These types of reactions are called endothermic reactions. This is when the system absorbs the heat from the surrounding. In this case, the system is the chemical reaction and the environment is the direct vicinity of the chemical reaction. You notice that the pack becomes cold because heat from the surrounding is being absorb.
The bonds involved in this substance includes ionic bonding and polar covalent.
Which should give the least vigorous reaction when dropped in water?
H2SO4 - sulpfhuric acid
Explanation:
(a) As per the mole concept, one mole of any atom contains 
atoms or molecules, that is, Avogadro's number of atoms.
Therefore, 1 mole of 
= 
molecules
= 
molecules
1 mole of 
= molecules
= molecules
1 mole of 
= molecules
= molecules
Hence, there are equal number of molecules present in the given atoms.
(b) Mass of each given atom will be calculated as follows.
Mass = no. of moles × molar mass
As one molecule of contains 2 atoms of hydrogen.
So, mass of 1 mole of = 
= 2.016 g
mass of 1 mole of = 
= 31.996 g
mass of 1 mole of = 
= 37.996 g
Thus, we can conclude that has the greatest mass.
The atomic structure of an atom is well explained experimentally by different experiments like Rutherford's experiment etc
Based on these experiments it was determined that the atoms contains a central part known as nucleus which contains the positively charged sub atomic particles protons and neutral sub atomic particles neutron.
There is empty space around the nucleus in which negatively charged subatomic particles are found known as electrons.
So answers are
a) protons
b) neutrons
c) electrons
