Question

How do you calculate the total number of protons and electrons in a polyatomic ion? Do you have to memorize it?

Answer 1

To solve this we must know that the groups of atoms that are covalently bonded and have a positive or negative charge caused by formation of an ionic bond with another ion are known as polyatomic ions. The polyatomic ion behaves as a single unit.

Complete step by step answer:
- We know that the groups of atoms that are covalently bonded and have a positive or negative charge caused by formation of an ionic bond with another ion are known as polyatomic ions.
- To calculate the total number of protons in a polyatomic ion we must know the number of protons of each atom in the polyatomic ion. We know that the atomic number of an element is equal to the number of protons. Thus, we can calculate the total number protons in a polyatomic ion from the atomic number of all the atoms in the polyatomic ions.
- To calculate the total number of protons in a polyatomic ion we must know the number of electrons of each atom in the polyatomic ion. We need to subtract or add the appropriate number of electrons depending on the positive or negative charge on the polyatomic ion.
- We know that the atomic number of an element is equal to the number of electrons. Thus, we can calculate the total number electrons in a polyatomic ion from the atomic number of all the atoms in the polyatomic ions and the charge on it.

For example consider the nitrate ion i.e. ${\text{NO}}_3^ -$.
- The atomic number of nitrogen is 7. Thus, the nitrogen atom has 7 protons and 7 electrons. The atomic number of oxygen is 8. Thus, each oxygen atom has 8 protons and 8 electrons. Thus, the total number of protons in ${\text{NO}}_3^ -$ ion $= \left( {1 \times 7} \right) + \left( {3 \times 8} \right) = 37 + 24 = 31$
- The total charge on the nitrate ion is $- 1$. Thus, add one electron to the total number of electrons. Thus, the total number of electrons in ${\text{NO}}_3^ -$ ion $= \left( {1 \times 7} \right) + \left( {3 \times 8} \right) + \left( 1 \right) = 37 + 24 + 1 = 32$. Thus, the total number of protons and electrons in a polyatomic ion can be calculated from the atomic number of atoms in the polyatomic ion. We do not have to memorize it.

Note: Remember that the total number of protons and electrons in a polyatomic ion can be calculated from the atomic number of atoms in the polyatomic ion. The atomic number is equal to the number of protons and the number of electrons. If the charge on the polyatomic ion is positive then subtract the electrons and if the charge on the polyatomic ion is negative then add the electrons.