# what is atomic number of an atom having -1 charge,18 electrons and 20 neutrons?

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If the ion has a -1 charge, and has 18 electrons, this means that it typically would have 17 electrons in its neutral state. And, therefore, it would require 17 protons. The element with 17 protons is chlorine. The specific isotope is chlorine-37.

Atomic number: 17

• The Atomic number is the number of protons in the atom and it is equivelent to the number of electron too. In this atom, the number of electrons is 18 but then it is mentioned that the atom is really an ion with a negative charge which means an electron has been added to develop a stable outer shell and in this case it's a halogen....you would this the atomic number would be 18 but really it's not for the atom it's for the ion so u subtract the number of electrons added which in this case is 1 from the total number of electrons which is 18 and the answer will be 17 which is the atomic number of the atom...

• The -1 charge means there is an extra electron that the atom has added to complete its valence shell. To determine the number of protons and the atomic number, simply remove the extra electron which would give you 17 electrons for a neutral atom which consists of equal numbers of electrons and protons. Hence, the atomic number is also 17. Neutrons do not figure into this part of the atom, they figure into the atomic weight of the atom and the particular isotope of this element, chlorine.

Source(s): I am an engineer
• If the atom has a -1 charge then it has one more electron than protons. Therefore the number of protons is 17 and the element is atomic number 17. (The neutrons have nothing to do with the atomic number).

• Anonymous