Water is molecule formed by the covalent bonding of two atoms of hydrogen with one atom of oxygen. The chemical and physical properties of water are different from each of it's constituent elements Oxygen and Hydrogen. The properties of a drop of water will depend on the simplest unit of water, which is the water molecule 
. Therefore, the smallest unit of the water drop that retains all the physical and chemical properties exhibited by a sample of water is the molecule.
It increases across a period and decreases down a group. A good way to remember this is that fluorine is the most electronegative atom, and it's to the top right of the table.
The atoms of hydrogen that are present in 7.63 g of ammonia(NH3)
find the moles of NH3 =mass/molar mass
7.63 g/ 17 g/mol = 0.449 moles
since there is 3 atoms of H in NH3 the moles of H = 0.449 x 3 = 1.347 moles
by use of 1 mole = 6.02 x10^23 atoms
what about 1.347 moles
= 1.347 moles/1 moles x 6.02 x10^23 atoms = 8.11 x10^23 atoms of Hydrogen
