Answer: A) [tex]K^+[/tex] has 18 electrons.
Explanation: Number of electrons in an atom equals to its atomic number. A cation is formed when electrons are lost from an atom and the anion is formed when electrons are gained by an atom.
Atomic number of K(potassium) is 19. [tex]K^+[/tex] is formed when one electron is lost from K.
[tex]K\rightarrow K^++1e^-[/tex]
As one electron is lost from K to form [tex]K^+[/tex] , the number of electrons will be 18.
The atomic number of Cl(chlorine) is 17. In choice B, we have chlorine ion, [tex]Cl^-[/tex] . Since it has -1 charge, the number of electrons for this ion will be 18 and not 17.
[tex]Cl+1e^-\rightarrow Cl^-[/tex]
Atomic number of B(boron) is 5 means it has 5 electrons. In choice C, we have [tex]B^3^+[/tex] which is formed when 3 electrons are lost from B.
[tex]B\rightarrow B^3^++3e^-[/tex]
As 3 electrons are lost, there will only 2 electrons remaining.
Atomic number of Ra(Radium) is 88 means it has 88 electrons. [tex]Ra^2^+[/tex] is formed when 2 electrons are lost from Ra.
[tex]Ra\rightarrow Ra^2^++2e^-[/tex]
As two electrons are lost, there will be 86 electrons remaining.
So, the only correct choice is A. [tex]K^+[/tex] has 18 electrons.
