How do I work out the number of electrons without an atomic number?

atomice mass = no of protons + no. of neutrons in an atom
if u subtract atomic mass and no. of neutrons like this:-
atomic mass - no. of neutrons = no. of protons

now as u know the no. of protons and electrons in an atom are SAME
so if u know the no. of protons , then u will automatically knw the number of electrons

You can figure out the atomic number from the information given.

Most of an atom's mass is in the nucleus.  Electrons don't have much mass.  Multiply mass of a neutron by # of neutrons in atom.  Subtract from atomic mass.  You get mass of protons in the atom.  Divide by mass of a proton.  You should get something very close to a whole number.  This is the number of protons in the atom and thus your atomic number.

P.S.  There seem to be some people who do not like my instructions for working out the atomic number.  If there is an error, please let me know.

atomic mass = number of protons + number of neutrons
thus
number of protons = atomic number =  atomic mass - number of neutrons

number of protons = number of electrons

The question is checking your understanding of atomic terms and definitions

Atomic mass = number of protons + number of neutrons -  You have the atomic mass
You have the number of neutrons.

Therefore: Number of protons = atomic mass- number of neutrons.

The number of electrons is equal to the number of protons.

So you are able to work out the answer.



.

atomic mass=mo.of neutrons+no.of protons

if you know the atomic no.of that element it will become easy to find not only the no.of electrons but  
also it's E.C(electronic configuration)

Ian giving you a hint:
find your answer through this formula :



1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d
6s 6p
7s


Thankyou