How does an atom of chlorine-37 become a chloride ion with a -1 charge?
17
35.45
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a. the atom gains 1 electron, to make a total of 21.
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b. the atom gains 1 electron, to make a total of 18.
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c. the atom loses 1 proton, to make a total of 36.
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d. the atom loses 1 proton, to make a total of 16.

B. The atom gains 1 electron, to make a total of 18 electrons.

Explanation:

Chlorine is the 17th element in the periodic table, so it has atomic number 17:

Z = 17

This means that a neutral atom of chlorine has 17 protons and 17 electrons.

When a chlorine atom gains 1 electron, its electric charge (initially zero) becomes -1, since the electron has negative charge of -1 (in elemntary charge units). This also means that the number of electrons in the ion is now

17 + 1 = 18

So the correct answer is

B. The atom gains 1 electron, to make a total of 18 electrons