Question

How do we assign charge to atoms and molecules? Answer with reference to the water molecule.

Answer 1

Look at the parent water molecule.....

Explanation:

We write `H-O-H`; and this is neutral molecule, and the Lewis structure reflects this neutrality. Each hydrogen has 1 proton, 1 positively charged particle in its nucleus; and this is matched by the ONE of the 2 electrons conceived to form the `H-O` bond. Hence hydrogen is neutral.

The other electron devolves to the oxygen. Now oxygen has 8 positive charges in its nucleus. And a neutral oxygen should therefore bring 8 electrons to the table. Well, there are 2 inner core electrons (i.e. the `1s^2` electrons), 2 electrons from the `2xxO-H` bonds, and FOUR electrons from the lone pairs, Lone pair electrons, and formal charges, devolve SOLELY to the contributing atom for the purpose of charge assignment. The oxygen in water is in fact electrostatically neutral. With me?

And now look at `H_3O:^+`; each hydrogen has the ONE electron, so these are neutral, but the oxygen now has 3 electrons from the `O-H` bonds, 2 lone pair electrons, and 2 inner core electrons; 7 electrons in total and THUS a formal positive charge.

And so at an atomic level we conceive of the distribution of charge for the hydronium ion as follows.....

`underbrace(HCl(g))_"NEUTRAL" + underbrace(H_2O(l))_"NEUTRAL"rarrunderbrace(H_3O^+ + Cl^-)_"OVERALL NEUTRAL"`

As with any chemical reaction, both MASS and CHARGE have been CONSERVED (what does this mean in this context?), but clearly charge separation has occurred to give discrete positive and negative ions. The hydronium ion as written is more of a conception than an actual entity (it is possibly `H_9O_4^+`) but our representation is still valid and useful.

From a prior answer on this question....which considers the formal charge of methane, which of course is neutral.

Just to add that for the purposes of assigning formal charge, we can go back to very old ideas that we learn when are introduced to bonding. In a covalent bond, electrons ARE SHARED between nuclei. An ionic bond is between a formal anion, and a formal cation, and thus involves the prior TRANSFER of electron.

If we take say methane, with FOUR `C-H` covalent bonds, we split this up to give `4xxdotH`, and a carbon atom with 4 valence electrons. The hydrogen ATOMS are neutral, because they have a positive nuclear charge, and also an electronic charge. And hence the hydrogens are neutral.

Likewise, the carbon atom claims 4 electrons from the four `C-H` bonds, and has 2 inner core electrons (i.e. formally the `1s^2` electrons. And so each carbon has 6 electrons, and there are 6 positive nuclear charges. The carbon atom in methane is also thus electronically neutral.

Also see this old answer.