ATOMS 4U

Generally we know that atomic size increases down the group from top to bottom due to increase in the number of valance shell . But then why the atomic size of gallium is less than aluminum ?  Because , in case of gallium due to presence of d-orbitals , size of gallium  decreases to greater extend . D orbitals have poor shielding effect . It means that they cannot shield the nuclear force on outer most electrons. Due to decrease in shielding effect and increase in nuclear force , size of Gallium becomes  smaller than Aluminum . But remember that , this case only for Gallium , remaining 4th period elements follow the same trend , that is atomic size increase down the group . Effects of decrease in size of Gallium  ●Increase in ionization enthalpy  Due to decrease in size of Gallium , it's Ionization enthalpy increases . Smaller the size , more will be nuclear force on valance electrons . Therefore more energy requires to remove electron from the outermost shell ....