Answer:- 
molecules.
Solution:- The grams of tetrabromomethane are given and it asks to calculate the number of molecules.
It is a two step unit conversion problem. In the first step, grams are converted to moles on dividing the grams by molar mass.
In second step, the moles are converted to molecules on multiplying by Avogadro number.
Molar mass of 
= 12+4(79.9) = 331.6 g per mol
let's make the set up using dimensional analysis:
= molecules
So, there will be molecules in 250 grams of .
Answer:
The electronegativity from order of least to highest is:
Ne, Ca, Fe, F
Explanation:
Elements in the periodic table have been arranged based on their level of electronegativity (which is the ability of an atom to attract electrons).
According to Paulings scale of rating elements based on their electronegativity, the electronegativity value of Fe, Ca, Ne, and F are 1.83, 1, 0 and 3.98 respectively.
Hence, based on Pauling scale, the order of electronegativity from least to highest is:
Ne > Ca > Fe > F
Control rods are used<span> in </span>nuclear<span> reactors to </span>control<span> the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.</span>
