[XM8]

Hi guys,

I have a test tomorrow in chemistry about the atom and there are a few questions I'd like to clear up.

Now I've been reading up on something called an "elementary charge" and I can't qutie grasp what it means. Does it mean that a single proton's charge is 1.602 x 10−19 C and a single electron's charge is −1.602 x 10−19 C ?

I also read something about "e" and "-e", correct me if I'm wrong but e refers to the proton's charge and the -e refers to the electron's charge (due to the minus), right?

Moreover what mixes me up the most is the issue of mass.

In my lesson we wrote that the mass of an electron is 9,1 x 10-31 kg

Now if I want to calculate the mass of the electrons in lets say an aliminium atom, would it be correct if I calculated 13 x 9,1 x 10-19 kg?

I'm also not sure on how to calculate the mass of a nucleus. In my lesson I wrote that I have to multiply the number of nucleons with the mass of a nucleon. Does that make sense? I thought nucleons were protons + neutrons, so how does a nucleon have a mass?

Thanks,

-Xm8

[Perito]

Does it mean that a single proton's charge is 1.602 x 10−19 C and a single electron's charge is −1.602 x 10−19 C ?

That sounds correct.

I also read something about "e" and "-e", correct me if I'm wrong but e refers to the proton's charge and the -e refers to the electron's charge (due to the minus), right?

Correct. I usually think of e as the charge. +e (or just e) is, the charge on the proton while -e is the charge on the electron.

In my lesson we wrote that the mass of an electron is 9,1 x 10-31 kg

Now if I want to calculate the mass of the electrons in an aliminium atom, would it be correct if I calculated 13 x 9,1 x 10-19 kg?

That's also correct in a *neutral* aluminum (watch your spelling) atom. I don't think I've ever been asked to calculate the mass of electrons in an atom.

I'm also not sure on how to calculate the mass of a nucleus. In my lesson I wrote that I have to multiply the number of nucleons with the mass of a nucleon. Does that make sense? I thought nucleons were protons + neutrons, so how does a nucleon have a mass?

A proton and a neutron contain most of the mass of the atom. Electrons comprise a very tiny amount of the mass. Protons and neutrons have almost the same mass. Therefore, you are correct that you multiply the number of nucleons (protons + neutrons) by the mass of one nucleon.

I think you've got it! Good luck.

[XM8]

Hey Perito,

First let me thank you for your helpful answer. I took the test a few hours ago and unfortunately I didn't do as well as I'd hoped.

I was asked to calculate the aproxamative mass of an atom of gold.

What I wrote was : Mnucleus = Mprotons + Mneutrons
Mnucleus = (1,673 x 10^-27 x 79) + (1,675 x 10^-27 x 197)

The only problem is that instead of multiplying the mass of the 79 protons present in the atom of gold with the 118 neutrons, I multiplied 79 protons with the number of nucleons, that being 179.

That probably ruined the whole exercise. Plus I wrote that all of the atom's mass is in the nucleus, which is not true. As you mentioned most of it is in the nucleus, how ever some of it is in the electrons, their mass being 9,1 x 10^-31 kg.


I was also asked to write the symbol of an atom of iron. The periodic table of elements was on the wall behind me, I did manage to find it, however I was so busy doing the other questions I forgot to write the answer - two simple letters : "Fe". :(


Oh well I hope I'll do better next time.

Thanks again for your help,

-Xm8