Why is the mass number of chlorine not a whole number?

[Jadaav]

I got a question :

Why is the nucleon of Chlorine 35.5 ? Why is is not a whole number ?

 

[Borek]

35.5 is not an atomic mass, it is atomic weight of the element.

 

[Jadaav]

What's the difference between atomic mass and atomic weight of an element ?

 

[Borek]

Have you ever heard about google and wikipedia?

 

[Jadaav]

Yeah, I searched in Wikipedia.

 

[Borek]

And you still don't know what is a difference between the two? Can you quote or write - in your own words - one line definitions of both?

 

[Jadaav]

Atomic mass is the mass of 1 atom comprising of its protons, neutrons and electrons.

Atomic weight is the average mass of all the atoms of an element that exist to 1/12 of the mass of an atom of Carbon - 12.

Am I right here ?

Wikipedia says this:

"The atomic mass is the total mass of protons, neutrons and electrons in a single atom (when the atom is motionless)"

Why does it specify when the atom is motionless ? Does it make a difference if its in motion ?

"Atomic weight (symbol: Ar) is a dimensionless physical quantity,..."

What is a dimensionless physical quantity ?

 

[Borek]

Am I right here ?

Yes.

Now, get back to your question and see if you can answer it.

Why does it specify when the atom is motionless ? Does it make a difference if its in motion?

Yes - moving objects get heavier (although for the effect to be observable their speed must be real high). Google for "relativistic mass".

What is a dimensionless physical quantity ?

One that has no units. Mass has units of kg (pound, stone...), length has units of m (feet, cable...), atomic weight has no units.

 

[Jadaav]

Now I got the answer to my question. It is not a whole number because it is the average mass of the element. When making averages, we often encounter decimals.

 

[Borek]

Wasn't that hard

 

[Jadaav]

Yeah, thanks to you