How many atoms are in a sample of chromium with a mass of 13 grams?

Let's find the number of moles first.  
Number of moles = mass/molar mass
 Molar mass = 51.9961
 Number of moles = 13/ 51.9961 = 0.2500.
 1 mole of any particle contains 6.02 * 10^(23) atoms. 
 Hence 0.2500 moles contains X 
 Hence X = 0.25 * 6.022 * 10^(23) = 1.5055 * 10^(23)

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).