is completed at neon `(2s^2 , 2p^6)`.

● Thus there are `8` elements in the second period.

`=>` The third period `(n = 3)` begins at sodium, and the added electron enters a `3s` orbital.

● Successive filling of `3s` and `3p` orbitals gives rise to the third period of `8` elements from sodium to argon.

`=>` The fourth period `(n = 4)` starts at potassium, and the added electrons fill up the `4s` orbital.

● Here before the `4p` orbital is filled, filling up of `3d` orbitals becomes energetically favourable and we come across the so called `3d` `text(transition series)` of elements.

● This starts from scandium `(Z = 21)` which has the electronic configuration `3d^1 , 4s^2`.

● The `3d` orbitals are filled at zinc `(Z=30)` with electronic configuration `3d^(10) , 4s^2`.

● The fourth period ends at krypton with the filling up of the `4p` orbitals.

● Altogether we have `18` elements in this fourth period.

`=>` The fifth period `(n = 5)` beginning with rubidium is similar to the fourth period and contains the `4d` transition series starting at yttrium `(Z = 39)`.

● This period ends at xenon with the filling up of the `5p` orbitals.

● The sixth period `(n = 6)` contains `32` elements and successive electrons enter `6s, 4f, 5d` and `6p` orbitals, in the order — filling up of the `4f` orbitals begins with cerium `(Z = 58)` and ends at lutetium `(Z = 71)` to give the `4f`-inner transition series which is called the `text(lanthanoid series)`.

`=>` The seventh period `(n = 7)` is similar to the sixth period with the successive filling up of the `7s, 5f, 6d` and `7p` orbitals and includes most of the man-made radioactive elements.

● This period will end at the element with atomic number `118` which would belong to the noble gas family.

● Filling up of the `5f` orbitals after actinium `(Z = 89)` gives the `5f`-inner transition series known as the `text(actinoid series)`.

`=>` The `4f` and `5f`-inner transition series of elements are placed separately in the Periodic Table to maintain its structure and to preserve the principle of classification by keeping elements with similar properties in a single column.

Groupwise Electronic Configurations :

`=>` Elements in the same vertical column or group have similar valence shell electronic configurations, the same number of electrons in the outer orbitals, and similar properties.

`text(Example :)` The Group 1 elements (alkali metals) all have `ns^1` valence shell electronic configuration as shown in fig.

`text(Note :)` Therefore, properties of an element have periodic dependence upon its atomic number and not on relative atomic mass.

Valence electrons- electrons in outermost shell/energy level (main group elements)

`=>` The valence electrons are the electrons in the outer shell of an atom. The valence electrons are involved in forming bonds to adjacent atoms.
`=>` For neutral atoms, the number of valence electrons is equal to the atom's main group number.
number of valence electrons(neutral atoms)= main group number