Aparticle with 12 electrons has a -2 charge. how many protons does it have.

Well, it has 10 protons.

For, a atom in its neutral state has the same number of protons as electrons. Thatswhy, it has a neutral charge. The protons have a positive charge (1+); the elctrons have a negative charge(1-). So, the charges cancel out. Like, a atom has 11 electrons and 11 protons. See-

11 e- = -11
11 p+ = +11

Then, . So, no charge. Its neutral. It happens in case of every atom.

And, if an atom has (2-) charge that means it has two extra electrons. Two more electron than the number of protons. It once had the same number of protons and electrons. Then two more electrons give it a (2-) charge. Thatswhy, if we subtract 2 from 12, we can get the number of protons.

We can calculate the number of protons as, 

Okay, what's the proof? Want it?

We know there are 10 protons and 12 electrons. Let's see it.



Eureka! That's it. I got it. It's right.

AND ANOTHER QUESTION ARISES, If all atoms have the same number of protons and electrons then, when does the number of electron decrease or increase? How do the atoms come to have positive or negative charge?

Well, this happens because of ionic bonding. I can't explain all these stuff, right here. To obtain more information on bonding, search it on GOOGLE.