Cyanide (c≡n−) blocks complex iv of the electron transport chain. suggest a hypothesis for what happens to the etc when complex iv stops working. your hypothesis should explain why cyanide poisoning in humans is fatal.

Cytochrome iv allow the transfer of electron from cytochrome a₃ present in cytochrome iv that reduce oxygen on mitochondrial membrane for ATP synthesis.  

Hypothesis: cyanide blocks the cytochrome iv, therefore, oxygen will not be utilized which is important to derive energy for the cell to live.

So binding of cyanide inhibit the electron transfer to the oxygen atom, therefore, the electron transport chain will be blocked at the terminal position. This blockage does not allow the use of oxygen atom to derive energy for the cell and result in the rapid death of the cell.

So cyanide poisoning in humans is fatal because it blocks ETC by restricting the transfer of electron from cyt iv to oxygen and ultimately blocks ATP synthesis which is required by the cell to live.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.