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An ion $X^{-}$ contains 18 electrons and 18 neutrons. The atomic number of the element X is ____.

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Detailed Solution

An ion

X^{-}

contains 18 electrons and 18 neutrons. The atomic number of the element X is 17.
Ion is the species which is formed when a neutral atom accepts electrons. For example, when fluorine with electron number 9 accepts electron, its electron number is 10 and it becomes

F^{-}

ion. For a given element, electron number 18 is obtained after the gain of the electron. Hence, the original number of electrons is

18 - 1 = 17

. Hence, 17 is the atomic number and the given element X must be chlorine.

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