Explanation:
Group 18 is also known as noble gases. Elements of this group are helium, neon, argon, krypton, xenon, and radon.
Period 1 means value of n = 1. Period 2 means value of n = 2. Period 3 means value of n = 3. Period 4 means value of n = 4 and so on, where n is valence shell.
Electronic configuration of helium is [tex]1s^{2}[/tex].
Electronic configuration of neon is [tex]1s^{2}2s^{2}2p^{6}[/tex].
Electronic configuration of argon is [tex]1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}[/tex].
Electronic configuration of krypton is [tex]1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}[/tex].
Electronic configuration of xenon is [tex]1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^{6}[/tex].
Electronic configuration of radon is [tex]1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^{2}4p^{6}4d^{10}5s^{2}5p^{6}4f^{14}5d^{10}6s^{2}6p^{6}[/tex].
Therefore, we can conclude that krypton is the atom which is a noble gas in group 18, row 4.
