Hydronium ion contains hydrogen and oxygen atoms. Lewis structure of hydronium ion (H3O+) contains three O-H bonds. There is +1 charge and one lone pair on oxygen atom in H3O+ lewis structure.
Each hydrogen atom has linked with oxygen atom. Only one lone pair exist on oxygen atom. Therefore, there is a +1 charge on oxygen atom in the lewis structure of H3O+ ion.
When we draw a lewis structure, there are several guidelines to follow. Number of steps can be changed according the complexity of the molecule or ion. Because H3O+ is an ion, we will have more steps than drawing a simple molecule. However those all steps are mentioned and explained in detail in this tutorial for your knowledge.
There are two elements in hydronium ion; hydrogen and oxygen. Hydrogen is a group IA element in the periodic table and only has one electron in its last shell (valence shell). Oxygen is a group VIA element in the periodic table and contains six electrons in its last shell. Now, we know how many electrons are there in valence shells of hydrogen and iodine atoms.
Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells
Total electron pairs are determined by dividing the number total valence electrons by two. For, H3O+ ion, Total pairs of electrons are four in their valence shells.
Because, hydrogen cannot be a center atom in a molecule or ion, oxygen becomes the center atom.
After determining the center atom and sketch of H3O+ ion, we can start to mark lone pairs on atoms. Remember that, there are total of four electron pairs.
There is +1 charge on oxygen atom in the drawn lewis structure.
Because hydrogen atom cannot make double bonds, we cannot convert lone pair to a bond. Also, there are not enough lone pairs on atom to convert as bonds to reduce charges. That means, we have already obtained the lewis structure.