Question

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

A. K+ has 18 electrons.
B. Cl- has 17 electrons.
C. B3+ has 5 electrons.
D. Ra2+ has 88 electrons.

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.