Find total number of third period element having less ionisation energy than the element of next atomic number.

Question

TestHub · Accepted Answer

The third-period elements are Na, Mg, Al, Si, P, S, Cl, Ar. Ionization energy generally increases across a period, but exceptions occur.

Mg ( $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2}$ ) has higher IE than Al ( $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{1}$ ) due to stable half-filled/fully-filled subshell.

P ( $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{3}$ ) has higher IE than S ( $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{4}$ ).

The elements with less ionization energy than the element of the next atomic number are Na, Al, S, Cl, Ar.

Thus, there are 5 such elements.

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