Question

Justify the given statement with suitable examples "the properties of the elements are a periodic function of their atomic numbers".

Answer 1

Therefore are numbrous physical properties suchas melting points, boiling points heats of fusion and vaporisation energy of atomisation etc, which show periodic variations.
the cause of periodiocty in properties is the repetition of similar outer electronic configurations after certain regular intervals , e.g., all the elements of 1st group (alkali metals) have similar outer electronic configuration i.e.,$n{s}^1$
${}_{3}Li=1s^2,2s^1{}^{}{}_{11}Na=1s^2,2s^2,2p^6,3s^1{}^{}{}_{19}K=1s^2,2s^2,2p^6,3s^2,3p^6,4s^1{}^{}{}_{37}Rb=1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,5s^1{}^{}{}_{55}Cs=1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10},5s^2,5p^6,6s^1{}^{}{}_{87}Fr=1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10},4f^{14}5s^2,5p^6,5d^{10},6s^2,6p^6,7s^1$
Therefore due to similar outermost shell electronic configuration all alkali metals have similar properties. e.g, sodium and potassium both are soft and reactive metals, they all from basic oxides and their basic character increases down the grouop. They all form unipositive ion by the lose of one electron.
Similarly, all the elements of 17th group (haiogens) have similar outermost shell electronic configuration, i.e. $n{s}^2,n{p}^6$ and thus possess similar properties.
${}_{9}F=1s^2,2s^2,2p^5{}^{}{}_{17}Cl=1s^2,2s^2,2p^6,3s^2,3p^5{}^{}{}_{36}Br=1s^2,2s^2,2p^{6}3s^2,3p^6,3d^{10},4s^2,4p^6{}^{}{}_{53}I=1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10}5s^2,5p^5{}^{}{}_{85}At=1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10}4f^{14},5s^2,5p^6,5d^{10},6s^2,6p^5$