Atoms in Group 18 elements are inert (chemically unreactive) because . A they combined to form molecules. B they have no valence electrons. C they have filled inner energy levels. D they have a full valence shell.

When you say full valence shell, are you talking about a valence electron shell?

I am learning about atoms and i know a little bit

thats correct

Explanation:

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =