IS there a nice person anywhere that can explain this to me?

[Guest Starry_Night]

Hello,

 

I really need help. I got a test tomorrow, and I need to know how to figure out what the atomic number, mass number, number of protons, number of nuetrons and number of electrons of an element. (Use sodium as an example.) Please help me figure it out step by step. Thank you to all who answer!!!

 

thanks,

Starry_night

[blike]

will u have access to a periodic table?

[John]

I'm assuming you'll have access to a periodic table.

 

The atomic number of an atom will always be a whole number, because the atomic number is equal to the number of protons in the atom's nucleus (which is also equal to the number of electrons orbiting the nucleus). For example, if you look in the box containing sodium (Na) on your periodic table, you'll see that its atomic number is 11.

 

The mass number is usually written on the periodic table as a decimal number taken to a few decimal places. For instance, sodium's atomic mass number is 22.989770 (your periodic table might only take the number to three or four decimal places).

 

As I mentioned earlier, the atomic number is equal to the number of protons (or the number of electrons) in the atom. In order to find the number of neutrons, round the atomic mass to the nearest whole number (23 in sodium's case), then subtract the atomic number. Using this for sodium, you subtract 11 (the atomic number) from 23 (the atomic mass number). 23-11 = 12, so there are 12 neutrons in sodium's nucleus.

[fafalone]

btw, you find the mass number by the weighted average of common isotopes.

[Ragnarak]

basically as already stated:

 

no. of protons = no. of electrons = atomic number

 

ie. 11

 

no. of neutrons = 'what's left after you account for the protons' = atomic mass - no. of protons

 

ie. 23-11

[fafalone]

We usually don't say atomic number is equal to the number of electrons, since H+ is still H, but has no electrons.

[Ragnarak]

ok then, slight addition to take into account not just the element but the ions too:

 

no. of electrons = (no. of protons) - (charge on ion*)

 

 

 

 

 

 

 

 

*which is 0 for stable elements