What fraction of this beryllium atom's mass comes from its electrons?

Question

confused on how to find the mass of electrons? 

Answer 1

In order to answer that question, you need to identify the specific isotope of Be you are talking about.  Is it Be-9 or Be-10?  Or another isotope?  Be-9 is the only stable isotope of Be, so let's look at that one...

 

There are 4 protons because it is Be and its atomic number is 4.  Therefore, there are also 4 electrons in the neutral atom.  Be-9 has a nuclear mass of 9, which means total protons + neutrons = 9.  Since we already know there are 4 protons, there must be 5 neutrons in this particular isotope.

 

Protons and Neutrons weigh "approximately" 1 amu, but the exact masses can be found in your textbook.  Electrons are very tiny compared to protons and neutrons, but they do still have a small mass.  Add up all of the masses for 

4 protons = 4 x 1.0073 amu = 4.0292 amu

5 neutrons = 5 x 1.0087 amu = 5.0435 amu

4 electrons = 4 x 5.486 x 10^-4 amu = 0.002194 amu

Total these up, and this Be atom would weigh 9.0749 amu

 

As you can see, the electrons account for a very small portion of the mass, but they still contribute to the total.  

As a percentage, they are 0.002194 amu / 9.0749 amu = 0.0002418  x 100% = 0.02418 %

WAY less than 1% of the mass!!!

 

Hope that helps... 

Answer 2

Electrons do not contribute to the atomic mass.

 

They are zer0 amu