Electronic Configuration in Periods and Groups

The electronic configuration of an atom is the numerical representation of the arrangement of electrons distributed in the orbitals of the atom. This determines the position of an element in the periodic table and in turn its chemical behavior. It explains how the atoms are held together by the chemical bonds, and the peculiar trends which are observed in the rows and columns of the periodic table.

Electronic Configuration in Periods

• The value of n, the principal quantum number, for the valence shell is the period of the element.

• Different energy levels can accommodate different number of electrons.

• The maximum number of electrons that a given energy level can accommodate is given by 2n², where n is the energy level. So the first energy level (K shell) can hold up to 2 electrons, second level (L shell) can hold up to 8 electrons, and third level (M shell) can also hold 18 electrons and so on.

• Thus, the first period with n = 1 can hold up to 2 elements by filling the lowest level 1s. The elements are Hydrogen and Helium with electronic configuration 1s¹ and 1s² This marks the complete filling of K shell.

• The second period starts with Lithium and Beryllium which have 3 and 4 electrons and hence the last electrons enter the level 2s and they have an electronic configuration of 1s²2s¹ and 1s²2s² This is followed by the start of the 2p orbital filling. It starts with Boron (1s²2s²2p¹) and ends with Neon (1s²2s²2p⁶) which marks the completion of L shell. Thus, 8 elements are present in the second period.

• The third period starts with Sodium and ends at Argon while successively filling 3s and 3p orbitals. 8 elements are present in this period too.

• The fourth period with n = 4 starts by filling the level 4s. It begins with Potassium. However, we know that 3d orbital is to be filled before filling of 4p orbital starts. This marks the beginning of the 3d transition elements with Scandium (electronic configuration- [Ar]3d¹4s²). At Zinc (electronic configuration- [Ar]3d¹⁰4s²), the 3d orbital is filled.

• The fifth period with n = 5 starts by filling the level 5s. This period consists of the 4d transition series which starts with Yttrium. Xenon ends the period by completely filling the 5p orbital.

• The sixth period with n = 6 holds 32 elements with electrons filling 6s, 4f, 5d and 6p orbitals. Cerium marks the electrons entering 4f orbital giving rise to the 4f-inner transition elements, called the lanthanide series.

• The seventh period with n = 7 includes the man-made radioactive elements with electrons filling 7s, 5f, 6d and 7p orbitals. Similar to period 6, this period also leads to the filling of electrons to 5f orbital, giving rise to the 5f-inner transition elements known as the actinide series.

Electronic Configuration in Groups

Elements in the same group have the same number of electrons in their outermost shell leading to similar valence shell electronic configuration. Thus, we observe a similar trend in the properties and chemistry of the elements in the same group.