Isoeletronic

[AlexandraZuniga1L]

How can atoms and ions have the same number of electrons?

[Jovian Cheung 1K]

For example, Cl (which has 17 electrons) gains one electron to form Cl^- (which has 18 electrons). Notice that Ar, a noble gas, also has 18 electrons. There are many many such examples! :-)
To be fair, an atom of an element won't have the same number of electrons as its own ion(s). It can, however, have the same number of electrons as the ion of another element. It's almost like a coincidence; don't worry too much about it yet.

[Andrea- 3J]

isoelectronic: having the same number of electrons

An atom is neutral, as they contain the same number of protons and electrons.
An ion is electronically charged particle. When electrons are added to a neutral atom it becomes a positive ion. When electrons are removed from a neutral atom it becomes a negative ion.

For instance, when N, nitrogen which has 6 electrons loses an electron to become N- which has 5 electrons. Boron, B has 5 electrons. Therefore, an atom and an ion have the same number of electrons.

[Carlos De La Torre 2L]

While Atoms and ions may have the same amount of electrons it is important to remember they still have different properties, due the the difference in protons in the nucleus, so while ions may be isoelectronic, they can have different sizes and so forth.