Question

Calculate the number of electrons, protons and neutrons in the following species. (i) Phosphorus atom  (ii) Phosphide ion (P^3-)  (iii) Magnesium ion (Mg²⁺)

Mass number : P = 31, Mg = 24

Atomic numbers : P = 15, Mg = 12

Answer 1

(i) Phosphorus atom 

Number of electrons = Atomic number = 15 

Number of protons = Atomic number = 15 

Number of neutrons = Mass number - Atomic number = 31 - 15 = 13. 

(ii) Phosphide ion (P^3-). 

Phosphide ion (P^3-) = Phosphorus atom + 3 electrons 

P^3- = P + 3e^- 

Thus, phosphide ion has same number of protons and neutrons as phosphorus atom but has three electrons more. 

Number of electrons = 15 + 3 = 18 

Number of protons = 15 

Number of neutrons = 31 - 15 = 16 

(iii) Magnesium ion (Mg²⁺) 

Mg²⁺ ion is formed by the loss of two electrons by Mg atom. Therefore, it has two electrons less than the number of electrons is Mg atom. 

Number of electrons = 12 - 2 = 10 

Number of protons = 12 

Number of neutrons = (24 - 12) = 12