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Q & A: Atomic numbers: protons, neutrons, and electrons

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Most recent answer: 12/04/2011
Q:
Do you think you can tell me a little bit about how to find out the number of electrons, protons, and neutrons in a atom?? Thanks So Much! :-)
- Julie (age 15)
East Longmeadow High School, Massachusetts
A:
Hi Julie!
If you're asking this question, I'm sure that you have access to a periodic table of the elements somewhere (if not, there's one at that you can look at) If you look on the periodic table, you will see that there is a number by each element. That number is the "atomic number" and is equal to the number of protons in the atom. If the atom is neutral (not an ion), then there are the same number of electrons as protons.
Now, for more difficult things. Imagine the atomic symbol of an element, say Helium, He. Now imagine a little square around the He. The square has four corners, and sometimes there will be numbers in those corners, like:

.....x....x
.......He
.....x....x

Each of these spots may have a number that tells you something:

.....4....1+
.......He
....2

Let's start with the lower left corner, where the "2" is. This is the atomic number, or the number of protons.
Now look at the upper left corner, where the "4" is. This number corresponds to the number of protons plus the number of neutrons. Use the top left to find the number of neutrons. We see that our atom has 2 protons, so we just subtract: 4 - 2 = 2 neutrons.
The upper right corner has to do with charge and number of electrons. This may be blank - just think of a blank spot meaning "0." If it is not blank, the number there is how many more protons than electrons that the atom has. This number may be positive or negative (you could have a 3- there, instead of a 3+, for example). In our atom, we have a "1+". That means there is one more proton than electron. Take the number of protons (from the upper left) and use this:
1+ means there is one more proton than electron. There are 2 protons. 2 - 1 is 1, so there is one electron.
If the number were negative (Let's say 2-), then you would say "there are two less protons than electrons" and:
2 - (-2) = 2 + 2 = 4 so there would be four electrons in this case.

Hope this helps!
-Sara

(published on 10/22/2007)

Follow-Up #1: atomic notation

Q:
This explanation is not correct. The 2 and 4 are switched.
- Tom (age 22)
Ca
A:
hey thanks, I think that's fixed now.

Mike W.

(published on 12/04/2011)

Follow-up on this answer.