99pts. i can't find isotope, pottasium-15 or k-15. is this even possible, i have a fill-in-the-blanks worksheet with a table like example:

element name: nitrogen-20 symbol: n-20 atomic number: 7 atomic mass: 20 charge: 0 protons: 7 neutrons: 13 electrons: 7

i remember finding the isotope number, nitrogen-20 by adding the protons and neutrons, then i get a new atomic mass. but on here pottasium-15 is confusing! what neutrons do i add from 19 protons to get 15? sometimes the teacher throws in impossible problems.

Yes, that's what [s]he has done this time.

First of all, the number of protons in every atom of an element
is the same.  That's the "Atomic number" of the element.  If
somehow the nucleus of the atom gained or lost a proton, then
that would change the Atomic number of the atom, and it would
be an atom of a different element.

Potassium is element #19.  Every atom of Potassium has 19 protons
in its nucleus.  Any atom that has 19 protons in its nucleus is an atom
of Potassium, and any atom that has more or less than 19 protons in
its nucleus is an atom of some other element, not Potassium.

In addition to protons, atoms also have some neutrons in the
nucleus.  Neutrons have no charge, so they don't change the
atomic number of the atom.  But they have mass ... almost the
same as the mass of a proton ... so the neutrons add to the atom's
"atomic weight".  The atomic weight of an atom is

                       (number of protons) plus (number of neutrons).

The numbers you see next to the names are the atomic weights. 
Notice that the atomic weight of an atom of Potassium can't be
less than 19 ... in order for it to be Potassium in the first place,
it has 19 protons, so its atomic weight would be 19 even if it had
no neutrons at all. 

There can be no such thing as an atom of Potassium-15.

Atoms of the same element can have different numbers of
neutrons in their nucleii.  Those are called "isotopes" of the
element.

There are three isotopes of Potassium that occur in nature ...
atoms with 19 protons plus 20, 21, or 22 neutrons.  So the
atomic weight of any naturally occurring atom of Potassium
is either 39, 40, or 41.   If you ever see a big chart that lists
natural isotopes of the elements, then in slot-#19, you'll see
Potassium-39, Potassium-40, and Potassium-41.

I hope my explanation is good enough for 50 points.