Hint Electronegativity is a periodic trend that varies across the periodic table . It gives us an idea or application of the electronic charge that the electrons possess in an atom.
Electronegativity measures the tendency of an atom to draw in a shared pair of electrons. An atom's electronegativity is littered with both its number and therefore the distance at which its valence electrons reside from the charged nucleus. Electronegativity increases as you progress across the table from left to right. This happens thanks to a greater charge on the nucleus, causing the electron bonding pairs to be very drawn to atoms placed further right the table. Electronegativity decreases as you progress down the table.
(a) First period : It refers as to the filling of electrons in the first energy shell i.e., (K shell), n = 1, i.e. 1s is the only orbital present which can accommodate only two electrons, therefore the first period contains only two elements. Hydrogen and Helium.
Second period : It corresponds to the 2nd energy shell (L shell) i.e. n = 2 where electrons get filled up. Since this shell has four orbitals (one 2s and three 2p) can accommodate eight electrons therefore the second period contains eight elements. It starts with Li (Z = 3) which has three electrons where two electrons are in the 1s orbital and the third electron enters the 2s orbital.
Third period : This period corresponds to the third shell i.e., n = 3 where electrons get filled. It contains nine orbitals ( 3s, 3p and 3d). However, 3d-orbitals have high energy so filled after 4s-orbital due to the energy level diagram of multielectron atoms. They have only eight elements.
Fourth period : It is used to refer to the filling up of electrons in the fourth energy level i.e., n = 4. After filling 4s orbital filling of five 3d-orbital stars that can accommodate ten electrons. Before the 4p-orbital is filled 3d orbitals are filled first being energetically more favourable and we come across the so called 3d transition series of elements. This series starts at scandium (Z = 21) with electronic configuration 3d 4s. Therefore, the filling of 4p orbitals starts at gallium (Z = 31) and ends at krypton (Z = 36.
Fifth period : Fifth period corresponds to the fifth shell i.e., n = 5. Like the fourth period it also has nine orbitals (one 5s, three 5p and five 4d) which can accommodate eighteen electrons i.e., it can have eighteen elements. The fifth period starts at rubidium (Z = 37) where one electron enters the 5s-orbital After 5s-orbital filling, the filling of 4d-orbital begins which starts at yttrium (Z = 39) and ends at cadmium (Z = 48). These ten elements are known as 4d-transition series. Therefore the filling of 5p-orbital starts from indium (Z = 49) and ends at xenon (Z = 54).
Sixth period : It corresponds to the filling of the sixth energy level i.e., n = 6. Since in this period only sixteen orbitals (one 6s, seven 4f, five 5d and three 6p) are available, thereby the sixth period contains thirty two elements. After filling the 6s-orbital, the next electron enters the 5d-orbitals against the Aufbau principle and thereafter the filling of seven 4f-orbital begins with cerium (Z = 58) and ends up with lutetium (Z = 71). These fourteen elements constitute the first (or 4f) inner transition series called lanthanide series or lanthanide.
Seventh period : It corresponds to the filling of the seventh energy shell, i.e., n = 7. Like the sixth period, it also contains thirty two elements due to the sixteen orbitals (one 7s, seven 5f, five 6d and three 7p). This includes most of the man made radioactive elements. The filling of orbitals starts with 7s where The first electron enters into 7s of Fr(Z = 87).
Note: Electronegativity decreases down the group as atomic size increases down the group and increases along the period as atomic size decreases.