1.

An element "X" has mass number 35 and the numberof neutrons, is 18. Identify the group number and periodof "X".(CBSE Sample Paper 2018)

Answer»

Given :Mass number of element is 35Number of neutrons= 18

Formula to find number of protons: Mass number = number of protons + number of neutrons.35=× +18×=35 - 18X=17.Number of protons =17 and number of electrons also will be 17 as atom is electrically neutral.

Hence Element is chlorine

Electronic configuraton:1s2 2s2 2p6 3s2 3p5

Group number= 17Period number =3Valency =1



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