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# Chemistry HELP! I need to find protons neutrons and electrons out of a formula!?

Hi i need to work out an equation, please help!

I need to find out how many protons - how many neutrons and how many electrons are in the formula

31, 15 P3-

Please help and elaborate on how you come to the answer? My chemistry teacher confused me!

### 1 Answer

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- 8 years agoBest Answer
A = atomic weight = 31 = p + n

p = no. of protons

n = no. of neutrons

p = 15

so, p + n = 15 + n = 31

so, n = 16.

now e = no. of electrons.

e = p when neutral, but here there is -3 charge, so there are 3 electrons in excess.

So.e = p + 3

e = p + 3 = 15 + 3 = 18.

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