Question

The atomic number of an element is 13. What will be the number of electrons in its ion ${{\text{X}}^ - }^3$.

Answer 1

Atomic number gives the number of protons and electrons in neutral elements. The charge on an atom is due to gain or loss of electrons by the element.

Complete step by step answer:
All matter is composed of small particles called atoms. The main fundamental particles are electron, proton and neutron. Electron was discovered in a cathode ray experiment by Thomson in 1887. Charge of one mole of electrons is about 96500 Coulomb or 1 Faraday. Proton was discovered in anode ray experiment by Goldstein in 1896. Neutron was discovered by James Chadwick in 1932. It is neutral and slightly heavier than proton. Atomic number is represented by Z. It is the number of protons present in the nucleus of an element or equal to number of electrons in neutral species of that element. For neutral atom; ${\text{Z}} = {\text{ number of proton}} = {\text{ number of electron}}$
For charged particles:
${\text{number of electron}} = {\text{ Z}} - \left( {{\text{charge of atom}}} \right)$ and Z is equals to number of protons only. Mass number is represented by A and it is the sum of the number of protons and neutrons in the nucleus of the element. A is the sum of the number of protons and the number of neutrons. An atom of the element is represented by zXᴬ.
The atomic number of elements is 13 as given in question. It means there are 13 protons in the nucleus of the element and 13 electrons in its neutral state. When an element loses an electron, its charge increases by the number of electrons it loses. When an element gains an electron, its charge decreases by the number of electrons it gains.
Now to find number of electron in its ion ${{\text{X}}^ - }^3$ ion
${\text{Number of electron}} = {\text{ Z}} - \left( {{\text{charge on atom}}} \right)$
$\Rightarrow {\text{Number of electron}} = {\text{ }}13 - \left( { - 3} \right) = 16$
Thus, there will be 16 electrons in ${{\text{X}}^ - }^3$ . The positive or negative charge on an atom depends on the number of electrons it loses or gains. The number of protons remains the same as it is not so easy to donate or accept protons.

Thus, the correct answer is 16 electrons.

Note:
Atomic number and mass number of an element is always a whole number. Of all the elementary particles in an atom, neutron is the heaviest and least stable particle. Actually an isolated neutron is unstable and disintegrates into an electron, proton and antineutrino.