ICSE Class 10 Answered

Hydronium ions have positive charge because it is combination of H⁺ , which is just a proton and a water molecule. To understand this, look at the H₂O molecule first. The oxygen atom has 6 protons and each of the hydrogen atoms have a proton. Hence the H₂O molecule has a total of 8 positively charged protons. Oxygen has 6 valence electrons and each of the hydrogen’s have 1 valence electron, directly involved in covalent bonding with the oxygen atom. Hence the H₂O molecule has a total of 8 negatively charged electrons. As the number of protons = number of electrons in the molecule, the molecule has no overall charge. When a lone proton forms a dative covalent bond with a lone pair on the oxygen atom of the H₂O molecule, there are now overall 9 protons and still the same 8 valence electrons in the molecule. Hence due to the imbalance between protons and valence electrons in H₃O, there is an overall positive charge on the molecule, hence why it exists as H₃O⁺ when a lone proton (no electrons) bonds with H₂O.

 

Just applying the similar concept in hydroxyl ions (OH^-), It is having more electrons than protons so it will have negative charge on it.