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