Question

In which of the following is the number of electrons the ion contains given correctly?

Answer 1

B. Cl- has 17 electrons.

Answer 2

Answer: A) $K^+$ 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. $K^+$ is formed when one electron is lost from K.

$K\rightarrow K^++1e^-$

As one electron is lost from K to form $K^+$ , the number of electrons will be 18.

The atomic number of Cl(chlorine) is 17. In choice B, we have chlorine ion, $Cl^-$ . Since it has -1 charge, the number of electrons for this ion will be 18 and not 17.

$Cl+1e^-\rightarrow Cl^-$

Atomic number of B(boron) is 5 means it has 5 electrons. In choice C, we have $B^3^+$ which is formed when 3 electrons are lost from B.

$B\rightarrow B^3^++3e^-$

As 3 electrons are lost, there will only 2 electrons remaining.

Atomic number of Ra(Radium) is 88 means it has 88 electrons. $Ra^2^+$ is formed when 2 electrons are lost from Ra.

$Ra\rightarrow Ra^2^++2e^-$

As two electrons are lost, there will be 86 electrons remaining.

So, the only correct choice is A. $K^+$ has 18 electrons.